這裡主要是先了解整個訊息傳遞的過程，知道這樣做的好處和必要性。而不是直接介紹裡面的幾個關鍵類，然後介紹這個機制，這樣容易頭暈。而且網路上已經有很多這樣的文章了，那些作者所站的高度對於我這種初學者來說有點高，我理解起來是比較稀裡糊塗的，所以這裡從一個問題出發，一步一步跟蹤程式碼，這裡只是搞清楚 handler 是怎麼跨執行緒收發訊息的，具體實現細節還是參考網上的那些大神的 Blog 比較權威。
PS. 本來是想分章節書寫，誰知道這一套軍體拳打下來收不住了，所以下面基本是以一種很流暢的過程解釋而不是很跳躍，細心看應該會對理解 Handler 機制有所收穫。
Q1: 假如有一個耗時的資料處理，而且資料處理的結果是對 UI 更新影響的，而 Android 中 UI 更新不是執行緒安全的，所以規定只能在主執行緒中更新。
下面我們有兩種選擇：
主執行緒版本：
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private Button btnTest;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_test);
init();
}
private void init() {
btnTest = (Button) findViewById(R.id.btn_test);
btnTest.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// 假裝資料處理
int i = 0;
for (i = 0; i < 10; i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// 假裝更新 UI
Log.d(TAG, "Handle it！" + i);
}
});
}
}複製程式碼直接在主執行緒中處理資料，接著直接根據處理結果更新 UI。我想弊端大家都看到了，小則 UI 卡頓，大則造成 ANR。
子執行緒版本：
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private Button btnTest;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_test);
init();
}
private void init() {
btnTest = (Button) findViewById(R.id.btn_test);
btnTest.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread(new Runnable() {
@Override
public void run() {
// 假裝資料處理
int i;
for (i = 0; i < 10; i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// 返回處理結果
handler.sendEmptyMessage(i);
}
}).start();
}
});
}
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
// 假裝更新 UI
Log.d(TAG, "Handle MSG = " + msg.what);
}
};
}複製程式碼這是一種典型的處理方式，開一個子執行緒處理資料，通過 Android 中提供的 Handler 機制進行跨執行緒通訊，把處理結果返回給主執行緒，進而更新 UI。這裡我們就是探討 Handler 是如何把資料傳送過去的。
到這裡，我們瞭解到的就是一個 Handler 的黑盒機制，子執行緒傳送，主執行緒接收。接下來，我們不介紹什麼 ThreadLocal、Looper 和 MessageQueue。而是直接從上面的程式碼引出它們的存在，從原理了解它們存在的必要性，然後在談它們內部存在的細節。
一切罪惡源於 handler.sendEmptyMessage();，最終找到以下函式 sendMessageAtTime(Message msg, long uptimeMillis)：
Handler.class
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(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);
}複製程式碼MessageQueue 出來了，我們避免不了了。裡面主要是 Message next() 和 enqueueMessage(Message msg, long when) 方法值得研究，但是現在還不是時候。
從 MessageQueue queue = mQueue; 中可以看出我們的 handler 物件裡面包含一個 mQueue 物件。至於裡面存的什麼怎麼初始化的現在也不用太關心。大概有個概念就是這是個訊息佇列，存的是訊息就行，具體實現細節後面會慢慢水落石出。
後面的程式碼就是說如果 queue 為空則列印 log 返回 false；否則執行 enqueueMessage(queue, msg, uptimeMillis); 入隊。那就好理解了，handler 傳送資訊其實是直接把資訊封裝進一個訊息佇列。
Handler.class
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}複製程式碼這裡涉及 Message，先說下這個類的三個成員變數：
/*package*/ Handler target;
/*package*/ Runnable callback;
/*package*/ Message next;複製程式碼所以 msg.target = this; 把當前 handler 傳給了 msg。
中間的 if 程式碼先忽略，先走主線：執行了 MessageQueue 的 enqueueMessage(msg, uptimeMillis);方法。接著看原始碼
MessageQueue.class
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) {
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;
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;
}
}
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;
}複製程式碼程式碼有點長，不影響主線的小細節就不介紹了，那些也很容易看懂的，但是原理還是值得分析。if (mQuitting)...，直接看看原始碼初始化賦值的函式是在 void quit(boolean safe) 函式裡面，這裡猜測可能是退出訊息輪訓，訊息輪訓的退出方式也是值得深究，不過這裡不影響主線就不看了。 msg.markInUse(); msg.when = when; 標記訊息在用而且繼續填充 msg，下面就是看註釋了。我們前面介紹的 Message 成員變數 next 就起作用了，把 msg 鏈在一起了。所以這裡的核心就是把 msg 以一種連結串列形式插進去。似乎這一波分析結束了，在這裡劃張圖總結下：
推薦自己根據所觀察到的變數賦值進行繪製圖畫，這樣印象更加深刻。
OK，訊息是存進去了，而且也是在 handler 所在的執行緒中。那麼到底怎麼取出資訊呢？也就是前面小例子
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
// 假裝更新 UI
Log.d(TAG, "Handle MSG = " + msg.what);
}
};複製程式碼handleMessage() 什麼時候呼叫？這裡基本斷了線索。但是如果你之前哪怕看過類似的一篇文章應該都知道其實在 Android 啟動時 main 函式就做了一些操作。這些操作是必要的，這也就是為什麼我們不能直接在子執行緒中 new Handler();。
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper(); // -------1
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler(); // -------2
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop(); // -------3
throw new RuntimeException("Main thread loop unexpectedly exited");
}複製程式碼可以看出這裡在獲取 sMainThreadHandler 之前進行了 Looper.prepareMainLooper(); 操作，之後進行了 Looper.loop(); 操作。
下面開始分析：
Loopr.class
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}複製程式碼前兩個方法是在自己建立 Looper 的時候用，第三個是主執行緒自己用的。由於這裡訊息傳遞以主執行緒為線索。prepare(false);說明了這是主執行緒，在 sThreadLocal.set(new Looper(quitAllowed)); 中的 quitAllowed 為 false 則說明主執行緒的 MessageQueue 輪訓不能 quit。這句程式碼裡還有 ThreadLocal 的 set() 方法。先不深究實現，容易暈，這裡需要知道的就是把一個 Looper 物件“放進”了 ThreadLocal，換句話說，通過 ThreadLocal 可以獲取不同的 Looper。
最後的 sThreadLocal.get(); 展示了 get 方法。說明到這時 Looper 已經存在啦。
現在看看 Looper 類的成員變數吧！
Looper.class
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;複製程式碼在這裡先介紹一下 ThreadLocal 的上帝視角吧。直接原始碼，可以猜測這是通過一個 ThreadLocalMap 的內部類對執行緒進行一種 map。傳進來的泛型 T 正是我們的 looper。所以 ThreadLocal 可以根據當前執行緒查詢該執行緒的 Looper，具體怎麼查詢推薦看原始碼，這裡就不介紹了。
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}複製程式碼分析到這裡，handler 和 looper 都有了，但是訊息還是沒有取出來？
這是看第三句 Looper.loop();。
Looper.class
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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();
for (;;) {
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);
}
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}複製程式碼一開始也是獲取 Looper，但是那麼多 Looper 怎麼知道這是哪個 Looper 呢？這先放著待會馬上解釋。把 loop() 函式主要功能搞懂再說。
接下來就是獲取 Looper 中的 MessageQueue了，等等，這裡提出一個疑問，前面說了 Handler 中也存在 MessageQueue，那這之間存在什麼關係嗎？（最後你會發現其實是同一個）
先往下看，一個死迴圈，也就是輪訓訊息嘍，中間有一句 msg.target.dispatchMessage(msg); 而前面介紹 msg.target 是 handler 型引數。所以和 handler 聯絡上了。
Handler.class
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}複製程式碼邏輯很簡單，總之就是調動了我們重寫的 handleMessage() 方法。
Step 1：Looper.prepare();
在 Looper 中有一個靜態變數 sThreadLocal，把建立的 looper “存在” 裡面，建立 looper 的同時建立 MessageQueue，並且和當前執行緒掛鉤。
Step 2：new Handler();
通過上帝 ThreadLocal，並根據當前執行緒，可獲取 looper，進而獲取 MessageQueue，Callback之類的。
```java
Handler.class
/**
- Use the {@link Looper} for the current thread with the specified callback interface
- and set whether the handler should be asynchronous.
*- Handlers are synchronous by default unless this constructor is used to make
- one that is strictly asynchronous.
*- Asynchronous messages represent interrupts or events that do not require global ordering
- with respect to synchronous messages. Asynchronous messages are not subject to
- the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*- @param callback The callback interface in which to handle messages, or null.
- @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
- each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*- @hide
*/
public Handler(Callback callback, boolean async) {
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()); }複製程式碼
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; // 前面的兩個 MessageQueue 聯絡起來了，疑問已解答。
mCallback = callback;
mAsynchronous = async;複製程式碼}``
這個函式可以說明在 new Handler() 之前該執行緒必需有 looper，所以要在這之前呼叫Looper.prepare();`。
Step 3：Looper.loop();
進行訊息迴圈。
基本到這裡整個過程應該是清楚了，這裡我畫下我的理解。
那麼我們現在來看一下 handler 是怎麼準確傳送資訊和處理資訊的。注意在 handler 傳送資訊之前，1、2、3 步已經完成。所以該獲取的執行緒已經獲取，直接往該執行緒所在的 MessageQueue 裡面塞資訊就行了，反正該資訊會在該 handler 所線上程的 looper 中迴圈，最終會通過訊息的 target 引數呼叫 dispatchMessage()，而在 dispatchMessage() 中會呼叫我們重寫的 handleMessage() 函式。
多謝閱讀