提到Android裡的訊息機制，便會提到Message、Handler、Looper、MessageQueue這四個類，我先簡單介紹以下這4個類
之間的愛恨情仇。
Message
訊息的封裝類，裡邊儲存了訊息的詳細資訊，以及要傳遞的資料
Handler
主要用在訊息的傳送上，有即時訊息，有延遲訊息，內部還提供了享元模式封裝了訊息物件池，能夠有效的減少重複物件的建立，留更多的記憶體做其他的事，
Looper
這個類內部持有一個MessageQueue物件，當建立Looper的時候，同時也會建立一個MessageQueue，然後Looper的主要工作就不斷的輪訓MessageQueue，輪到天荒地老的那種
MessageQueue
內部持有一個Message物件，採用單項鍊表的形式來維護訊息列隊。並且提供了入隊，出隊的基礎操作
舉個現實中的栗子，Message就相當於包裝好的快遞盒子，Handler就相當於傳送帶，MessageQueue就相當於快遞車，Looper就相當於快遞員，聯想一下，來個快遞盒子，biu丟到傳送帶上，傳送帶很智慧，直接傳送到快遞三輪車裡，然後快遞小哥送一波～，日夜交替，不分晝夜的工作，好傢伙，007工作制
訊息機制的初始化
好，我們把這4個傢伙從頭到位分析一遍，要想使用Android的訊息，首先要建立Looper物件，Android系統已經幫我們在UI執行緒內建立好了一個，我們可以看一下
public final class ActivityThread extends ClientTransactionHandler {
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
ActivityThread這個類大家應該不陌生吧，沒錯，他就是我們App的主執行緒管理類，我們看到他呼叫了 prepareMainLooper 來初始化，然後 loop，天荒地老的那種loop，這個loop，我們最後聊
我們看一下Looper內部提供的 prepareMainLooper 實現
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
上邊涉及到了3個方法，我都貼出來了，首先 quitAllowed 這個引數代表該Looper是否可以退出，我們主執行緒內的Looper是不允許退出的，所以封裝了 prepareMainLooper 方法和 prepare 方法已做區分，我們專案中平時用的都是 prepare 方法，因為是子執行緒，所以允許退出Looper，大家在子執行緒內用完記得呼叫quit哦~
這裡我們看Looper內部是通過ThreadLocal維護的Looper物件，也就是說每個執行緒都是相互獨立的。而且Looper做了限制，每個執行緒內部只能存在一個Looper物件，等同於每個執行緒內只能有一個MessageQueue
最後在Looper的構造方法內，建立了一個MessageQueue物件，整個Looper的初始化就結束了
建立訊息
我們準備好了Looper和MessageQueue後，就可以建立訊息啦，接下來我們建立一個訊息吧
//直接new物件，不推薦的方式
Message msg = new Message();
//推薦：內部是一個複用物件池
Message message = handler.obtainMessage();
message.what = 1;
message.obj = "hello world";
傳送訊息（入隊）
我們傳送訊息的時候，都是會藉助Handler的sendMessage就可以把訊息傳送到列隊裡了，我們往下看是如何完成的入隊操作吧，首先我們平時都是建立一個Handler，然後呼叫sendMessage就可以了
Handler handler = new Handler();
handler.sendMessage(message);
我們先看一下Handler的構造方法
public Handler() {
this(null, false);
}
public Handler(@Nullable Callback callback, boolean async) {
//FIND_POTENTIAL_LEAKS一直都是false，所以不用關心這個邏輯
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
//得到當前執行緒下的Looper物件
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
//從Loopper內部獲取一個列隊
mQueue = mLooper.mQueue;
// 回撥物件，我們平時寫的時候，一般都是用類整合的方式重寫 handleMessage 方法
mCallback = callback;
//標示當前Handler是否支援非同步訊息
mAsynchronous = async;
}
其實構造方法很簡單吶，就是獲取Looper物件，然後初始化列隊和回撥物件就完事了，我們繼續看sendMessage然後看訊息的入隊吧
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
通過內部的過載方法，一直呼叫到sendMessageAtTime方法，在這裡得到Handler內部的MessageQueue物件，然後呼叫了 enqueueMessage 方法準備入隊
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
這裡呼叫了MessageQueue的enqueueMessage方法真正入隊，我們繼續看一下
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
//如果當前退出狀態，則回收訊息，並返回訊息入隊失敗
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
//如果連結串列是空的，或者當前訊息的when小於表頭的when的時候，便會重新設定表頭
//這裡可以得知，訊息的順序是按照延遲時間，從小往大排序的
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//把msg放到連結串列最後
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
通過這個方法，我們瞭解到MessageQueue是通過Message的單鏈結構儲存的，然後每次入隊的時候，都會
通過這個enqueueMessage方法向連結串列的最末尾新增資料。
最後我們聊一下Looper下的loop方法吧
接下來我們看一下
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {
//queue的next會阻塞
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
//派發訊息，執行回撥handleMessage
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycleUnchecked();
}
}
Looper內的loop方法別看這麼多，大多數都是日誌相關的處理。其實他就兩件事
第一件事就是從列隊中通過next取出Message物件
第二件事就是通過Message物件上繫結的target物件dispatchMessage方法，來分發訊息
我們接下來看一下dispatchMessage方法，然後在看MessageQueue的next
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
灰常簡單，判斷CallBack物件。然後呼叫handleMessage就完事了，我們的Activity就收到資料了。
接下來我們看看MessageQueue的next是怎麼獲取列隊內的訊息的把。
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//沒有訊息的時候，或者有延遲訊息的時候會進行睡眠
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//當前時間小於訊息內記錄的時間，然後計算一個睡眠時間，跳出迴圈執行睡眠
if (now < msg.when) {
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
首先MessageQueue的訊息是用單連結串列的形式儲存，然後next函式做的事情就是死迴圈獲取訊息，
在獲取訊息的時候判斷一下訊息是否符合執行時間，如果不符合執行時間，就進入睡眠狀態等待訊息。
如果符合執行時間就直接返回Message給Looper進行分發，如果Message連結串列都為空。則睡眠時間是-1
代表無休止的睡眠。在無休止睡眠的狀態下，enqueueMessage的nativeWake方法，會進行一次喚醒，喚醒後next函式繼續執行，判斷返回訊息給Looper執行訊息分發