概述
做過iOS開發的同學相信對於GCD（Grand Central Dispatch）並不陌生，因為在平時多執行緒開發過程中GCD應該是使用最多的技術甚至它要比它的上層封裝NSOperation還要常用，其中最主要的原因是簡單易用功能強大。本文將從GCD的原理和使用兩個層面分析GCD的內容，本文會結合原始碼和例項分析使用GCD的注意事項，原始碼解讀部分主要通過註釋原始碼的方式方便進行原始碼分析，具體到細節通過在原始碼解釋說明。
開源的libdispatch
和前面一篇文章深入瞭解Runloop一樣GCD的程式碼是開源的(也可以直接從蘋果官網下載)，這樣要弄清GCD的很多實現原理就有了可能，所以文中不涉及的很多細節大家可以通過原始碼進行了解。下面讓我們看一下關於常見的幾個型別的原始碼：
佇列型別 dispatch_queue_t
dispatch_queue_t應該是平時接觸最多的一個GCD型別，比如說建立一個佇列，它返回的就是一個dispatch_queue_t型別：
dispatch_queue_t serialDispatch = dispatch_queue_create("com.cmjstudio.dispatch", nil);
通過檢視原始碼可以看到dispatch_queue_t的定義：
// 首先可以看到dispatch_queue_t本身只是dispatch_queue_s這個結構體指標
typedef struct dispatch_queue_s *dispatch_queue_t;
// 繼續檢視dispatch_queue_s定義，可以看到一個
DISPATCH_QUEUE_CLASS_HEADER的巨集定義
struct dispatch_queue_s {
DISPATCH_QUEUE_CLASS_HEADER(queue, void *__dq_opaque1);
/* 32bit hole on LP64 */
} DISPATCH_ATOMIC64_ALIGN;
// 檢視DISPATCH_QUEUE_CLASS_HEADER
#define DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__) \
_DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__); \
/* LP64 global queue cacheline boundary */ \
unsigned long dq_serialnum; \
const char *dq_label; \
DISPATCH_UNION_LE(uint32_t volatile dq_atomic_flags, \
const uint16_t dq_width, \
const uint16_t __dq_opaque2 \
); \
dispatch_priority_t dq_priority; \
union { \
struct dispatch_queue_specific_head_s *dq_specific_head; \
struct dispatch_source_refs_s *ds_refs; \
struct dispatch_timer_source_refs_s *ds_timer_refs; \
struct dispatch_mach_recv_refs_s *dm_recv_refs; \
}; \
int volatile dq_sref_cnt
// 展開_DISPATCH_QUEUE_CLASS_HEADER
#define _DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__) \
DISPATCH_OBJECT_HEADER(x); \
DISPATCH_UNION_LE(uint64_t volatile dq_state, \
dispatch_lock dq_state_lock, \
uint32_t dq_state_bits \
); \
__pointer_sized_field__
// 持續展開DISPATCH_OBJECT_HEADER
#define DISPATCH_OBJECT_HEADER(x) \
struct dispatch_object_s _as_do[0]; \
_DISPATCH_OBJECT_HEADER(x)
// 進一步檢視 _DISPATCH_OBJECT_HEADER
#define _DISPATCH_OBJECT_HEADER(x) \
struct _os_object_s _as_os_obj[0]; \
OS_OBJECT_STRUCT_HEADER(dispatch_##x); \
struct dispatch_##x##_s *volatile do_next; \
struct dispatch_queue_s *do_targetq; \
void *do_ctxt; \
void *do_finalizer
// 再檢視 OS_OBJECT_STRUCT_HEADER
#define OS_OBJECT_STRUCT_HEADER(x) \
_OS_OBJECT_HEADER(\
const void *_objc_isa, \
do_ref_cnt, \
do_xref_cnt); \
const struct x##_vtable_s *do_vtable
// 進一步檢視 _OS_OBJECT_HEADER
#define _OS_OBJECT_HEADER(isa, ref_cnt, xref_cnt) \
isa; /* must be pointer-sized */ \
int volatile ref_cnt; \
int volatile xref_cnt
上面的原始碼拆分過程儘管繁瑣但是每一步都可以在原始碼中順利的找到倒也不是太複雜。最終可以看到 dispatch_queue_t 本身儲存了我們平時常見的label、priority、specific等，本身就是isa指標和引用計數器等一些資訊。
需要說明的是 dispatch 版本眾多，如果檢視當前版本可以直接列印
DISPATCH_API_VERSION即可。
建立佇列 dispatch_queue_create
dispatch_queue_create 用於建立一個佇列，返回型別是上面分析過的dispatch_queue_t ，那麼現在看一下如何建立一個佇列：
dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
return _dispatch_lane_create_with_target(label, attr,
DISPATCH_TARGET_QUEUE_DEFAULT, true);
}
// 然後進一步檢視 _dispatch_lane_create_with_target 的程式碼
static dispatch_queue_t
_dispatch_lane_create_with_target(const char *label, dispatch_queue_attr_t dqa,
dispatch_queue_t tq, bool legacy)
{
dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);
//
// Step 1: Normalize arguments (qos, overcommit, tq)
//
dispatch_qos_t qos = dqai.dqai_qos;
#if !HAVE_PTHREAD_WORKQUEUE_QOS
if (qos == DISPATCH_QOS_USER_INTERACTIVE) {
dqai.dqai_qos = qos = DISPATCH_QOS_USER_INITIATED;
}
if (qos == DISPATCH_QOS_MAINTENANCE) {
dqai.dqai_qos = qos = DISPATCH_QOS_BACKGROUND;
}
#endif // !HAVE_PTHREAD_WORKQUEUE_QOS
_dispatch_queue_attr_overcommit_t overcommit = dqai.dqai_overcommit;
if (overcommit != _dispatch_queue_attr_overcommit_unspecified && tq) {
if (tq->do_targetq) {
DISPATCH_CLIENT_CRASH(tq, "Cannot specify both overcommit and "
"a non-global target queue");
}
}
if (tq && dx_type(tq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE) {
// Handle discrepancies between attr and target queue, attributes win
if (overcommit == _dispatch_queue_attr_overcommit_unspecified) {
if (tq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) {
overcommit = _dispatch_queue_attr_overcommit_enabled;
} else {
overcommit = _dispatch_queue_attr_overcommit_disabled;
}
}
if (qos == DISPATCH_QOS_UNSPECIFIED) {
qos = _dispatch_priority_qos(tq->dq_priority);
}
tq = NULL;
} else if (tq && !tq->do_targetq) {
// target is a pthread or runloop root queue, setting QoS or overcommit
// is disallowed
if (overcommit != _dispatch_queue_attr_overcommit_unspecified) {
DISPATCH_CLIENT_CRASH(tq, "Cannot specify an overcommit attribute "
"and use this kind of target queue");
}
} else {
if (overcommit == _dispatch_queue_attr_overcommit_unspecified) {
// Serial queues default to overcommit!
overcommit = dqai.dqai_concurrent ?
_dispatch_queue_attr_overcommit_disabled :
_dispatch_queue_attr_overcommit_enabled;
}
}
if (!tq) {
tq = _dispatch_get_root_queue(
qos == DISPATCH_QOS_UNSPECIFIED ? DISPATCH_QOS_DEFAULT : qos,
overcommit == _dispatch_queue_attr_overcommit_enabled)->_as_dq;
if (unlikely(!tq)) {
DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");
}
}
//
// Step 2: Initialize the queue
//
if (legacy) {
// if any of these attributes is specified, use non legacy classes
if (dqai.dqai_inactive || dqai.dqai_autorelease_frequency) {
legacy = false;
}
}
const void *vtable;
dispatch_queue_flags_t dqf = legacy ? DQF_MUTABLE : 0;
if (dqai.dqai_concurrent) {
vtable = DISPATCH_VTABLE(queue_concurrent);
} else {
vtable = DISPATCH_VTABLE(queue_serial);
}
switch (dqai.dqai_autorelease_frequency) {
case DISPATCH_AUTORELEASE_FREQUENCY_NEVER:
dqf |= DQF_AUTORELEASE_NEVER;
break;
case DISPATCH_AUTORELEASE_FREQUENCY_WORK_ITEM:
dqf |= DQF_AUTORELEASE_ALWAYS;
break;
}
if (label) {
const char *tmp = _dispatch_strdup_if_mutable(label);
if (tmp != label) {
dqf |= DQF_LABEL_NEEDS_FREE;
label = tmp;
}
}
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
dq->dq_label = label;
dq->dq_priority = _dispatch_priority_make((dispatch_qos_t)dqai.dqai_qos,
dqai.dqai_relpri);
if (overcommit == _dispatch_queue_attr_overcommit_enabled) {
dq->dq_priority |= DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
}
if (!dqai.dqai_inactive) {
_dispatch_queue_priority_inherit_from_target(dq, tq);
_dispatch_lane_inherit_wlh_from_target(dq, tq);
}
_dispatch_retain(tq);
dq->do_targetq = tq;
_dispatch_object_debug(dq, "%s", __func__);
return _dispatch_trace_queue_create(dq)._dq;
}
從原始碼註釋也可以看出主要有兩步操作，第一步是 Normalize arguments，第二部才是真正建立佇列，忽略一些引數規範化操作。首先_dispatch_get_root_queue用於獲取root佇列，它有兩個引數：一個是佇列優先順序（有6個：userInteractive>default>unspecified>userInitiated>utility>background），另一個是支援不支援過載overcommit（支援overcommit的佇列在建立佇列時無論系統是否有足夠的資源都會重新開一個執行緒），所以總共就有12個root佇列。對應的原始碼如下(其實是從一個陣列中獲取)：
DISPATCH_ALWAYS_INLINE DISPATCH_CONST
static inline dispatch_queue_global_t
_dispatch_get_root_queue(dispatch_qos_t qos, bool overcommit)
{
if (unlikely(qos < DISPATCH_QOS_MIN || qos > DISPATCH_QOS_MAX)) {
DISPATCH_CLIENT_CRASH(qos, "Corrupted priority");
}
return &_dispatch_root_queues[2 * (qos - 1) + overcommit];
}
至於12個root佇列可以檢視原始碼：
struct dispatch_queue_global_s _dispatch_root_queues[] = {
#define _DISPATCH_ROOT_QUEUE_IDX(n, flags) \
((flags & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) ? \
DISPATCH_ROOT_QUEUE_IDX_##n##_QOS_OVERCOMMIT : \
DISPATCH_ROOT_QUEUE_IDX_##n##_QOS)
#define _DISPATCH_ROOT_QUEUE_ENTRY(n, flags, ...) \
[_DISPATCH_ROOT_QUEUE_IDX(n, flags)] = { \
DISPATCH_GLOBAL_OBJECT_HEADER(queue_global), \
.dq_state = DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE, \
.do_ctxt = _dispatch_root_queue_ctxt(_DISPATCH_ROOT_QUEUE_IDX(n, flags)), \
.dq_atomic_flags = DQF_WIDTH(DISPATCH_QUEUE_WIDTH_POOL), \
.dq_priority = flags | ((flags & DISPATCH_PRIORITY_FLAG_FALLBACK) ? \
_dispatch_priority_make_fallback(DISPATCH_QOS_##n) : \
_dispatch_priority_make(DISPATCH_QOS_##n, 0)), \
__VA_ARGS__ \
}
_DISPATCH_ROOT_QUEUE_ENTRY(MAINTENANCE, 0,
.dq_label = "com.apple.root.maintenance-qos",
.dq_serialnum = 4,
),
_DISPATCH_ROOT_QUEUE_ENTRY(MAINTENANCE, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.maintenance-qos.overcommit",
.dq_serialnum = 5,
),
_DISPATCH_ROOT_QUEUE_ENTRY(BACKGROUND, 0,
.dq_label = "com.apple.root.background-qos",
.dq_serialnum = 6,
),
_DISPATCH_ROOT_QUEUE_ENTRY(BACKGROUND, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.background-qos.overcommit",
.dq_serialnum = 7,
),
_DISPATCH_ROOT_QUEUE_ENTRY(UTILITY, 0,
.dq_label = "com.apple.root.utility-qos",
.dq_serialnum = 8,
),
_DISPATCH_ROOT_QUEUE_ENTRY(UTILITY, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.utility-qos.overcommit",
.dq_serialnum = 9,
),
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT, DISPATCH_PRIORITY_FLAG_FALLBACK,
.dq_label = "com.apple.root.default-qos",
.dq_serialnum = 10,
),
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT,
DISPATCH_PRIORITY_FLAG_FALLBACK | DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.default-qos.overcommit",
.dq_serialnum = 11,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INITIATED, 0,
.dq_label = "com.apple.root.user-initiated-qos",
.dq_serialnum = 12,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INITIATED, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.user-initiated-qos.overcommit",
.dq_serialnum = 13,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INTERACTIVE, 0,
.dq_label = "com.apple.root.user-interactive-qos",
.dq_serialnum = 14,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INTERACTIVE, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.user-interactive-qos.overcommit",
.dq_serialnum = 15,
),
};
其實我們平時用到的全域性佇列也是其中一個root佇列，這個只要檢視dispatch_get_global_queue程式碼就可以了：
dispatch_queue_global_t
dispatch_get_global_queue(intptr_t priority, uintptr_t flags)
{
dispatch_assert(countof(_dispatch_root_queues) ==
DISPATCH_ROOT_QUEUE_COUNT);
if (flags & ~(unsigned long)DISPATCH_QUEUE_OVERCOMMIT) {
return DISPATCH_BAD_INPUT;
}
dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority);
#if !HAVE_PTHREAD_WORKQUEUE_QOS
if (qos == QOS_CLASS_MAINTENANCE) {
qos = DISPATCH_QOS_BACKGROUND;
} else if (qos == QOS_CLASS_USER_INTERACTIVE) {
qos = DISPATCH_QOS_USER_INITIATED;
}
#endif
if (qos == DISPATCH_QOS_UNSPECIFIED) {
return DISPATCH_BAD_INPUT;
}
return _dispatch_get_root_queue(qos, flags & DISPATCH_QUEUE_OVERCOMMIT);
}
可以很清楚的看到，dispatch_get_global_queue的本質就是呼叫_dispatch_get_root_queue，其中的flag只是一個蘋果予保留欄位，通常我們傳0（你可以試試傳1應該佇列建立失敗），而代入上面的陣列當使用dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)。如果列印這個返回結果可以看到：
<OS_dispatch_queue_global: com.apple.root.default-qos[0x1063cbf00] = { xref = -2147483648, ref = -2147483648, sref = 1, target = [0x0], width = 0xfff, state = 0x0060000000000000, in-barrier}>
首先通過上面陣列進行索引2 * (qos - 1) + overcommit = 2*(4-1)+0 = 6 ，可以索引得到 dq_serialnum=10的佇列，剛好label=com.apple.root.default-qos。至於qos引數為什麼是4呢？
DISPATCH_ALWAYS_INLINE
static inline dispatch_qos_t
_dispatch_qos_from_queue_priority(intptr_t priority)
{
switch (priority) {
case DISPATCH_QUEUE_PRIORITY_BACKGROUND: return DISPATCH_QOS_BACKGROUND;
case DISPATCH_QUEUE_PRIORITY_NON_INTERACTIVE: return DISPATCH_QOS_UTILITY;
case DISPATCH_QUEUE_PRIORITY_LOW: return DISPATCH_QOS_UTILITY;
case DISPATCH_QUEUE_PRIORITY_DEFAULT: return DISPATCH_QOS_DEFAULT;
case DISPATCH_QUEUE_PRIORITY_HIGH: return DISPATCH_QOS_USER_INITIATED;
default: return _dispatch_qos_from_qos_class((qos_class_t)priority);
}
#define DISPATCH_QOS_DEFAULT ((dispatch_qos_t)4)
}
然後我們分析一下dispatch_queue_create中的DISPATCH_VTABLE這個巨集：
#define DISPATCH_VTABLE(name) DISPATCH_OBJC_CLASS(name)
// 檢視DISPATCH_OBJC_CLASS
#define DISPATCH_OBJC_CLASS(name) (&DISPATCH_CLASS_SYMBOL(name))
// 進一步檢視 DISPATCH_CLASS_SYMBOL
#define DISPATCH_CLASS_SYMBOL(name) OS_dispatch_##name##_class
解析之後就是按佇列型別分別獲取不同佇列型別的類： OS_dispatch_queue_concurrent_class 和 OS_dispatch_queue_serial_class ，對比我們平時列印一個佇列的資訊(如下)，可以看到 OS_dispatch_queue_serial 或者 OS_dispatch_queue_concurrent_class :
<OS_dispatch_queue_serial: com.cmjstudio.dispatch[0x6000026a5a00] = { xref = 1, ref = 1, sref = 1, target = com.apple.root.default-qos.overcommit[0x108a4bf80], width = 0x1, state = 0x001ffe2000000000, in-flight = 0}>
接著看_dispatch_object_alloc和_dispatch_queue_init，分別用於申請對應型別的記憶體和初始化。首先看前者的實現:
// 注意對於iOS並不滿足 OS_OBJECT_HAVE_OBJC1
void *
_dispatch_object_alloc(const void *vtable, size_t size)
{
#if OS_OBJECT_HAVE_OBJC1
const struct dispatch_object_vtable_s *_vtable = vtable;
dispatch_object_t dou;
dou._os_obj = _os_object_alloc_realized(_vtable->_os_obj_objc_isa, size);
dou._do->do_vtable = vtable;
return dou._do;
#else
return _os_object_alloc_realized(vtable, size);
#endif
}
// 接著看 _os_object_alloc_realized
_os_object_t
_os_object_alloc_realized(const void *cls, size_t size)
{
dispatch_assert(size >= sizeof(struct _os_object_s));
return _os_objc_alloc(cls, size);
}
// 再看一下 _os_objc_alloc
static inline id
_os_objc_alloc(Class cls, size_t size)
{
id obj;
size -= sizeof(((struct _os_object_s *)NULL)->os_obj_isa);
while (unlikely(!(obj = class_createInstance(cls, size)))) {
_dispatch_temporary_resource_shortage();
}
return obj;
}
DISPATCH_NOINLINE
void
_dispatch_temporary_resource_shortage(void)
{
sleep(1);
__asm__ __volatile__(""); // prevent tailcall
}
然後看一下記憶體分配之後的初始化_dispatch_queue_init原始碼，也只是簡單的進行了初始化工作，不過值得一提的是dqai.dqai_concurrent ? DISPATCH_QUEUE_WIDTH_MAX : 1這個引數，DISPATCH_QUEUE_WIDTH_MAX其實看一下原始碼就知道是0x1000ull-2就是0xffe，而如果是序列佇列就是1，這也是為什麼可以在上面列印中看到width = 0x1的原因，width本身就是併發數的個數，對於序列佇列是1而對於併發佇列是不限制的(回過頭去看全域性佇列width為什麼是0xfff呢，因為它的width是#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1)
)=0x1000ull-1：
static inline dispatch_queue_class_t
_dispatch_queue_init(dispatch_queue_class_t dqu, dispatch_queue_flags_t dqf,
uint16_t width, uint64_t initial_state_bits)
{
uint64_t dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(width);
dispatch_queue_t dq = dqu._dq;
dispatch_assert((initial_state_bits & ~(DISPATCH_QUEUE_ROLE_MASK |
DISPATCH_QUEUE_INACTIVE)) == 0);
if (initial_state_bits & DISPATCH_QUEUE_INACTIVE) {
dq_state |= DISPATCH_QUEUE_INACTIVE + DISPATCH_QUEUE_NEEDS_ACTIVATION;
dq->do_ref_cnt += 2; // rdar://8181908 see _dispatch_lane_resume
if (dx_metatype(dq) == _DISPATCH_SOURCE_TYPE) {
dq->do_ref_cnt++; // released when DSF_DELETED is set
}
}
dq_state |= (initial_state_bits & DISPATCH_QUEUE_ROLE_MASK);
dq->do_next = DISPATCH_OBJECT_LISTLESS;
dqf |= DQF_WIDTH(width);
os_atomic_store2o(dq, dq_atomic_flags, dqf, relaxed);
dq->dq_state = dq_state;
dq->dq_serialnum =
os_atomic_inc_orig(&_dispatch_queue_serial_numbers, relaxed);
return dqu;
}
接著看dispatch_queue_create的dq->do_targetq = tq;這句話是什麼意思呢？這個其實是當使用dispatch_queue_create建立的自定義佇列（事實上包括主佇列和管理佇列，也就是非全域性佇列[可以看一下上面的原始碼全域性佇列並沒有設定do_targetq，但是事實上它本身就是root佇列]），都需要壓入到全域性佇列（這裡指的是root佇列）進行處理，這個目標佇列的目的就是允許我們將一個佇列放在另一個佇列裡執行任務。看一下上面建立自定義佇列的原始碼不難發現，如果是自定義一個序列佇列其實最終就是一個root佇列。
為了驗證上面關於主佇列也是root佇列的說法不放看一下主佇列的原始碼：
DISPATCH_INLINE DISPATCH_ALWAYS_INLINE DISPATCH_CONST DISPATCH_NOTHROW
dispatch_queue_main_t
dispatch_get_main_queue(void)
{
return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
}
// 進一步檢視 DISPATCH_GLOBAL_OBJECT
#define DISPATCH_GLOBAL_OBJECT(type, object) ((OS_OBJECT_BRIDGE type)&(object))
// 先看一下型別 dispatch_queue_main_t
#if defined(__DISPATCH_BUILDING_DISPATCH__) && !defined(__OBJC__)
typedef struct dispatch_queue_static_s *dispatch_queue_main_t;
#else
DISPATCH_DECL_SUBCLASS(dispatch_queue_main, dispatch_queue_serial);
#endif
// 然後檢視 _dispatch_main_q，可以看到真正的型別如下
struct dispatch_queue_static_s _dispatch_main_q = {
DISPATCH_GLOBAL_OBJECT_HEADER(queue_main),
#if !DISPATCH_USE_RESOLVERS
.do_targetq = _dispatch_get_default_queue(true),
#endif
.dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(1) |
DISPATCH_QUEUE_ROLE_BASE_ANON,
.dq_label = "com.apple.main-thread",
.dq_atomic_flags = DQF_THREAD_BOUND | DQF_WIDTH(1),
.dq_serialnum = 1,
};
// 檢視 _dispatch_get_default_queue原始碼
#define _dispatch_get_default_queue(overcommit) \
_dispatch_root_queues[DISPATCH_ROOT_QUEUE_IDX_DEFAULT_QOS + \
!!(overcommit)]._as_dq
// 檢視 DISPATCH_ROOT_QUEUE_IDX_DEFAULT_QOS
enum {
DISPATCH_ROOT_QUEUE_IDX_MAINTENANCE_QOS = 0,
DISPATCH_ROOT_QUEUE_IDX_MAINTENANCE_QOS_OVERCOMMIT,
DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_QOS,
DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_QOS_OVERCOMMIT,
DISPATCH_ROOT_QUEUE_IDX_UTILITY_QOS,
DISPATCH_ROOT_QUEUE_IDX_UTILITY_QOS_OVERCOMMIT,
DISPATCH_ROOT_QUEUE_IDX_DEFAULT_QOS,
DISPATCH_ROOT_QUEUE_IDX_DEFAULT_QOS_OVERCOMMIT,
DISPATCH_ROOT_QUEUE_IDX_USER_INITIATED_QOS,
DISPATCH_ROOT_QUEUE_IDX_USER_INITIATED_QOS_OVERCOMMIT,
DISPATCH_ROOT_QUEUE_IDX_USER_INTERACTIVE_QOS,
DISPATCH_ROOT_QUEUE_IDX_USER_INTERACTIVE_QOS_OVERCOMMIT,
_DISPATCH_ROOT_QUEUE_IDX_COUNT,
};
可以看到主佇列do_targetq也是一個root佇列（通過獲取_dispatch_root_queues），DISPATCH_ROOT_QUEUE_IDX_DEFAULT_QOS =6 所以 _dispatch_root_queues[6+1] 就是com.apple.root.default-qos.overcommit，不妨列印一些主佇列(如下)，可以看到target正是com.apple.root.default-qos.overcommit，而且width=1，其次由於dispatch_queue_main_t是對dispatch_queue_serial的重寫所以也是一個序列佇列：
<OS_dispatch_queue_main: com.apple.main-thread[0x1092dfb00] = { xref = -2147483648, ref = -2147483648, sref = 1, target = com.apple.root.default-qos.overcommit[0x1092dff80], width = 0x1, state = 0x001ffe9000000300, dirty, in-flight = 0, thread = 0x303 }>
到了這裡關於佇列的建立我們已經基本介紹完了，可以看到不管是自定義佇列、全域性佇列還是主佇列最終都直接或者間接的依賴12個root佇列來執行任務排程(儘管如此主佇列有自己的label，如果按照label計算總共16個，除了上面的12個，就是com.apple.main-thread還有兩個內部管理佇列com.apple.libdispatch-manager和com.apple.root.libdispatch-manager以及runloop的執行佇列)。下面看一下幾個常用的佇列任務的執行方法的原始碼，對於任務的執行GCD其實主要用兩個方法dispatch_sync和dispatch_async。
佇列和執行緒之間的關係
上面提到一個重要概念是overcommit，overcommit的佇列在佇列建立時會新建一個執行緒，非overcommit佇列建立佇列則未必建立執行緒。另外width=1意味著是序列佇列，只有一個執行緒可用，width=0xffe則意味著並行佇列，執行緒則是從執行緒池獲取，可用執行緒數是64個。
可以看到全域性佇列是非overcommit的（flat保留字只能傳0，如果預設優先順序則是com.apple.root.default-qos，但是width=0xffe是並行佇列）；主佇列是overcommit的com.apple.root.default-qos.overcommit，不過它是序列佇列，width=1，並且執行的這個執行緒只能是主執行緒；自定義序列佇列是overcommit的，預設優先順序則是 com.apple.root.default-qos.overcommit，並行佇列則是非overcommit的。
這裡看一下為什麼上面說並行佇列最大執行緒數是64個，不妨結合幾個例子來檢視：
/**
序列佇列只有一個執行緒，執行緒num > 2
**/
- (void)test1 {
dispatch_queue_t serialQueue = dispatch_queue_create("com.cmjstudio.dispatch", DISPATCH_QUEUE_SERIAL);
for (int i=0; i<1000; ++i) {
dispatch_async(serialQueue, ^{
NSLog(@"%@，%i",[NSThread currentThread],i); // only one thread（number = 3~66）
});
}
}
- (void)test2 {
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INITIATED, -1);
dispatch_queue_t serialQueue = dispatch_queue_create("com.cmjstudio.dispatch", attr);
for (int i=0; i<1000; ++i) {
dispatch_async(serialQueue, ^{
NSLog(@"%@，%i",[NSThread currentThread],i); // only one thread
});
}
}
/**
不管優先順序多高並行佇列有最多有64個執行緒，執行緒num在3~66，在一次輪詢中遇到高優先順序的會先執行
**/
- (void)test3 {
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.cmjstudio.dispatch", DISPATCH_QUEUE_CONCURRENT);
for (int i=0; i<1000; ++i) {
dispatch_async(concurrentQueue, ^{
NSLog(@"%@，%i",[NSThread currentThread],i); // 64 thread (num = 3~66)
});
}
}
// 全域性佇列是並行佇列(下面的demo會先輸出global然後是所有custom再是剩餘的global,整體遵循高優先順序先執行規則，個別低優先順序先輸出的原因是傳送global期間還沒有輪訓到高優先順序任務，一旦遇到就會先執行高優先順序任務)
- (void)test4 {
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
for (int i=0; i<100; ++i) {
dispatch_async(globalQueue, ^{
NSLog(@"global:%@，%i",[NSThread currentThread],i); // 64 thread (num = 3~66)
});
}
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_CONCURRENT, QOS_CLASS_USER_INITIATED, -1);
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.cmjstudio.dispatch", attr);
for (int i=0; i<100; ++i) {
dispatch_async(concurrentQueue, ^{
NSLog(@"custom:%@，%i",[NSThread currentThread],i); // 64 thread (num = 3~66)
});
}
}
/**
序列佇列和並行佇列都存線上程數多了1個，number最大到了67，不過序列佇列的任務不一定在67這個執行緒中而是會複用前面的任意一個執行緒。說明序列佇列加入時一定會建立一個執行緒
**/
- (void)test5 {
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_CONCURRENT, QOS_CLASS_USER_INITIATED, -1);
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.cmjstudio.dispatch", attr);
for (int i=0; i<100; ++i) {
dispatch_async(concurrentQueue, ^{
NSLog(@"concurrent:%@，%i",[NSThread currentThread],i);
});
}
dispatch_queue_t serialQueue = dispatch_queue_create("com.cmjstudio.dispatch", DISPATCH_QUEUE_SERIAL);
for (int i=0; i<100; ++i) {
dispatch_async(serialQueue, ^{
NSLog(@"serial:%@，%i",[NSThread currentThread],i);
});
}
}
/**
當一個序列佇列依附於一個並行佇列時（非overcommit，如果是overcommit佇列則會新建一個執行緒），執行緒最多恢復到了64個，並不會再新建一個執行緒了
**/
- (void)test6 {
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_CONCURRENT, QOS_CLASS_USER_INITIATED, -1);
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.cmjstudio.dispatch", attr);
for (int i=0; i<100; ++i) {
dispatch_async(concurrentQueue, ^{
NSLog(@"%@，%i",[NSThread currentThread],i);
});
}
dispatch_queue_t serialQueue = dispatch_queue_create("com.cmjstudio.dispatch", DISPATCH_QUEUE_SERIAL);
dispatch_set_target_queue(serialQueue, concurrentQueue);
for (int i=0; i<100; ++i) {
dispatch_async(serialQueue, ^{
NSLog(@"%@，%i",[NSThread currentThread],i);
});
}
}
可以看到對於 dispatch_asyn 的呼叫（同步操作執行緒都在主執行緒不再贅述）序列佇列是overcommit的，建立佇列會建立1個新的執行緒，並行佇列是非overcommit的，不一定會新建執行緒，會從執行緒池中的64個執行緒中獲取並使用。另外上面的dispatch_set_target_queue 操作和前面原始碼中的do_targetq是作用一樣的。
這樣以來反而序列佇列是開發中應該注意的，因為一旦新建一個序列佇列就會新建一個執行緒，避免在類似迴圈操作中新建序列佇列，這個上限是多少是任意多嗎？其實也不是最多新增512個（不算主執行緒,number從4開始到515）但是這明顯已經是災難性的了。另外對於多個同一優先順序的自定義序列佇列（比如：com.apple.root.default-qos.overcommit）對於 dispatch_asyn 呼叫又怎麼保證呼叫順序呢？儘管是overcommit可以建立多個執行緒，畢竟都在一個root佇列中執行，優先順序又是相同的。
先看一段程式碼：
-(void)test10{
dispatch_queue_t serialQueue1 = dispatch_queue_create("com.cmjstudio.dispatch1", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t serialQueue2 = dispatch_queue_create("com.cmjstudio.dispatch2", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t serialQueue3 = dispatch_queue_create("com.cmjstudio.dispatch3", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQueue1, ^{
NSLog(@"serialQueue1 async invoke:%@", [NSThread currentThread]);
});
dispatch_async(serialQueue2, ^{
NSLog(@"serialQueue2 async invoke:%@", [NSThread currentThread]);
});
dispatch_async(serialQueue3, ^{
NSLog(@"serialQueue3 async invoke:%@", [NSThread currentThread]);
});
}
三次執行順序依次如下：
2020-07-07 19:26:57.951602+0800 GCDBasic[68448:3758078] serialQueue2 async invoke:<NSThread: 0x600000e06500>{number = 4, name = (null)}
2020-07-07 19:26:57.951633+0800 GCDBasic[68448:3758079] serialQueue1 async invoke:<NSThread: 0x600000e37f00>{number = 6, name = (null)}
2020-07-07 19:26:57.951651+0800 GCDBasic[68448:3758076] serialQueue3 async invoke:<NSThread: 0x600000e3cc80>{number = 7, name = (null)}
2020-07-07 19:27:08.292555+0800 GCDBasic[68448:3758077] serialQueue1 async invoke:<NSThread: 0x600000e06480>{number = 3, name = (null)}
2020-07-07 19:27:08.292651+0800 GCDBasic[68448:3758271] serialQueue3 async invoke:<NSThread: 0x600000e37e80>{number = 8, name = (null)}
2020-07-07 19:27:08.292659+0800 GCDBasic[68448:3758273] serialQueue2 async invoke:<NSThread: 0x600000e30340>{number = 9, name = (null)}
2020-07-07 19:27:12.261150+0800 GCDBasic[68448:3758077] serialQueue1 async invoke:<NSThread: 0x600000e06480>{number = 3, name = (null)}
2020-07-07 19:27:12.261157+0800 GCDBasic[68448:3758273] serialQueue2 async invoke:<NSThread: 0x600000e30340>{number = 9, name = (null)}
2020-07-07 19:27:12.261162+0800 GCDBasic[68448:3758271] serialQueue3 async invoke:<NSThread: 0x600000e37e80>{number = 8, name = (null)}
確實單次執行都建立了新的執行緒（和前面說的 overcommit 是相符的），但是執行任務的順序可以說是隨機的，這個和執行緒排程有關，那麼如果有比較重的任務會不會造成影響呢？這個答案是如果都分別建立了佇列（overcommit）一般不會有影響，除非建立超過了512個，因為儘管是同一個root佇列但是會建立不同的執行緒，此時當前root佇列僅僅控制任務FIFO，但是並不是只有第一個任務執行完第二個任務才能開始，也就是說FIFO控制的是開始的節奏，但是任務在不同的thread執行不會阻塞。當然一個序列佇列中的多個非同步task是相互有執行順序的，比如下面的程式碼task2一定會被task1阻塞，但是都不會阻塞task3：
-(void)test11{
dispatch_queue_t serialQueue1 = dispatch_queue_create("com.cmjstudio.dispatch1", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQueue1, ^{
NSLog(@"task1 in");
[NSThread sleepForTimeInterval:5];
NSLog(@"serialQueue1-task1 async invoke:%@", [NSThread currentThread]);
});
dispatch_async(serialQueue1, ^{
NSLog(@"task2 in");
[NSThread sleepForTimeInterval:5];
NSLog(@"serialQueue1-task2 async invoke:%@", [NSThread currentThread]);
});
dispatch_queue_t serialQueue2 = dispatch_queue_create("com.cmjstudio.dispatch2", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQueue2, ^{
NSLog(@"task3 in");
NSLog(@"serialQueue2-task3 async invoke:%@", [NSThread currentThread]);
});
}
同步執行 dispatch_sync
DISPATCH_NOINLINE
void
dispatch_sync(dispatch_queue_t dq, dispatch_block_t work)
{
uintptr_t dc_flags = DC_FLAG_BLOCK;
if (unlikely(_dispatch_block_has_private_data(work))) {
return _dispatch_sync_block_with_privdata(dq, work, dc_flags);
}
_dispatch_sync_f(dq, work, _dispatch_Block_invoke(work), dc_flags);
}
// 進一步檢視 _dispatch_sync_f
DISPATCH_NOINLINE
static void
_dispatch_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func,
uintptr_t dc_flags)
{
_dispatch_sync_f_inline(dq, ctxt, func, dc_flags);
}
// 檢視 _dispatch_sync_f_inline
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_sync_f_inline(dispatch_queue_t dq, void *ctxt,
dispatch_function_t func, uintptr_t dc_flags)
{
if (likely(dq->dq_width == 1)) {
return _dispatch_barrier_sync_f(dq, ctxt, func, dc_flags);
}
if (unlikely(dx_metatype(dq) != _DISPATCH_LANE_TYPE)) {
DISPATCH_CLIENT_CRASH(0, "Queue type doesn't support dispatch_sync");
}
dispatch_lane_t dl = upcast(dq)._dl;
// Global concurrent queues and queues bound to non-dispatch threads
// always fall into the slow case, see DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE
if (unlikely(!_dispatch_queue_try_reserve_sync_width(dl))) {
return _dispatch_sync_f_slow(dl, ctxt, func, 0, dl, dc_flags);
}
if (unlikely(dq->do_targetq->do_targetq)) {
return _dispatch_sync_recurse(dl, ctxt, func, dc_flags);
}
_dispatch_introspection_sync_begin(dl);
_dispatch_sync_invoke_and_complete(dl, ctxt, func DISPATCH_TRACE_ARG(
_dispatch_trace_item_sync_push_pop(dq, ctxt, func, dc_flags)));
}
可以看到首先通過width判定是序列佇列還是併發佇列，如果是併發佇列則呼叫_dispatch_sync_invoke_and_complete，序列佇列則呼叫_dispatch_barrier_sync_f。先展開看一下序列佇列的同步執行原始碼：
DISPATCH_NOINLINE
static void
_dispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt,
dispatch_function_t func, uintptr_t dc_flags)
{
_dispatch_barrier_sync_f_inline(dq, ctxt, func, dc_flags);
}
// 看一下 _dispatch_barrier_sync_f_inline
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_barrier_sync_f_inline(dispatch_queue_t dq, void *ctxt,
dispatch_function_t func, uintptr_t dc_flags)
{
dispatch_tid tid = _dispatch_tid_self();
if (unlikely(dx_metatype(dq) != _DISPATCH_LANE_TYPE)) {
DISPATCH_CLIENT_CRASH(0, "Queue type doesn't support dispatch_sync");
}
dispatch_lane_t dl = upcast(dq)._dl;
// The more correct thing to do would be to merge the qos of the thread
// that just acquired the barrier lock into the queue state.
//
// However this is too expensive for the fast path, so skip doing it.
// The chosen tradeoff is that if an enqueue on a lower priority thread
// contends with this fast path, this thread may receive a useless override.
//
// Global concurrent queues and queues bound to non-dispatch threads
// always fall into the slow case, see DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE
if (unlikely(!_dispatch_queue_try_acquire_barrier_sync(dl, tid))) {
return _dispatch_sync_f_slow(dl, ctxt, func, DC_FLAG_BARRIER, dl,
DC_FLAG_BARRIER | dc_flags);
}
if (unlikely(dl->do_targetq->do_targetq)) {
return _dispatch_sync_recurse(dl, ctxt, func,
DC_FLAG_BARRIER | dc_flags);
}
_dispatch_introspection_sync_begin(dl);
_dispatch_lane_barrier_sync_invoke_and_complete(dl, ctxt, func
DISPATCH_TRACE_ARG(_dispatch_trace_item_sync_push_pop(
dq, ctxt, func, dc_flags | DC_FLAG_BARRIER)));
}
首先獲取執行緒id，然後處理死鎖的情況，因此這裡先看一下死鎖的情況：
DISPATCH_NOINLINE
static void
_dispatch_sync_f_slow(dispatch_queue_class_t top_dqu, void *ctxt,
dispatch_function_t func, uintptr_t top_dc_flags,
dispatch_queue_class_t dqu, uintptr_t dc_flags)
{
dispatch_queue_t top_dq = top_dqu._dq;
dispatch_queue_t dq = dqu._dq;
if (unlikely(!dq->do_targetq)) {
return _dispatch_sync_function_invoke(dq, ctxt, func);
}
pthread_priority_t pp = _dispatch_get_priority();
struct dispatch_sync_context_s dsc = {
.dc_flags = DC_FLAG_SYNC_WAITER | dc_flags,
.dc_func = _dispatch_async_and_wait_invoke,
.dc_ctxt = &dsc,
.dc_other = top_dq,
.dc_priority = pp | _PTHREAD_PRIORITY_ENFORCE_FLAG,
.dc_voucher = _voucher_get(),
.dsc_func = func,
.dsc_ctxt = ctxt,
.dsc_waiter = _dispatch_tid_self(),
};
_dispatch_trace_item_push(top_dq, &dsc);
__DISPATCH_WAIT_FOR_QUEUE__(&dsc, dq);
if (dsc.dsc_func == NULL) {
dispatch_queue_t stop_dq = dsc.dc_other;
return _dispatch_sync_complete_recurse(top_dq, stop_dq, top_dc_flags);
}
_dispatch_introspection_sync_begin(top_dq);
_dispatch_trace_item_pop(top_dq, &dsc);
_dispatch_sync_invoke_and_complete_recurse(top_dq, ctxt, func,top_dc_flags
DISPATCH_TRACE_ARG(&dsc));
}
// 看一下 __DISPATCH_WAIT_FOR_QUEUE__
DISPATCH_NOINLINE
static void
__DISPATCH_WAIT_FOR_QUEUE__(dispatch_sync_context_t dsc, dispatch_queue_t dq)
{
uint64_t dq_state = _dispatch_wait_prepare(dq);
if (unlikely(_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter))) {
DISPATCH_CLIENT_CRASH((uintptr_t)dq_state,
"dispatch_sync called on queue "
"already owned by current thread");
}
// Blocks submitted to the main thread MUST run on the main thread, and
// dispatch_async_and_wait also executes on the remote context rather than
// the current thread.
//
// For both these cases we need to save the frame linkage for the sake of
// _dispatch_async_and_wait_invoke
_dispatch_thread_frame_save_state(&dsc->dsc_dtf);
if (_dq_state_is_suspended(dq_state) ||
_dq_state_is_base_anon(dq_state)) {
dsc->dc_data = DISPATCH_WLH_ANON;
} else if (_dq_state_is_base_wlh(dq_state)) {
dsc->dc_data = (dispatch_wlh_t)dq;
} else {
_dispatch_wait_compute_wlh(upcast(dq)._dl, dsc);
}
if (dsc->dc_data == DISPATCH_WLH_ANON) {
dsc->dsc_override_qos_floor = dsc->dsc_override_qos =
(uint8_t)_dispatch_get_basepri_override_qos_floor();
_dispatch_thread_event_init(&dsc->dsc_event);
}
dx_push(dq, dsc, _dispatch_qos_from_pp(dsc->dc_priority));
_dispatch_trace_runtime_event(sync_wait, dq, 0);
if (dsc->dc_data == DISPATCH_WLH_ANON) {
_dispatch_thread_event_wait(&dsc->dsc_event); // acquire
} else {
_dispatch_event_loop_wait_for_ownership(dsc);
}
if (dsc->dc_data == DISPATCH_WLH_ANON) {
_dispatch_thread_event_destroy(&dsc->dsc_event);
// If _dispatch_sync_waiter_wake() gave this thread an override,
// ensure that the root queue sees it.
if (dsc->dsc_override_qos > dsc->dsc_override_qos_floor) {
_dispatch_set_basepri_override_qos(dsc->dsc_override_qos);
}
}
}
// 展開 _dq_state_drain_locked_by
DISPATCH_ALWAYS_INLINE
static inline bool
_dq_state_drain_locked_by(uint64_t dq_state, dispatch_tid tid)
{
return _dispatch_lock_is_locked_by((dispatch_lock)dq_state, tid);
}
// 然後看一下 _dispatch_lock_is_locked_by
DISPATCH_ALWAYS_INLINE
static inline bool
_dispatch_lock_is_locked_by(dispatch_lock lock_value, dispatch_tid tid)
{
// equivalent to _dispatch_lock_owner(lock_value) == tid
return ((lock_value ^ tid) & DLOCK_OWNER_MASK) == 0;
}
佇列push以後就是用_dispatch_lock_is_locked_by判斷將要排程的和當前等待的佇列是不是同一個，如果相同則返回YES，產生死鎖DISPATCH_CLIENT_CRASH；如果沒有產生死鎖，則執行 _dispatch_trace_item_pop()出佇列執行。如何執行排程呢，需要看一下_dispatch_sync_invoke_and_complete_recurse？
DISPATCH_NOINLINE
static void
_dispatch_sync_invoke_and_complete_recurse(dispatch_queue_class_t dq,
void *ctxt, dispatch_function_t func, uintptr_t dc_flags
DISPATCH_TRACE_ARG(void *dc))
{
_dispatch_sync_function_invoke_inline(dq, ctxt, func);
_dispatch_trace_item_complete(dc);
_dispatch_sync_complete_recurse(dq._dq, NULL, dc_flags);
}
// 看一下 _dispatch_sync_function_invoke_inline
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_sync_function_invoke_inline(dispatch_queue_class_t dq, void *ctxt,
dispatch_function_t func)
{
dispatch_thread_frame_s dtf;
_dispatch_thread_frame_push(&dtf, dq);
_dispatch_client_callout(ctxt, func);
_dispatch_perfmon_workitem_inc();
_dispatch_thread_frame_pop(&dtf);
}
// 看一下 _dispatch_client_callout
void
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
@try {
return f(ctxt);
}
@catch (...) {
objc_terminate();
}
}
可以比較清楚的看到最終執行f函式，這個就是外界傳過來的回撥block。
非同步呼叫 dispatch_async
void
dispatch_async(dispatch_queue_t dq, dispatch_block_t work)
{
dispatch_continuation_t dc = _dispatch_continuation_alloc();
uintptr_t dc_flags = DC_FLAG_CONSUME;
dispatch_qos_t qos;
qos = _dispatch_continuation_init(dc, dq, work, 0, dc_flags);
_dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}
// 檢視 _dispatch_continuation_init 程式碼，主要進行block初始化
DISPATCH_ALWAYS_INLINE
static inline dispatch_qos_t
_dispatch_continuation_init(dispatch_continuation_t dc,
dispatch_queue_class_t dqu, dispatch_block_t work,
dispatch_block_flags_t flags, uintptr_t dc_flags)
{
void *ctxt = _dispatch_Block_copy(work);
dc_flags |= DC_FLAG_BLOCK | DC_FLAG_ALLOCATED;
if (unlikely(_dispatch_block_has_private_data(work))) {
dc->dc_flags = dc_flags;
dc->dc_ctxt = ctxt;
// will initialize all fields but requires dc_flags & dc_ctxt to be set
return _dispatch_continuation_init_slow(dc, dqu, flags);
}
dispatch_function_t func = _dispatch_Block_invoke(work);
if (dc_flags & DC_FLAG_CONSUME) {
func = _dispatch_call_block_and_release;
}
return _dispatch_continuation_init_f(dc, dqu, ctxt, func, flags, dc_flags);
}
// 另外檢視 _dispatch_continuation_async
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_continuation_async(dispatch_queue_class_t dqu,
dispatch_continuation_t dc, dispatch_qos_t qos, uintptr_t dc_flags)
{
#if DISPATCH_INTROSPECTION
if (!(dc_flags & DC_FLAG_NO_INTROSPECTION)) {
_dispatch_trace_item_push(dqu, dc);
}
#else
(void)dc_flags;
#endif
return dx_push(dqu._dq, dc, qos);
}
// 進一步檢視 dx_push
#define dx_push(x, y, z) dx_vtable(x)->dq_push(x, y, z)
// 本質是呼叫dx_vtable的dq_push(其實就是呼叫物件的do_push)，進一步檢視 dq_push，我們假設是global_queue進行非同步呼叫可以看到：
DISPATCH_VTABLE_SUBCLASS_INSTANCE(queue_global, lane,
.do_type = DISPATCH_QUEUE_GLOBAL_ROOT_TYPE,
.do_dispose = _dispatch_object_no_dispose,
.do_debug = _dispatch_queue_debug,
.do_invoke = _dispatch_object_no_invoke,
.dq_activate = _dispatch_queue_no_activate,
.dq_wakeup = _dispatch_root_queue_wakeup,
.dq_push = _dispatch_root_queue_push,
);
可以看到dx_push已經到了_dispatch_root_queue_push，這是可以接著檢視_dispatch_root_queue_push：
DISPATCH_NOINLINE
void
_dispatch_root_queue_push(dispatch_queue_global_t rq, dispatch_object_t dou,
dispatch_qos_t qos)
{
#if DISPATCH_USE_KEVENT_WORKQUEUE
dispatch_deferred_items_t ddi = _dispatch_deferred_items_get();
if (unlikely(ddi && ddi->ddi_can_stash)) {
dispatch_object_t old_dou = ddi->ddi_stashed_dou;
dispatch_priority_t rq_overcommit;
rq_overcommit = rq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
if (likely(!old_dou._do || rq_overcommit)) {
dispatch_queue_global_t old_rq = ddi->ddi_stashed_rq;
dispatch_qos_t old_qos = ddi->ddi_stashed_qos;
ddi->ddi_stashed_rq = rq;
ddi->ddi_stashed_dou = dou;
ddi->ddi_stashed_qos = qos;
_dispatch_debug("deferring item %p, rq %p, qos %d",
dou._do, rq, qos);
if (rq_overcommit) {
ddi->ddi_can_stash = false;
}
if (likely(!old_dou._do)) {
return;
}
// push the previously stashed item
qos = old_qos;
rq = old_rq;
dou = old_dou;
}
}
#endif
#if HAVE_PTHREAD_WORKQUEUE_QOS
if (_dispatch_root_queue_push_needs_override(rq, qos)) {
return _dispatch_root_queue_push_override(rq, dou, qos);
}
#else
(void)qos;
#endif
_dispatch_root_queue_push_inline(rq, dou, dou, 1);
}
// 多數情況下符合HAVE_PTHREAD_WORKQUEUE_QOS，會執行_dispatch_root_queue_push_override（對比的是qos與root佇列的qos是否一致，基本上都不一致的。）
DISPATCH_NOINLINE
static void
_dispatch_root_queue_push_override(dispatch_queue_global_t orig_rq,
dispatch_object_t dou, dispatch_qos_t qos)
{
bool overcommit = orig_rq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
dispatch_queue_global_t rq = _dispatch_get_root_queue(qos, overcommit);
dispatch_continuation_t dc = dou._dc;
if (_dispatch_object_is_redirection(dc)) {
// no double-wrap is needed, _dispatch_async_redirect_invoke will do
// the right thing
dc->dc_func = (void *)orig_rq;
} else {
dc = _dispatch_continuation_alloc();
dc->do_vtable = DC_VTABLE(OVERRIDE_OWNING);
dc->dc_ctxt = dc;
dc->dc_other = orig_rq;
dc->dc_data = dou._do;
dc->dc_priority = DISPATCH_NO_PRIORITY;
dc->dc_voucher = DISPATCH_NO_VOUCHER;
}
_dispatch_root_queue_push_inline(rq, dc, dc, 1);
}
// 上面_dispatch_object_is_redirection函式其實就是return _dispatch_object_has_type(dou,DISPATCH_CONTINUATION_TYPE(ASYNC_REDIRECT));所以自定義佇列會走這個if語句，如果是dispatch_get_global_queue不會走if語句。展開 _dispatch_root_queue_push_inline。注意_dispatch_root_queue_push_inline中的if把任務裝進佇列，大多數不走進if語句。但是第一個任務進來之前還是滿足這個條件式的，會進入這個條件語句去啟用佇列來執行裡面的任務，後面再加入的任務因為佇列被啟用了，所以也就不太需要再進入這個佇列了，所以相對來說啟用佇列只要一次
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_root_queue_push_inline(dispatch_queue_global_t dq,
dispatch_object_t _head, dispatch_object_t _tail, int n)
{
struct dispatch_object_s *hd = _head._do, *tl = _tail._do;
if (unlikely(os_mpsc_push_list(os_mpsc(dq, dq_items), hd, tl, do_next))) {
return _dispatch_root_queue_poke(dq, n, 0);
}
}
// 我們可以看到，我們裝入到自定義的任務都被扔到其掛靠的root佇列裡去了，所以我們我們自己建立的佇列只是一個代理人身份，繼續檢視 _dispatch_root_queue_poke 原始碼
DISPATCH_NOINLINE
void
_dispatch_root_queue_poke(dispatch_queue_global_t dq, int n, int floor)
{
if (!_dispatch_queue_class_probe(dq)) {
return;
}
#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
if (likely(dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE))
#endif
{
if (unlikely(!os_atomic_cmpxchg2o(dq, dgq_pending, 0, n, relaxed))) {
_dispatch_root_queue_debug("worker thread request still pending "
"for global queue: %p", dq);
return;
}
}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
return _dispatch_root_queue_poke_slow(dq, n, floor);
}
// 繼續檢視 _dispatch_root_queue_poke_slow
DISPATCH_NOINLINE
static void
_dispatch_root_queue_poke_slow(dispatch_queue_global_t dq, int n, int floor)
{
int remaining = n;
#if !defined(_WIN32)
int r = ENOSYS;
#endif
_dispatch_root_queues_init();
_dispatch_debug_root_queue(dq, __func__);
_dispatch_trace_runtime_event(worker_request, dq, (uint64_t)n);
#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE)
#endif
{
_dispatch_root_queue_debug("requesting new worker thread for global "
"queue: %p", dq);
r = _pthread_workqueue_addthreads(remaining,
_dispatch_priority_to_pp_prefer_fallback(dq->dq_priority));
(void)dispatch_assume_zero(r);
return;
}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
dispatch_pthread_root_queue_context_t pqc = dq->do_ctxt;
if (likely(pqc->dpq_thread_mediator.do_vtable)) {
while (dispatch_semaphore_signal(&pqc->dpq_thread_mediator)) {
_dispatch_root_queue_debug("signaled sleeping worker for "
"global queue: %p", dq);
if (!--remaining) {
return;
}
}
}
bool overcommit = dq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
if (overcommit) {
os_atomic_add2o(dq, dgq_pending, remaining, relaxed);
} else {
if (!os_atomic_cmpxchg2o(dq, dgq_pending, 0, remaining, relaxed)) {
_dispatch_root_queue_debug("worker thread request still pending for "
"global queue: %p", dq);
return;
}
}
int can_request, t_count;
// seq_cst with atomic store to tail <rdar://problem/16932833>
t_count = os_atomic_load2o(dq, dgq_thread_pool_size, ordered);
do {
can_request = t_count < floor ? 0 : t_count - floor;
if (remaining > can_request) {
_dispatch_root_queue_debug("pthread pool reducing request from %d to %d",
remaining, can_request);
os_atomic_sub2o(dq, dgq_pending, remaining - can_request, relaxed);
remaining = can_request;
}
if (remaining == 0) {
_dispatch_root_queue_debug("pthread pool is full for root queue: "
"%p", dq);
return;
}
} while (!os_atomic_cmpxchgvw2o(dq, dgq_thread_pool_size, t_count,
t_count - remaining, &t_count, acquire));
#if !defined(_WIN32)
pthread_attr_t *attr = &pqc->dpq_thread_attr;
pthread_t tid, *pthr = &tid;
#if DISPATCH_USE_MGR_THREAD && DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (unlikely(dq == &_dispatch_mgr_root_queue)) {
pthr = _dispatch_mgr_root_queue_init();
}
#endif
do {
_dispatch_retain(dq); // released in _dispatch_worker_thread
while ((r = pthread_create(pthr, attr, _dispatch_worker_thread, dq))) {
if (r != EAGAIN) {
(void)dispatch_assume_zero(r);
}
_dispatch_temporary_resource_shortage();
}
} while (--remaining);
#else // defined(_WIN32)
#if DISPATCH_USE_MGR_THREAD && DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (unlikely(dq == &_dispatch_mgr_root_queue)) {
_dispatch_mgr_root_queue_init();
}
#endif
do {
_dispatch_retain(dq); // released in _dispatch_worker_thread
uintptr_t hThread = 0;
while (!(hThread = _beginthreadex(NULL, /* stack_size */ 0, _dispatch_worker_thread_thunk, dq, STACK_SIZE_PARAM_IS_A_RESERVATION, NULL))) {
if (errno != EAGAIN) {
(void)dispatch_assume(hThread);
}
_dispatch_temporary_resource_shortage();
}
#if DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (_dispatch_mgr_sched.prio > _dispatch_mgr_sched.default_prio) {
(void)dispatch_assume_zero(SetThreadPriority((HANDLE)hThread, _dispatch_mgr_sched.prio) == TRUE);
}
#endif
CloseHandle((HANDLE)hThread);
} while (--remaining);
#endif // defined(_WIN32)
#else
(void)floor;
#endif // DISPATCH_USE_PTHREAD_POOL
}
到了這裡可以清楚的看到對於全域性佇列使用_pthread_workqueue_addthreads開闢執行緒，對於其他佇列使用pthread_create開闢新的執行緒。那麼任務執行的程式碼為什麼沒看到？其實_dispatch_root_queues_init中會首先執行第一個任務：
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_root_queues_init(void)
{
dispatch_once_f(&_dispatch_root_queues_pred, NULL,
_dispatch_root_queues_init_once);
}
// 看一下dispatch_once_f就不展開了，可以看一下下面dispatch_once的分析，這裡看一下 _dispatch_root_queues_init_once
static void
_dispatch_root_queues_init_once(void *context DISPATCH_UNUSED)
{
_dispatch_fork_becomes_unsafe();
#if DISPATCH_USE_INTERNAL_WORKQUEUE
size_t i;
for (i = 0; i < DISPATCH_ROOT_QUEUE_COUNT; i++) {
_dispatch_root_queue_init_pthread_pool(&_dispatch_root_queues[i], 0,
_dispatch_root_queues[i].dq_priority);
}
#else
int wq_supported = _pthread_workqueue_supported();
int r = ENOTSUP;
if (!(wq_supported & WORKQ_FEATURE_MAINTENANCE)) {
DISPATCH_INTERNAL_CRASH(wq_supported,
"QoS Maintenance support required");
}
if (unlikely(!_dispatch_kevent_workqueue_enabled)) {
r = _pthread_workqueue_init(_dispatch_worker_thread2,
offsetof(struct dispatch_queue_s, dq_serialnum), 0);
#if DISPATCH_USE_KEVENT_WORKLOOP
} else if (wq_supported & WORKQ_FEATURE_WORKLOOP) {
r = _pthread_workqueue_init_with_workloop(_dispatch_worker_thread2,
(pthread_workqueue_function_kevent_t)
_dispatch_kevent_worker_thread,
(pthread_workqueue_function_workloop_t)
_dispatch_workloop_worker_thread,
offsetof(struct dispatch_queue_s, dq_serialnum), 0);
#endif // DISPATCH_USE_KEVENT_WORKLOOP
#if DISPATCH_USE_KEVENT_WORKQUEUE
} else if (wq_supported & WORKQ_FEATURE_KEVENT) {
r = _pthread_workqueue_init_with_kevent(_dispatch_worker_thread2,
(pthread_workqueue_function_kevent_t)
_dispatch_kevent_worker_thread,
offsetof(struct dispatch_queue_s, dq_serialnum), 0);
#endif
} else {
DISPATCH_INTERNAL_CRASH(wq_supported, "Missing Kevent WORKQ support");
}
if (r != 0) {
DISPATCH_INTERNAL_CRASH((r << 16) | wq_supported,
"Root queue initialization failed");
}
#endif // DISPATCH_USE_INTERNAL_WORKQUEUE
}
// 繼續檢視
DISPATCH_NOINLINE
static void
_dispatch_workloop_worker_thread(uint64_t *workloop_id,
dispatch_kevent_t *events, int *nevents)
{
if (!workloop_id || !dispatch_assume(*workloop_id != 0)) {
return _dispatch_kevent_worker_thread(events, nevents);
}
if (!events || !nevents) {
// events for worker thread request have already been delivered earlier
return;
}
if (!dispatch_assume(*nevents && *events)) return;
dispatch_wlh_t wlh = (dispatch_wlh_t)*workloop_id;
_dispatch_adopt_wlh(wlh);
_dispatch_wlh_worker_thread(wlh, *events, nevents);
_dispatch_preserve_wlh_storage_reference(wlh);
}
// 檢視 _dispatch_worker_thread2
static void
_dispatch_worker_thread2(pthread_priority_t pp)
{
bool overcommit = pp & _PTHREAD_PRIORITY_OVERCOMMIT_FLAG;
dispatch_queue_global_t dq;
pp &= _PTHREAD_PRIORITY_OVERCOMMIT_FLAG | ~_PTHREAD_PRIORITY_FLAGS_MASK;
_dispatch_thread_setspecific(dispatch_priority_key, (void *)(uintptr_t)pp);
dq = _dispatch_get_root_queue(_dispatch_qos_from_pp(pp), overcommit);
_dispatch_introspection_thread_add();
_dispatch_trace_runtime_event(worker_unpark, dq, 0);
int pending = os_atomic_dec2o(dq, dgq_pending, relaxed);
dispatch_assert(pending >= 0);
_dispatch_root_queue_drain(dq, dq->dq_priority,
DISPATCH_INVOKE_WORKER_DRAIN | DISPATCH_INVOKE_REDIRECTING_DRAIN);
_dispatch_voucher_debug("root queue clear", NULL);
_dispatch_reset_voucher(NULL, DISPATCH_THREAD_PARK);
_dispatch_trace_runtime_event(worker_park, NULL, 0);
}
// 檢視 _dispatch_root_queue_drain
DISPATCH_NOT_TAIL_CALLED // prevent tailcall (for Instrument DTrace probe)
static void
_dispatch_root_queue_drain(dispatch_queue_global_t dq,
dispatch_priority_t pri, dispatch_invoke_flags_t flags)
{
#if DISPATCH_DEBUG
dispatch_queue_t cq;
if (unlikely(cq = _dispatch_queue_get_current())) {
DISPATCH_INTERNAL_CRASH(cq, "Premature thread recycling");
}
#endif
_dispatch_queue_set_current(dq);
_dispatch_init_basepri(pri);
_dispatch_adopt_wlh_anon();
struct dispatch_object_s *item;
bool reset = false;
dispatch_invoke_context_s dic = { };
#if DISPATCH_COCOA_COMPAT
_dispatch_last_resort_autorelease_pool_push(&dic);
#endif // DISPATCH_COCOA_COMPAT
_dispatch_queue_drain_init_narrowing_check_deadline(&dic, pri);
_dispatch_perfmon_start();
while (likely(item = _dispatch_root_queue_drain_one(dq))) {
if (reset) _dispatch_wqthread_override_reset();
_dispatch_continuation_pop_inline(item, &dic, flags, dq);
reset = _dispatch_reset_basepri_override();
if (unlikely(_dispatch_queue_drain_should_narrow(&dic))) {
break;
}
}
// overcommit or not. worker thread
if (pri & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) {
_dispatch_perfmon_end(perfmon_thread_worker_oc);
} else {
_dispatch_perfmon_end(perfmon_thread_worker_non_oc);
}
#if DISPATCH_COCOA_COMPAT
_dispatch_last_resort_autorelease_pool_pop(&dic);
#endif // DISPATCH_COCOA_COMPAT
_dispatch_reset_wlh();
_dispatch_clear_basepri();
_dispatch_queue_set_current(NULL);
}
// 檢視 _dispatch_continuation_pop_inline 這個是出佇列操作，這裡注意一下首先看了有沒有vtable（_dispatch_object_has_vtable），這裡解釋了為什麼dispatch_barrier_async儘管主要流程和dispatch_async一模一樣但是無法應用到全域性佇列的原因，因為全域性佇列沒有v_table結構會直接像dispatch_async一樣執行
DISPATCH_ALWAYS_INLINE_NDEBUG
static inline void
_dispatch_continuation_pop_inline(dispatch_object_t dou,
dispatch_invoke_context_t dic, dispatch_invoke_flags_t flags,
dispatch_queue_class_t dqu)
{
dispatch_pthread_root_queue_observer_hooks_t observer_hooks =
_dispatch_get_pthread_root_queue_observer_hooks();
if (observer_hooks) observer_hooks->queue_will_execute(dqu._dq);
flags &= _DISPATCH_INVOKE_PROPAGATE_MASK;
if (_dispatch_object_has_vtable(dou)) {
dx_invoke(dou._dq, dic, flags);
} else {
_dispatch_continuation_invoke_inline(dou, flags, dqu);
}
if (observer_hooks) observer_hooks->queue_did_execute(dqu._dq);
}
// 檢視 _dispatch_continuation_invoke_inline ，這裡`_dispatch_client_callout`就是真正的執行block操作 ，當然還有一種情況這裡還不會走就是_dispatch_continuation_with_group_invoke，這個後面的dispatch_group會用到
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_continuation_invoke_inline(dispatch_object_t dou,
dispatch_invoke_flags_t flags, dispatch_queue_class_t dqu)
{
dispatch_continuation_t dc = dou._dc, dc1;
dispatch_invoke_with_autoreleasepool(flags, {
uintptr_t dc_flags = dc->dc_flags;
// Add the item back to the cache before calling the function. This
// allows the 'hot' continuation to be used for a quick callback.
//
// The ccache version is per-thread.
// Therefore, the object has not been reused yet.
// This generates better assembly.
_dispatch_continuation_voucher_adopt(dc, dc_flags);
if (!(dc_flags & DC_FLAG_NO_INTROSPECTION)) {
_dispatch_trace_item_pop(dqu, dou);
}
if (dc_flags & DC_FLAG_CONSUME) {
dc1 = _dispatch_continuation_free_cacheonly(dc);
} else {
dc1 = NULL;
}
if (unlikely(dc_flags & DC_FLAG_GROUP_ASYNC)) {
_dispatch_continuation_with_group_invoke(dc);
} else {
_dispatch_client_callout(dc->dc_ctxt, dc->dc_func);
_dispatch_trace_item_complete(dc);
}
if (unlikely(dc1)) {
_dispatch_continuation_free_to_cache_limit(dc1);
}
});
_dispatch_perfmon_workitem_inc();
}
另外對於_dispatch_continuation_init的程式碼中的並沒有對其進行展開，其實_dispatch_continuation_init中的func就是_dispatch_call_block_and_release(原始碼如下)，它在dx_push呼叫時包裝進了qos。
void
_dispatch_call_block_and_release(void *block)
{
void (^b)(void) = block;
b();
Block_release(b);
}
dispatch_async程式碼實現看起來比較複雜，因為其中的資料結構較多，分支流程控制比較複雜。不過思路其實很簡單，用連結串列儲存所有提交的 block（先進先出，，在佇列本身維護了一個連結串列新加入block放到連結串列尾部），然後在底層執行緒池中，依次取出 block 並執行。
類似的可以看到dispatch_barrier_async原始碼和dispatch_async幾乎一致，僅僅多了一個標記位DC_FLAG_BARRIER，這個標記位用於在取出任務時進行判斷，正常的非同步呼叫會依次取出，而如果遇到了DC_FLAG_BARRIER則會返回，所以可以等待所有任務執行結束執行dx_push（不過提醒一下dispatch_barrier_async必須在自定義佇列才有用，原因是global佇列沒有v_table結構，同時不要試圖在主佇列呼叫，否則會crash)：
void
dispatch_barrier_async(dispatch_queue_t dq, dispatch_block_t work)
{
dispatch_continuation_t dc = _dispatch_continuation_alloc();
uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_BARRIER;
dispatch_qos_t qos;
qos = _dispatch_continuation_init(dc, dq, work, 0, dc_flags);
_dispatch_continuation_async(dq, dc, qos, dc_flags);
}
單次執行dispatch_once
下面的程式碼在objc開發中應該很常見，這種方式可以保證instance只會建立一次：
+ (instancetype)sharedInstance {
static MyClass *instance;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
instance = [[MyClass alloc] init];
});
return instance;
}
不放分析一下dispatch_once的原始碼：
void
dispatch_once(dispatch_once_t *val, dispatch_block_t block)
{
dispatch_once_f(val, block, _dispatch_Block_invoke(block));
}
// 展開 dispatch_once_f
DISPATCH_NOINLINE
void
dispatch_once_f(dispatch_once_t *val, void *ctxt, dispatch_function_t func)
{
dispatch_once_gate_t l = (dispatch_once_gate_t)val;
#if !DISPATCH_ONCE_INLINE_FASTPATH || DISPATCH_ONCE_USE_QUIESCENT_COUNTER
uintptr_t v = os_atomic_load(&l->dgo_once, acquire);
if (likely(v == DLOCK_ONCE_DONE)) {
return;
}
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
if (likely(DISPATCH_ONCE_IS_GEN(v))) {
return _dispatch_once_mark_done_if_quiesced(l, v);
}
#endif
#endif
if (_dispatch_once_gate_tryenter(l)) {
return _dispatch_once_callout(l, ctxt, func);
}
return _dispatch_once_wait(l);
}
// 如果 os_atomic_load為 DLOCK_ONCE_DONE 則直接返回，否則進入_dispatch_once_gate_tryenter，在這裡首先判斷物件是否儲存過，如果儲存過則則標記為unlock
DISPATCH_ALWAYS_INLINE
static inline bool
_dispatch_once_gate_tryenter(dispatch_once_gate_t l)
{
return os_atomic_cmpxchg(&l->dgo_once, DLOCK_ONCE_UNLOCKED,
(uintptr_t)_dispatch_lock_value_for_self(), relaxed);
}
// 如果沒有儲存過則執行 _dispatch_once_callout，主要是執行block
DISPATCH_NOINLINE
static void
_dispatch_once_callout(dispatch_once_gate_t l, void *ctxt,
dispatch_function_t func)
{
_dispatch_client_callout(ctxt, func);
_dispatch_once_gate_broadcast(l);
}
// 執行過block則呼叫 _dispatch_once_gate_broadcast
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_once_gate_broadcast(dispatch_once_gate_t l)
{
dispatch_lock value_self = _dispatch_lock_value_for_self();
uintptr_t v;
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
v = _dispatch_once_mark_quiescing(l);
#else
v = _dispatch_once_mark_done(l);
#endif
if (likely((dispatch_lock)v == value_self)) return;
_dispatch_gate_broadcast_slow(&l->dgo_gate, (dispatch_lock)v);
}
// 在 _dispatch_once_gate_broadcast 中由於執行完畢，使用_dispatch_once_mark_don畢標記為done
DISPATCH_ALWAYS_INLINE
static inline uintptr_t
_dispatch_once_mark_done(dispatch_once_gate_t dgo)
{
return os_atomic_xchg(&dgo->dgo_once, DLOCK_ONCE_DONE, release);
}
swift中實現dispatch_once
說到這裡，從swift3.0以後已經沒辦法使用dispach_once了，其實原因很簡單因為在swift1.x的static var/let屬性就已經是dispatch_once在後臺執行的了，所以對於單例的建立沒有必要顯示呼叫了。但是有時候其他情況我們還是需要使用單次執行怎麼辦呢？代替方法：使用全域性變數（例如建立一個物件例項或者初始化成一個立即執行的閉包：let g = {}();_ = g;），當然習慣於dispatch_once的朋友有時候並不適應這種方法，這裡給出一個比較簡單的方案:
public extension DispatchQueue {
private static var _onceTracker = [String]()
public class func once(file: String = #file, function: String = #function, line: Int = #line, block:(Void)->Void) {
let token = file + ":" + function + ":" + String(line)
once(token: token, block: block)
}
/**
Executes a block of code, associated with a unique token, only once. The code is thread safe and will
only execute the code once even in the presence of multithreaded calls.
- parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
- parameter block: Block to execute once
*/
public class func once(token: String, block:(Void)->Void) {
objc_sync_enter(self)
defer { objc_sync_exit(self) }
if _onceTracker.contains(token) {
return
}
_onceTracker.append(token)
block()
}
}
延遲執行 dispatch_after
dispatch_after也是一個常用的延遲執行的方法，比如常見的使用方法是：
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"...");
});
在檢視dispatch_after原始碼之前先看一下另一個內容事件源dispatch_source_t，其實dispatch_source_t是一個很少讓開發者和GCD聯想到一起的一個型別，它本身也有對應的建立方法dispatch_source_create（事實上它的使用甚至可以追蹤到Runloop）。多數開發者認識dispatch_source_t都是通過定時器，很多文章會教你如何建立一個比較準確的定時器，比如下面的程式碼：
dispatch_source_t timerSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
dispatch_source_set_timer(timerSource, dispatch_time(DISPATCH_TIME_NOW, 0), 3*NSEC_PER_SEC, 0);
dispatch_source_set_event_handler(timerSource, ^{
NSLog(@"dispatch_source_t...");
});
dispatch_resume(timerSource);
self->source = timerSource;
如果你知道上面一個定時器如何執行的那麼下面看一下dispatch_after應該就比較容易明白了：
void
dispatch_after(dispatch_time_t when, dispatch_queue_t queue,
dispatch_block_t work)
{
_dispatch_after(when, queue, NULL, work, true);
}
// 檢視 _dispatch_after
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_after(dispatch_time_t when, dispatch_queue_t dq,
void *ctxt, void *handler, bool block)
{
dispatch_timer_source_refs_t dt;
dispatch_source_t ds;
uint64_t leeway, delta;
if (when == DISPATCH_TIME_FOREVER) {
#if DISPATCH_DEBUG
DISPATCH_CLIENT_CRASH(0, "dispatch_after called with 'when' == infinity");
#endif
return;
}
delta = _dispatch_timeout(when);
if (delta == 0) {
if (block) {
return dispatch_async(dq, handler);
}
return dispatch_async_f(dq, ctxt, handler);
}
leeway = delta / 10; // <rdar://problem/13447496>
if (leeway < NSEC_PER_MSEC) leeway = NSEC_PER_MSEC;
if (leeway > 60 * NSEC_PER_SEC) leeway = 60 * NSEC_PER_SEC;
// this function can and should be optimized to not use a dispatch source
ds = dispatch_source_create(&_dispatch_source_type_after, 0, 0, dq);
dt = ds->ds_timer_refs;
dispatch_continuation_t dc = _dispatch_continuation_alloc();
if (block) {
_dispatch_continuation_init(dc, dq, handler, 0, 0);
} else {
_dispatch_continuation_init_f(dc, dq, ctxt, handler, 0, 0);
}
// reference `ds` so that it doesn't show up as a leak
dc->dc_data = ds;
_dispatch_trace_item_push(dq, dc);
os_atomic_store2o(dt, ds_handler[DS_EVENT_HANDLER], dc, relaxed);
dispatch_clock_t clock;
uint64_t target;
_dispatch_time_to_clock_and_value(when, &clock, &target);
if (clock != DISPATCH_CLOCK_WALL) {
leeway = _dispatch_time_nano2mach(leeway);
}
dt->du_timer_flags |= _dispatch_timer_flags_from_clock(clock);
dt->dt_timer.target = target;
dt->dt_timer.interval = UINT64_MAX;
dt->dt_timer.deadline = target + leeway;
dispatch_activate(ds);
}
程式碼並不是太複雜，無時間差則直接呼叫dispatch_async，否則先建立一個dispatch_source_t，不同的是這裡的型別並不是DISPATCH_SOURCE_TYPE_TIMER而是_dispatch_source_type_after，檢視原始碼不難發現它只是dispatch_source_type_s型別的一個常量和_dispatch_source_type_timer並沒有明顯區別：
const dispatch_source_type_s _dispatch_source_type_after = {
.dst_kind = "timer (after)",
.dst_filter = DISPATCH_EVFILT_TIMER_WITH_CLOCK,
.dst_flags = EV_DISPATCH,
.dst_mask = 0,
.dst_timer_flags = DISPATCH_TIMER_AFTER,
.dst_action = DISPATCH_UNOTE_ACTION_SOURCE_TIMER,
.dst_size = sizeof(struct dispatch_timer_source_refs_s),
.dst_create = _dispatch_source_timer_create,
.dst_merge_evt = _dispatch_source_merge_evt,
};
而和dispatch_activate()其實和dispatch_resume() 是一樣的開啟定時器。那麼為什麼看不到dispatch_source_set_event_handler來給timer設定handler呢？不放看一下dispatch_source_set_event_handler的原始碼：
void
dispatch_source_set_event_handler(dispatch_source_t ds,
dispatch_block_t handler)
{
_dispatch_source_set_handler(ds, handler, DS_EVENT_HANDLER, true);
}
// 檢視 _dispatch_source_set_handler
DISPATCH_NOINLINE
static void
_dispatch_source_set_handler(dispatch_source_t ds, void *func,
uintptr_t kind, bool is_block)
{
dispatch_continuation_t dc;
dc = _dispatch_source_handler_alloc(ds, func, kind, is_block);
if (_dispatch_lane_try_inactive_suspend(ds)) {
_dispatch_source_handler_replace(ds, kind, dc);
return _dispatch_lane_resume(ds, false);
}
dispatch_queue_flags_t dqf = _dispatch_queue_atomic_flags(ds);
if (unlikely(dqf & DSF_STRICT)) {
DISPATCH_CLIENT_CRASH(kind, "Cannot change a handler of this source "
"after it has been activated");
}
// Ignore handlers mutations past cancelation, it's harmless
if ((dqf & DSF_CANCELED) == 0) {
_dispatch_ktrace1(DISPATCH_PERF_post_activate_mutation, ds);
if (kind == DS_REGISTN_HANDLER) {
_dispatch_bug_deprecated("Setting registration handler after "
"the source has been activated");
} else if (func == NULL) {
_dispatch_bug_deprecated("Clearing handler after "
"the source has been activated");
}
}
dc->dc_data = (void *)kind;
_dispatch_barrier_trysync_or_async_f(ds, dc,
_dispatch_source_set_handler_slow, 0);
}
可以看到最終還是封裝成一個dispatch_continuation_t進行同步或者非同步呼叫，而上面_dispatch_after直接構建了dispatch_continuation_t進行執行。
取消延遲執行的任務
使用dispatch_after還有一個問題就是取消問題，當然通常遇到了這種問題大部分答案就是使用下面的方式：
[self performSelector:@selector(myDelayedMethod) withObject: self afterDelay: desiredDelay];
[NSObject cancelPreviousPerformRequestsWithTarget: self selector:@selector(myDelayedMethod) object: self];
不過如果你使用的是iOS 8及其以上的版本，那麼其實是可以取消的（如下），當然如果你還在支援iOS 8以下的版本不妨試試這個自定義的dispatch_cancelable_block_t類：
dispatch_block_t block = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
NSLog(@"dispatch_after...");
});
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 3*NSEC_PER_SEC), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), block);
// 取消
dispatch_block_cancel(block);
如果你用的是swift那麼恭喜你，很簡單：
let dispatchItem = DispatchWorkItem {
handler()
}
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Double(Int64(3 * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC), execute: dispatchItem)
// 取消
dispatchItem.cancel()
dispatch_semaphore
訊號量是執行緒同步操作中很常用的一個操作，常用的幾個型別：
dispatch_semaphore_t：訊號量型別
dispatch_semaphore_create：建立一個訊號量
dispatch_semaphore_wait：傳送一個等待訊號，訊號量-1，當訊號量為0阻塞執行緒，大於0則開始執行後面的邏輯（也就是說執行dispatch_semaphore_wait前如果訊號量<=0則阻塞，否則正常執行後面的邏輯）
dispatch_semaphore_signal：傳送喚醒訊號，訊號量會+1
比如我們有個操作foo()在非同步執行緒已經開始執行，同時可能使用者會手動再次觸發動作bar()，但是bar依賴foo完成則可以使用訊號量:
- (void)foo {
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 這裡執行其他任務。。。
// TODO:
// 執行完傳送訊號
dispatch_semaphore_signal(semaphore);
});
self->semaphore = semaphore;
}
- (void)bar {
// 等待上面的操作完成，如果60s還沒有完成則超時繼續執行下面的邏輯
dispatch_semaphore_wait(self.semaphore, dispatch_time(DISPATCH_TIME_NOW, 60*NSEC_PER_SEC));
// 這裡執行其他任務。。。但是依賴上面的操作完成
// TODO:
}
那麼訊號量是如何實現的呢，不妨看一下它的原始碼：
// 首先看一下dispatch_semaphore_t，沒錯和上面一樣本質就是 dispatch_semaphore_s，dsema_value代表當前訊號量，dsema_orig表示初始訊號量
DISPATCH_CLASS_DECL(semaphore, OBJECT);
struct dispatch_semaphore_s {
DISPATCH_OBJECT_HEADER(semaphore);
intptr_t volatile dsema_value;
intptr_t dsema_orig;
_dispatch_sema4_t dsema_sema;
};
// 檢視 dispatch_semaphore_create 原始碼，其實並不複雜建立分配DISPATCH_VTABLE結構的空間，設定初始訊號量，但是可以清楚的看到同樣指定了目標佇列，這是一個優先順序為`DISPATCH_QUEUE_PRIORITY_DEFAULT`的非過載佇列
dispatch_semaphore_t
dispatch_semaphore_create(intptr_t value)
{
dispatch_semaphore_t dsema;
// If the internal value is negative, then the absolute of the value is
// equal to the number of waiting threads. Therefore it is bogus to
// initialize the semaphore with a negative value.
if (value < 0) {
return DISPATCH_BAD_INPUT;
}
dsema = _dispatch_object_alloc(DISPATCH_VTABLE(semaphore),
sizeof(struct dispatch_semaphore_s));
dsema->do_next = DISPATCH_OBJECT_LISTLESS;
dsema->do_targetq = _dispatch_get_default_queue(false);
dsema->dsema_value = value;
_dispatch_sema4_init(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
dsema->dsema_orig = value;
return dsema;
}
// 下面看一下 dispatch_semaphore_wait，首先`os_atomic_dec2o`訊號量減一，當然遞減之後訊號量大於等於0它其實什麼也不做繼續執行就好了，但是如果不滿足執行_dispatch_semaphore_wait_slow 等待訊號量喚醒或者timeout超時
dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
long value = os_atomic_dec2o(dsema, dsema_value, acquire);
if (likely(value >= 0)) {
return 0;
}
return _dispatch_semaphore_wait_slow(dsema, timeout);
}
// 看一下 _dispatch_semaphore_wait_slow 原始碼，這裡首先對於兩種極端情況：如果是DISPATCH_TIME_NOW則執行訊號量+1並返回超時訊號，DISPATCH_TIME_FOREVER則一直等待，預設則呼叫 `_dispatch_sema4_timedwait`
DISPATCH_NOINLINE
static intptr_t
_dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema,
dispatch_time_t timeout)
{
long orig;
_dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
switch (timeout) {
default:
if (!_dispatch_sema4_timedwait(&dsema->dsema_sema, timeout)) {
break;
}
// Try to undo what the fast path did to dsema->dsema_value
DISPATCH_FALLTHROUGH;
case DISPATCH_TIME_NOW:
orig = dsema->dsema_value;
while (orig < 0) {
if (os_atomic_cmpxchgvw2o(dsema, dsema_value, orig, orig + 1,
&orig, relaxed)) {
return _DSEMA4_TIMEOUT();
}
}
// Another thread called semaphore_signal(). Drain the wakeup.
DISPATCH_FALLTHROUGH;
case DISPATCH_TIME_FOREVER:
_dispatch_sema4_wait(&dsema->dsema_sema);
break;
}
return 0;
}
// 檢視 _dispatch_sema4_timedwait 呼叫mach的核心函式semaphore_timedwait等待收到訊號直至超時
bool
_dispatch_sema4_timedwait(_dispatch_sema4_t *sema, dispatch_time_t timeout)
{
mach_timespec_t _timeout;
kern_return_t kr;
do {
uint64_t nsec = _dispatch_timeout(timeout);
_timeout.tv_sec = (__typeof__(_timeout.tv_sec))(nsec / NSEC_PER_SEC);
_timeout.tv_nsec = (__typeof__(_timeout.tv_nsec))(nsec % NSEC_PER_SEC);
kr = semaphore_timedwait(*sema, _timeout);
} while (unlikely(kr == KERN_ABORTED));
if (kr == KERN_OPERATION_TIMED_OUT) {
return true;
}
DISPATCH_SEMAPHORE_VERIFY_KR(kr);
return false;
}
// 最後看一下 dispatch_semaphore_signal，首先訊號量+1,如果訊號量大於0就什麼也不做（通常到了這裡dispatch_semaphore_wait還沒呼叫），否則執行 _dispatch_semaphore_signal_slow
intptr_t
dispatch_semaphore_signal(dispatch_semaphore_t dsema)
{
long value = os_atomic_inc2o(dsema, dsema_value, release);
if (likely(value > 0)) {
return 0;
}
if (unlikely(value == LONG_MIN)) {
DISPATCH_CLIENT_CRASH(value,
"Unbalanced call to dispatch_semaphore_signal()");
}
return _dispatch_semaphore_signal_slow(dsema);
}
// 檢視 _dispatch_semaphore_signal_slow ，呼叫核心`semaphore_signal`喚醒執行緒，如apple api描述“如果喚醒執行緒則返回非0，否則返回0”
DISPATCH_NOINLINE
intptr_t
_dispatch_semaphore_signal_slow(dispatch_semaphore_t dsema)
{
_dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
_dispatch_sema4_signal(&dsema->dsema_sema, 1);
return 1;
}
// 檢視 _dispatch_sema4_signal 原始碼
void
_dispatch_sema4_signal(_dispatch_sema4_t *sema, long count)
{
do {
kern_return_t kr = semaphore_signal(*sema);
DISPATCH_SEMAPHORE_VERIFY_KR(kr);
} while (--count);
}
訊號量是一個比較重要的內容，合理使用可以讓你的程式更加的優雅，比如說一個常見的情況：大家知道
PHImageManager.requestImage是一個釋放消耗記憶體的方法，有時我們需要批量獲取到圖片執行一些操作的話可能就沒辦法直接for迴圈，不然記憶體會很快爆掉，因為每個requestImage操作都需要佔用大量記憶體，即使外部巢狀autoreleasepool也不一定可以及時釋放（想想for執行的速度，釋放肯定來不及），那麼requestImage又是一個非同步操作，如此只能讓一個操作執行完再執行另一個迴圈操作才能解決。也就是說這個問題就變成for迴圈內部的非同步操作序列執行的問題。要解決這個問題有幾種思路：1.使用requestImage的同步請求照片 2.使用遞迴操作一個操作執行完再執行另外一個操作移除for操作 3.使用訊號量解決。當然第一個方法並非普適，有些非同步操作並不能輕易改成同步操作，第二個方法相對普適，但是遞迴呼叫本身因為要改變原來的程式碼結構看起來不是那麼優雅，自然當前討論的訊號量是更好的方式。我們假設requestImage是一個bar(callback:((_ image)->
Void))操作,整個請求是一個foo(callback:((_ images)->Void))那麼它的實現方式如下:
- (void)foo:(CallbackWithImages)callback {
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_async(globalQueue, ^{
NSMutableArray *array = [[NSMutableArray alloc] init];
for (int i=0; i<100; ++i) {
[self bar:^(UIImage *image){
[array addObject:image];
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}
dispatch_async(dispatch_get_main_queue(), ^{
callback([array copy]);
});
});
}
- (void)bar:(CallbackWithImage)callback {
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(globalQueue, ^{
callback([UIImage new]);
});
}
訊號量常見crash
可以看到訊號量在做執行緒同步時簡單易用，不過有時候不經意間容易出錯，比如下面的程式碼會出現EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)錯誤，原因是之前的訊號量還在使用:
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
dispatch_semaphore_wait(semaphore, dispatch_time(DISPATCH_TIME_NOW, 1000*NSEC_PER_SEC));
// semaphore = dispatch_semaphore_create(0);
為什麼會這樣呢？原因和上面dispatch_semaphore_create中的DISPATCH_VTABLE(semaphore)有關係，這個巨集我們上面分析過，最終展開就是OS_dispatch_semaphore_class例項的引用，那麼它的例項是什麼呢？它當然是通過_dispatch_object_alloc建立的，沿著查詢_dispatch_object_alloc的原始碼可以找到下面的程式碼：
static inline id
_os_objc_alloc(Class cls, size_t size)
{
id obj;
size -= sizeof(((struct _os_object_s *)NULL)->os_obj_isa);
while (unlikely(!(obj = class_createInstance(cls, size)))) {
_dispatch_temporary_resource_shortage();
}
return obj;
}
不難看出就是依靠class_createInstance建立一個OS_dispatch_semaphore_class例項，這個程式碼在libdispatch是找不到的，它在runtime原始碼中。不過在這裡可以找到它的例項的定義(其實類似的通過vtable結構建立的例項都包含在libdispatch的init.c中)：
DISPATCH_VTABLE_INSTANCE(semaphore,
.do_type = DISPATCH_SEMAPHORE_TYPE,
.do_dispose = _dispatch_semaphore_dispose,
.do_debug = _dispatch_semaphore_debug,
.do_invoke = _dispatch_object_no_invoke,
);
不難看出這個物件是包含一個dispose方法的，就是_dispatch_semaphore_dispose，我們可以看到它的原始碼，其實這裡對我們排查問題最重要的就是if條件語句，訊號量的當前值小於初始化，會發生閃退，因為訊號量已經被釋放了，如果此時沒有crash其實就會意味著一直有執行緒在訊號量等待：
void
_dispatch_semaphore_dispose(dispatch_object_t dou,
DISPATCH_UNUSED bool *allow_free)
{
dispatch_semaphore_t dsema = dou._dsema;
if (dsema->dsema_value < dsema->dsema_orig) {
DISPATCH_CLIENT_CRASH(dsema->dsema_orig - dsema->dsema_value,
"Semaphore object deallocated while in use");
}
_dispatch_sema4_dispose(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
}
dispatch_group
dispatch_group常常用來同步多個任務（注意和dispatch_barrier_sync不同的是它可以是多個佇列的同步），所以其實上面先分析dispatch_semaphore也是這個原因，它本身是依靠訊號量來完成的同步管理。典型的用法如下：
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, globalQueue, ^{
sleep(10);
NSLog(@"任務1完成");
});
dispatch_group_async(group, globalQueue, ^{
NSLog(@"任務2完成");
});
dispatch_group_notify(group, globalQueue, ^{
NSLog(@"兩個任務全部完成");
});
dispatch_async(globalQueue, ^{
// 等待5s超時後繼續執行，此時dispatch_group中的任務未必全部完成，注意：dispatch_group_wait是同步操作必須放到非同步佇列否則阻塞當前執行緒
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, 5*NSEC_PER_SEC));
NSLog(@"等待到了上限，開始執行。。。");
});
下面看一下dispatch_group相關的原始碼：
// 和其他物件一樣，dispatch_group_t的本質就是 dispatch_group_s指標，這裡重點關注一下dg_state和dg_bits是一個計數器
struct dispatch_group_s {
DISPATCH_OBJECT_HEADER(group);
DISPATCH_UNION_LE(uint64_t volatile dg_state,
uint32_t dg_bits,
uint32_t dg_gen
) DISPATCH_ATOMIC64_ALIGN;
struct dispatch_continuation_s *volatile dg_notify_head;
struct dispatch_continuation_s *volatile dg_notify_tail;
};
// 檢視 dispatch_group_create
dispatch_group_t
dispatch_group_create(void)
{
return _dispatch_group_create_with_count(0);
}
// 展開 _dispatch_group_create_with_count,其實就是一個dispatch_group_s物件，指定了do_targetq是預設佇列並且不支援過載
DISPATCH_ALWAYS_INLINE
static inline dispatch_group_t
_dispatch_group_create_with_count(uint32_t n)
{
dispatch_group_t dg = _dispatch_object_alloc(DISPATCH_VTABLE(group),
sizeof(struct dispatch_group_s));
dg->do_next = DISPATCH_OBJECT_LISTLESS;
dg->do_targetq = _dispatch_get_default_queue(false);
if (n) {
os_atomic_store2o(dg, dg_bits,
(uint32_t)-n * DISPATCH_GROUP_VALUE_INTERVAL, relaxed);
os_atomic_store2o(dg, do_ref_cnt, 1, relaxed); // <rdar://22318411>
}
return dg;
}
// 首先看一下 dispatch_group_enter，它的核心就是os_atomic_sub_orig2o對dg_bits進行-1操作
void
dispatch_group_enter(dispatch_group_t dg)
{
// The value is decremented on a 32bits wide atomic so that the carry
// for the 0 -> -1 transition is not propagated to the upper 32bits.
uint32_t old_bits = os_atomic_sub_orig2o(dg, dg_bits,
DISPATCH_GROUP_VALUE_INTERVAL, acquire);
uint32_t old_value = old_bits & DISPATCH_GROUP_VALUE_MASK;
if (unlikely(old_value == 0)) {
_dispatch_retain(dg); // <rdar://problem/22318411>
}
if (unlikely(old_value == DISPATCH_GROUP_VALUE_MAX)) {
DISPATCH_CLIENT_CRASH(old_bits,
"Too many nested calls to dispatch_group_enter()");
}
}
// 然後看一下 dispatch_group_leave，核心就是os_atomic_add_orig2o執行dg_state+1操作，如果+1之後還等於0那麼說明之前沒有呼叫`dispatch_group_enter`，就裡會crash，當然這裡核心在 `_dispatch_group_wake`
void
dispatch_group_leave(dispatch_group_t dg)
{
// The value is incremented on a 64bits wide atomic so that the carry for
// the -1 -> 0 transition increments the generation atomically.
uint64_t new_state, old_state = os_atomic_add_orig2o(dg, dg_state,
DISPATCH_GROUP_VALUE_INTERVAL, release);
uint32_t old_value = (uint32_t)(old_state & DISPATCH_GROUP_VALUE_MASK);
if (unlikely(old_value == DISPATCH_GROUP_VALUE_1)) {
old_state += DISPATCH_GROUP_VALUE_INTERVAL;
do {
new_state = old_state;
if ((old_state & DISPATCH_GROUP_VALUE_MASK) == 0) {
new_state &= ~DISPATCH_GROUP_HAS_WAITERS;
new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
} else {
// If the group was entered again since the atomic_add above,
// we can't clear the waiters bit anymore as we don't know for
// which generation the waiters are for
new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
}
if (old_state == new_state) break;
} while (unlikely(!os_atomic_cmpxchgv2o(dg, dg_state,
old_state, new_state, &old_state, relaxed)));
return _dispatch_group_wake(dg, old_state, true);
}
if (unlikely(old_value == 0)) {
DISPATCH_CLIENT_CRASH((uintptr_t)old_value,
"Unbalanced call to dispatch_group_leave()");
}
}
// 檢視 _dispatch_group_wake原始碼，到了這裡通常就是出了排程組，如果有notify等待則執行notify遍歷並且在對應佇列中執行，如果有wait任務則喚醒其執行任務（注意這裡比較牛叉的`_dispatch_wake_by_address`可以根據地址進行函式呼叫，本身是呼叫的WakeByAddressAll這個系統呼叫）
DISPATCH_NOINLINE
static void
_dispatch_group_wake(dispatch_group_t dg, uint64_t dg_state, bool needs_release)
{
uint16_t refs = needs_release ? 1 : 0; // <rdar://problem/22318411>
if (dg_state & DISPATCH_GROUP_HAS_NOTIFS) {
dispatch_continuation_t dc, next_dc, tail;
// Snapshot before anything is notified/woken <rdar://problem/8554546>
dc = os_mpsc_capture_snapshot(os_mpsc(dg, dg_notify), &tail);
do {
dispatch_queue_t dsn_queue = (dispatch_queue_t)dc->dc_data;
next_dc = os_mpsc_pop_snapshot_head(dc, tail, do_next);
_dispatch_continuation_async(dsn_queue, dc,
_dispatch_qos_from_pp(dc->dc_priority), dc->dc_flags);
_dispatch_release(dsn_queue);
} while ((dc = next_dc));
refs++;
}
if (dg_state & DISPATCH_GROUP_HAS_WAITERS) {
_dispatch_wake_by_address(&dg->dg_gen);
}
if (refs) _dispatch_release_n(dg, refs);
}
// 檢視 dispatch_group_async原始碼，dispatch_continuation_t僅僅是封裝任務，核心是_dispatch_continuation_group_async
void
dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq,
dispatch_block_t db)
{
dispatch_continuation_t dc = _dispatch_continuation_alloc();
uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_GROUP_ASYNC;
dispatch_qos_t qos;
qos = _dispatch_continuation_init(dc, dq, db, 0, dc_flags);
_dispatch_continuation_group_async(dg, dq, dc, qos);
}
// 展開 _dispatch_continuation_group_async，這裡重點記住以下 dispatch_group_enter()方法，至於_dispatch_continuation_async前面已經介紹過
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_continuation_group_async(dispatch_group_t dg, dispatch_queue_t dq,
dispatch_continuation_t dc, dispatch_qos_t qos)
{
dispatch_group_enter(dg);
dc->dc_data = dg;
_dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}
// 繼續檢視 dispatch_group_notify，注意這裡將`dispatch_block_t`儲存到了共享資料 dispatch_continuation_t中
void
dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
dispatch_block_t db)
{
dispatch_continuation_t dsn = _dispatch_continuation_alloc();
_dispatch_continuation_init(dsn, dq, db, 0, DC_FLAG_CONSUME);
_dispatch_group_notify(dg, dq, dsn);
}
// 展開 _dispatch_group_notify，其實最主要的方法就是`_dispatch_group_wake`
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
dispatch_continuation_t dsn)
{
uint64_t old_state, new_state;
dispatch_continuation_t prev;
dsn->dc_data = dq;
_dispatch_retain(dq);
prev = os_mpsc_push_update_tail(os_mpsc(dg, dg_notify), dsn, do_next);
if (os_mpsc_push_was_empty(prev)) _dispatch_retain(dg);
os_mpsc_push_update_prev(os_mpsc(dg, dg_notify), prev, dsn, do_next);
if (os_mpsc_push_was_empty(prev)) {
os_atomic_rmw_loop2o(dg, dg_state, old_state, new_state, release, {
new_state = old_state | DISPATCH_GROUP_HAS_NOTIFS;
if ((uint32_t)old_state == 0) {
os_atomic_rmw_loop_give_up({
return _dispatch_group_wake(dg, new_state, false);
});
}
});
}
}
簡單的說就是dispatch_group_async和dispatch_group_notify本身就是和dispatch_group_enter、dispatch_group_leave沒有本質區別，後者相對更加靈活。當然這裡還有一個重要的操作就是dispatch_group_wait，還沒有看：
// os_atomic_rmw_loop2o不斷遍歷，如果(old_state & DISPATCH_GROUP_VALUE_MASK) == 0表示執行完，直接返回0，如果當前如果超時立即返回，其他情況呼叫_dispatch_group_wait_slow
intptr_t
dispatch_group_wait(dispatch_group_t dg, dispatch_time_t timeout)
{
uint64_t old_state, new_state;
os_atomic_rmw_loop2o(dg, dg_state, old_state, new_state, relaxed, {
if ((old_state & DISPATCH_GROUP_VALUE_MASK) == 0) {
os_atomic_rmw_loop_give_up_with_fence(acquire, return 0);
}
if (unlikely(timeout == 0)) {
os_atomic_rmw_loop_give_up(return _DSEMA4_TIMEOUT());
}
new_state = old_state | DISPATCH_GROUP_HAS_WAITERS;
if (unlikely(old_state & DISPATCH_GROUP_HAS_WAITERS)) {
os_atomic_rmw_loop_give_up(break);
}
});
return _dispatch_group_wait_slow(dg, _dg_state_gen(new_state), timeout);
}
// 檢視 _dispatch_group_wait_slow，最終呼叫 _dispatch_wait_on_address 直至 __ulock_wait
DISPATCH_NOINLINE
static intptr_t
_dispatch_group_wait_slow(dispatch_group_t dg, uint32_t gen,
dispatch_time_t timeout)
{
for (;;) {
int rc = _dispatch_wait_on_address(&dg->dg_gen, gen, timeout, 0);
if (likely(gen != os_atomic_load2o(dg, dg_gen, acquire))) {
return 0;
}
if (rc == ETIMEDOUT) {
return _DSEMA4_TIMEOUT();
}
}
}
多個非同步操作同步
上面第一個dispatch_group例子介紹的情況很簡單，任務本身都是同步的，只是將一個同步任務放到了dispatch_group_async中，現實中這個操作可能是一個網路請求，你現在想讓10個請求都完成後再執行某個操作怎麼辦（網路請求假設方法是request(url:String,complete:Callback)）？你現在不可能在網路請求方法內部做出修改了，怎麼保證操作同步呢?
之前看到過這種操作：
- (void)foo {
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
for (int i=0; i<5; ++i) {
dispatch_group_async(group, globalQueue, ^{
[self bar:^{
printf("task\n");
dispatch_semaphore_signal(semaphore);
}];
});
}
dispatch_group_notify(group, globalQueue, ^{
for (int i=0; i<5; ++i) {
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}
NSLog(@"complete");
});
}
- (void)bar:(Callback)complete {
dispatch_queue_t queue = dispatch_queue_create(DISPATCH_QUEUE_PRIORITY_DEFAULT, nil);
dispatch_async(queue, ^{
sleep(arc4random_uniform(5));
complete();
});
}
其實這種方法基本沒有用dispatch_group，直接用訊號量就可以解決，有了上面的分析使用dispatch_enter和dispatch_leave就可以了。
- (void)foo {
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
for (int i=0; i<5; ++i) {
dispatch_group_enter(group);
[self bar:^{
printf("task\n");
dispatch_group_leave(group);
}];
}
dispatch_group_notify(group, globalQueue, ^{
NSLog(@"complete");
});
}
dispatch_apply
dispatch_apply設計的主要目的是提高並行能力（注意不是併發，等同於Swift中的DispatchQueue.concurrentPerform），所以一般我們用來並行執行多個結構類似的任務，比如：
void
dispatch_apply(size_t iterations, dispatch_queue_t dq, void (^work)(size_t))
{
dispatch_apply_f(iterations, dq, work,
(dispatch_apply_function_t)_dispatch_Block_invoke(work));
}
// 檢視 dispatch_apply_f，對於width=1或者單核心數cpu其實這個是一個同步呼叫，核心方法就是_dispatch_apply_f
DISPATCH_NOINLINE
void
dispatch_apply_f(size_t iterations, dispatch_queue_t _dq, void *ctxt,
void (*func)(void *, size_t))
{
if (unlikely(iterations == 0)) {
return;
}
dispatch_thread_context_t dtctxt =
_dispatch_thread_context_find(_dispatch_apply_key);
size_t nested = dtctxt ? dtctxt->dtc_apply_nesting : 0;
dispatch_queue_t old_dq = _dispatch_queue_get_current();
dispatch_queue_t dq;
if (likely(_dq == DISPATCH_APPLY_AUTO)) {
dq = _dispatch_apply_root_queue(old_dq)->_as_dq;
} else {
dq = _dq; // silence clang Nullability complaints
}
dispatch_qos_t qos = _dispatch_priority_qos(dq->dq_priority) ?:
_dispatch_priority_fallback_qos(dq->dq_priority);
if (unlikely(dq->do_targetq)) {
// if the queue passed-in is not a root queue, use the current QoS
// since the caller participates in the work anyway
qos = _dispatch_qos_from_pp(_dispatch_get_priority());
}
int32_t thr_cnt = (int32_t)_dispatch_qos_max_parallelism(qos,
DISPATCH_MAX_PARALLELISM_ACTIVE);
if (likely(!nested)) {
nested = iterations;
} else {
thr_cnt = nested < (size_t)thr_cnt ? thr_cnt / (int32_t)nested : 1;
nested = nested < DISPATCH_APPLY_MAX && iterations < DISPATCH_APPLY_MAX
? nested * iterations : DISPATCH_APPLY_MAX;
}
if (iterations < (size_t)thr_cnt) {
thr_cnt = (int32_t)iterations;
}
struct dispatch_continuation_s dc = {
.dc_func = (void*)func,
.dc_ctxt = ctxt,
.dc_data = dq,
};
dispatch_apply_t da = (__typeof__(da))_dispatch_continuation_alloc();
da->da_index = 0;
da->da_todo = iterations;
da->da_iterations = iterations;
da->da_nested = nested;
da->da_thr_cnt = thr_cnt;
#if DISPATCH_INTROSPECTION
da->da_dc = _dispatch_continuation_alloc();
*da->da_dc = dc;
da->da_dc->dc_flags = DC_FLAG_ALLOCATED;
#else
da->da_dc = &dc;
#endif
da->da_flags = 0;
if (unlikely(dq->dq_width == 1 || thr_cnt <= 1)) {
return dispatch_sync_f(dq, da, _dispatch_apply_serial);
}
if (unlikely(dq->do_targetq)) {
if (unlikely(dq == old_dq)) {
return dispatch_sync_f(dq, da, _dispatch_apply_serial);
} else {
return dispatch_sync_f(dq, da, _dispatch_apply_redirect);
}
}
dispatch_thread_frame_s dtf;
_dispatch_thread_frame_push(&dtf, dq);
_dispatch_apply_f(upcast(dq)._dgq, da, _dispatch_apply_invoke);
_dispatch_thread_frame_pop(&dtf);
}
// 檢視 _dispatch_apply_f
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_apply_f(dispatch_queue_global_t dq, dispatch_apply_t da,
dispatch_function_t func)
{
int32_t i = 0;
dispatch_continuation_t head = NULL, tail = NULL;
pthread_priority_t pp = _dispatch_get_priority();
// The current thread does not need a continuation
int32_t continuation_cnt = da->da_thr_cnt - 1;
dispatch_assert(continuation_cnt);
for (i = 0; i < continuation_cnt; i++) {
dispatch_continuation_t next = _dispatch_continuation_alloc();
uintptr_t dc_flags = DC_FLAG_CONSUME;
_dispatch_continuation_init_f(next, dq, da, func,
DISPATCH_BLOCK_HAS_PRIORITY, dc_flags);
next->dc_priority = pp | _PTHREAD_PRIORITY_ENFORCE_FLAG;
next->do_next = head;
head = next;
if (!tail) {
tail = next;
}
}
_dispatch_thread_event_init(&da->da_event);
// FIXME: dq may not be the right queue for the priority of `head`
_dispatch_trace_item_push_list(dq, head, tail);
_dispatch_root_queue_push_inline(dq, head, tail, continuation_cnt);
// Call the first element directly
_dispatch_apply_invoke_and_wait(da);
}
// 展開 _dispatch_apply_invoke_and_wait
DISPATCH_NOINLINE
static void
_dispatch_apply_invoke_and_wait(void *ctxt)
{
_dispatch_apply_invoke2(ctxt, DISPATCH_APPLY_INVOKE_WAIT);
_dispatch_perfmon_workitem_inc();
}
// 檢視 _dispatch_apply_invoke2，主要是迴圈使用_dispatch_perfmon_workitem_inc呼叫任務，同時在最後一個任務呼叫完恢復執行緒 _dispatch_thread_event_signal
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_apply_invoke2(dispatch_apply_t da, long invoke_flags)
{
size_t const iter = da->da_iterations;
size_t idx, done = 0;
idx = os_atomic_inc_orig2o(da, da_index, acquire);
if (unlikely(idx >= iter)) goto out;
// da_dc is only safe to access once the 'index lock' has been acquired
dispatch_apply_function_t const func = (void *)da->da_dc->dc_func;
void *const da_ctxt = da->da_dc->dc_ctxt;
_dispatch_perfmon_workitem_dec(); // this unit executes many items
// Handle nested dispatch_apply rdar://problem/9294578
dispatch_thread_context_s apply_ctxt = {
.dtc_key = _dispatch_apply_key,
.dtc_apply_nesting = da->da_nested,
};
_dispatch_thread_context_push(&apply_ctxt);
dispatch_thread_frame_s dtf;
dispatch_priority_t old_dbp = 0;
if (invoke_flags & DISPATCH_APPLY_INVOKE_REDIRECT) {
dispatch_queue_t dq = da->da_dc->dc_data;
_dispatch_thread_frame_push(&dtf, dq);
old_dbp = _dispatch_set_basepri(dq->dq_priority);
}
dispatch_invoke_flags_t flags = da->da_flags;
// Striding is the responsibility of the caller.
do {
dispatch_invoke_with_autoreleasepool(flags, {
_dispatch_client_callout2(da_ctxt, idx, func);
_dispatch_perfmon_workitem_inc();
done++;
idx = os_atomic_inc_orig2o(da, da_index, relaxed);
});
} while (likely(idx < iter));
if (invoke_flags & DISPATCH_APPLY_INVOKE_REDIRECT) {
_dispatch_reset_basepri(old_dbp);
_dispatch_thread_frame_pop(&dtf);
}
_dispatch_thread_context_pop(&apply_ctxt);
// The thread that finished the last workitem wakes up the possibly waiting
// thread that called dispatch_apply. They could be one and the same.
if (!os_atomic_sub2o(da, da_todo, done, release)) {
_dispatch_thread_event_signal(&da->da_event);
}
out:
if (invoke_flags & DISPATCH_APPLY_INVOKE_WAIT) {
_dispatch_thread_event_wait(&da->da_event);
_dispatch_thread_event_destroy(&da->da_event);
}
if (os_atomic_dec2o(da, da_thr_cnt, release) == 0) {
#if DISPATCH_INTROSPECTION
_dispatch_continuation_free(da->da_dc);
#endif
_dispatch_continuation_free((dispatch_continuation_t)da);
}
}
執行緒和鎖
在GCD中其實總共有兩個執行緒池進行執行緒管理，一個是主執行緒池，另一個是除了主執行緒池之外的執行緒池。主執行緒池由序列為1的主佇列管理，使用objc.io上的一幅圖表示如下：
大家都知道使用dispatch_sync很有可能會發生死鎖那麼這是為什麼呢？
不妨回顧一下dispatch_sync的過程：
dispatch_sync
_dispatch_sync_f:區分併發還是序列佇列，如果是序列佇列
_dispatch_barrier_sync_f:是不是同一個佇列，如果是
_dispatch_sync_f_slow
重點在_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter)這個條件，成立則會發生死鎖，那麼它成立的條件就是((lock_value ^ tid) & DLOCK_OWNER_MASK) == 0首先lock_value和tid進行異或操作，相同為0不同為1，然後和DLOCK_OWNER_MASK（0xfffffffc）進行按位與操作，一個為0則是0，所以若干lock_value和tid相同則會發生死鎖。
補充
關於上面原始碼中的幾個巨集定義
__builtin_expect
__builtin_expect是一個針對編譯器優化的內建函式，讓編譯更加優化。比如說我們會寫這種程式碼：
if a {
print(a)
} else {
print(b)
}
如果我們更加傾向於使用a那麼可將其設為預設值，極特殊情況下才會使用b條件。CPU讀取指定是多條一起載入的，可能先載入進來的是a，那麼如果遇到執行b的情況則再載入b，那麼對於條件a的情況就造成了效能浪費。long __builtin_expect (long EXP, long C) 第一個引數是要預測變數，第二個引數是預測值，這樣__builtin_expect(a,false)說明多數情況a應該是false，極少數情況可能是true，這樣不至於造成效能浪費。其實對於編譯器在彙編時會優化成if !a的形式：
if !a {
print(b)
} else {
print(a)
}
likely和unlikely
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
看了likely和unlikely可以瞭解,likely表示更大可能成立，unlikely表示更大可能不成立。likely就是 if(likely(x == 0)) 就是if (x==0)。
os_atomic_cmpxchg
第二個引數與第一個引數值比較，如果相等，第三個引數的值替換第一個引數的值。如果不相等，把第一個引數的值賦值到第二個引數上。
#define os_atomic_cmpxchg(p, e, v, m) \
({ _os_atomic_basetypeof(p) _r = (e); \
atomic_compare_exchange_strong_explicit(_os_atomic_c11_atomic(p), \
&_r, v, memory_order_##m, memory_order_relaxed); })
os_atomic_store2o
將第二個引數儲存到第一個引數中
#define os_atomic_store2o(p, f, v, m) \
os_atomic_store(&(p)->f, (v), m)
#define os_atomic_store(p, v, m) \
atomic_store_explicit(_os_atomic_c11_atomic(p), v, memory_order_##m)
os_atomic_inc_orig
第一個引數賦值為1
#define os_atomic_inc_orig(p, m) \
os_atomic_add_orig((p), 1, m)
#define os_atomic_add_orig(p, v, m) \
_os_atomic_c11_op_orig((p), (v), m, add, +)
#define _os_atomic_c11_op_orig(p, v, m, o, op) \
atomic_fetch_##o##_explicit(_os_atomic_c11_atomic(p), v, \
memory_order_##m)
os_atomic_inc_orig2o
第二個引數加1並返回
#define os_atomic_inc_orig2o(p, f, m) \
os_atomic_add_orig2o(p, f, 1, m)
#define os_atomic_add_orig2o(p, f, v, m) \
os_atomic_add_orig(&(p)->f, (v), m)
os_atomic_dec2o
第二個引數-1並返回
#define os_atomic_dec2o(p, f, m) \
os_atomic_sub2o(p, f, 1, m)
#define os_atomic_sub2o(p, f, v, m) \
os_atomic_sub(&(p)->f, (v), m)
序列、併發、並行
dispatch_barrier_async
dispatch_apply()