Android的消息機制（Handler、Looper、MessageQueue）

註：本文源碼基於android7.0

先大概概括一下消息機制：

這裏有三個角色，Handler、looper、messageQueue。Handler負責發消息和處理消息，Looper負責從MessageQueue中取出消息給Handler處理，MessageQueue則負責存儲Handler發過來的消息。

這個機制主要是將一個任務切換到指定線程中執行，就像我們在子線程中更新UI界面，就可以用Handler。多說一句，特殊情況下子線程中也是可以更新UI的，具體可以看這篇文章，大概意思就是在ViewRootImpl未完全初始化之前更新Android 中子線程真的不能更新 UI 嗎？。

我們從Handler的sendMessage方法入手開始分析：

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, systemClock.uptimeMillis() + delayMillis);
    }

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);
    }

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

Looper我們就從Looper.prepare方法來分析

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));
    }

接下來我們看Looper.loop方法

public static void loop() {
	//獲取到當前的Looper
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
	//從Looper中得到MessageQueue
        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();

	//不斷的從MessageQueue取消息，當沒有消息時，next方法會阻塞，直到消息隊列被標記為退出
	//這樣next方法會返回null
        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.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
		//從這裏看到了Handler的消息最終交給它的dispatchMessage方法來處理了
            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();
        }
    }

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