Android源碼學習(2) Handler之Looper

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Looper準備

Handler執行個體化時，會從當前線程擷取Looper，從而獲得MessageQueue，用於發送訊息。然後，線程不是生來就有Looper對象的，需要線上程執行中調用靜態方法Looper.prepare()，最終會調用到如下靜態方法：

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));}

靜態變數sThreadLocal是範型類ThreadLocal<Looper>的執行個體，用於儲存線程與其Looper之間的映射關係：sThreadLocal以自身為key，將Looper執行個體放入當前線程的ThreadLocalMap中(欄位名為threadlocals，ThreadLocalMap底層是通過hash表實現的)。由於任何線程存放Looper都是以sThreadLocal為key，所以任意線程最多隻能有一個Looper。

public void set(T value) {    Thread t = Thread.currentThread();    ThreadLocalMap map = getMap(t);    if (map != null)        map.set(this, value);    else        createMap(t, value);}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();}

Looper迴圈

準備完成之後，通過調用Looper.loop()進入looper迴圈。Looper在死迴圈中，不斷從MessageQueue中擷取訊息，交給Handler的dispatchMessage進行處理。loop函數的關鍵代碼是黃色背景標記的地區：

 1 public static void loop() { 2     final Looper me = myLooper(); 3     if (me == null) { 4         throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread."); 5     } 6     final MessageQueue queue = me.mQueue; 7  8     // Make sure the identity of this thread is that of the local process, 9     // and keep track of what that identity token actually is.10     Binder.clearCallingIdentity();11     final long ident = Binder.clearCallingIdentity();12 13     for (;;) {14         Message msg = queue.next(); // might block15         if (msg == null) {16             // No message indicates that the message queue is quitting.17             return;18         }19 20         // This must be in a local variable, in case a UI event sets the logger21         final Printer logging = me.mLogging;22         if (logging != null) {23             logging.println(">>>>> Dispatching to " + msg.target + " " +24                     msg.callback + ": " + msg.what);25         }26 27         final long traceTag = me.mTraceTag;28         if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {29             Trace.traceBegin(traceTag, msg.target.getTraceName(msg));30         }31         try {32             msg.target.dispatchMessage(msg);33         } finally {34             if (traceTag != 0) {35                 Trace.traceEnd(traceTag);36             }37         }38 39         if (logging != null) {40             logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);41         }42 43         // Make sure that during the course of dispatching the44         // identity of the thread wasn‘t corrupted.45         final long newIdent = Binder.clearCallingIdentity();46         if (ident != newIdent) {47             Log.wtf(TAG, "Thread identity changed from 0x"48                     + Long.toHexString(ident) + " to 0x"49                     + Long.toHexString(newIdent) + " while dispatching to "50                     + msg.target.getClass().getName() + " "51                     + msg.callback + " what=" + msg.what);52         }53 54         msg.recycleUnchecked();55     }56 }

每個Message被處理完之後，會被自動回收，放回Message池中。

Looper退出

通過調用quit或者quitSafely退出Looper迴圈。底層是調用MessageQueue的quit函數實現：

 1 void quit(boolean safe) { 2     if (!mQuitAllowed) { 3         throw new IllegalStateException("Main thread not allowed to quit."); 4     } 5  6     synchronized (this) { 7         if (mQuitting) { 8             return; 9         }10         mQuitting = true;11 12         if (safe) {13             removeAllFutureMessagesLocked();14         } else {15             removeAllMessagesLocked();16         }17 18         // We can assume mPtr != 0 because mQuitting was previously false.19         nativeWake(mPtr);20     }21 }

quit調用removeAllMessageLocked，該函數會直接移除MessageQueue中所有尚未處理的Message；quitSafely調用removeAllFutureMessageLocked，該函數只會移除執行時刻晚於當前時刻的Message(即，Message.when > now)。

Android源碼學習(2) Handler之Looper