一、android的消息機制概述

1 消息機制原理

即Handler的運行機制,配合底層的Looper和MessageQueue，完成消息的發送與接收工作過程。handler發送Message給Looper輪循器，Looper輪循器被喚醒並開始遍歷MessageQueue，輪詢取出消息並讓handler來分發消息，最終完成消息的傳遞與處理。

2 幾個名詞的意義

Looper 消息輪循
MessageQueue 消息隊列，底層由鏈表實現
Message 消息對象
Handler 消息發送、接收、處理

3 Message的獲取與發送的幾種方法

發送消息

1、handler.sendMessage(msg);

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2、msg.setTarget(target)； //設置發送給哪個Handler
msg.sendToTarget();

獲取消息

1、handler.obtainMessage();

obtainMessage()方法實際調用了Message.obtain方法

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2、Message msg = Message.obtain(handler);// obtain（handler）調用了Message.obtain()方法
或
Message msg = Message.obtain();
msg.setTarget(target)

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3、Message message=new Message();

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Message.obtain()方法如下

 /**
 * Return a new Message instance from the global pool. Allows us to
 * avoid allocating new objects in many cases.
 * 消息池有消息則直接返回消息
 * 否則直接新建一個消息返回
 */
public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

---

二、 Handler消息機制運行流程

1 Looper初始化與開啟輪循

ActivityThread（UI線程）的main方法中初始化Looper

public static void main(String[] args) {
    SamplingProfilerIntegration.start();


...
// 初始化Looper
    Looper.prepareMainLooper();

    ActivityThread thread = new ActivityThread();
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

 ...
// 開啟輪循
    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

Looper初始化

prepareMainLooper方法

 public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

prepare(boolean quitAllowed)

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)); 
}

myLooper() , 從sThreadLocal中獲取Looper對象

 public static Looper myLooper() {
    return sThreadLocal.get();
}

Looper開啟輪循

loop() 方法開啟輪循，等待處理消息
1、通過queue.next()取出消息
2、通過queue.enqueueMessage向消息隊列插入一條消息

/**
 * 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();
// 取出消息，若無消息則阻塞
    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
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

// 分發消息
        msg.target.dispatchMessage(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);
        }
// 回收消息
        msg.recycle();
    }
}

2 Handler的構造方法

/**
 * @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) {
   ...
// 獲取Looper對象 
    mLooper = Looper.myLooper(); 
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
// 獲得消息隊列
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

3 消息的發送與插入消息隊列

發送消息的幾種方法最終都會調用sendMessageDelayed方法

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

sendMessageDelayed方法又調用了sendMessageAtTime方法

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);
}

在sendMessageAtTime中調用enqueueMessage方法插入消息

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    // 設置要發送給那個handler對象
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    // 插入消息
    return queue.enqueueMessage(msg, uptimeMillis);
}

在enqueueMessage方法中通過queue.enqueueMessage向消息隊列插入消息

boolean enqueueMessage(Message msg, long when) {
    if (msg.isInUse()) {
        throw new AndroidRuntimeException(msg + " This message is already in use.");
    }
    if (msg.target == null) {
        throw new AndroidRuntimeException("Message must have a target.");
    }

    synchronized (this) {
        if (mQuitting) {
            RuntimeException e = new RuntimeException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w("MessageQueue", e.getMessage(), e);
            return false;
        }

        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) {  // 遍歷到最後一個元素
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next  插入新消息
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

4 消息取出與分發

Looper的loop方法取出消息，並讓Handler來分發消息，即msg.target.dispatchMessage(msg);其中msg.target即Handler對象

/**
 * 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);
    }
}

1、首先會判斷msg.callback是否為空，msg.callback即是handler.post的那個Runnable對象，不為空則調用 handleCallback(msg);

注意：handler.post（）並沒有開啟一個新線程，Runnable的run方法運行於Handler所在線程

private static void handleCallback(Message message) {
    message.callback.run();
}

當msg.callback為空時,要判斷mCallback是否為空，mCallback是一個回調接口

 public interface Callback {
    public boolean handleMessage(Message msg);
}

Handler有如下構造

     public Handler(Callback callback) {
        this(callback, false);
    }

可以用來構造Handler，如下

Handler h=new Handler(new Handler.Callback() {

        @Override
        public boolean handleMessage(Message msg) {
            // TODO Auto-generated method stub
            return false;
        }
    });

2、mCallback不為空時，則調用mCallback.handleMessage(msg),即調用了Callback中的handleMessage方法，如下handleMessage1方法

3、mCallback為空時，則調用了handler的handleMessage方法，即調用了handleMessage2方法

    Handler h = new Handler(new Handler.Callback() {

    // handleMessage1
        @Override
        public boolean handleMessage(Message msg) {
            // TODO Auto-generated method stub
            return false;
        }
    }) {

    // handleMessage2
        @Override
        public void handleMessage(Message msg) {
            // TODO Auto-generated method stub
            super.handleMessage(msg);
        }

    };

---

三、分析 runOnUiThread原理

public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

1 通過 mHandler.post方法發送一個消息

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

2 通過getPostMessage方法初始化一個消息對象，並給它的callback賦值

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

3 Handler在dispatchMessage中處理消息，此時msg.callback不為空，msg.callback即是handler.post的那個Runnable對象，此時handleCallback(msg);會被調用

View的post方法原理類似。

四、幾個思考

1、handler.post（Runnable r）有開啟一個新線程嗎?

追蹤源碼發現，最終是通過 message.callback.run();來開啟任務的，並沒有開啟一個新線程，Runnable的run方法運行於Handler所在線程 

2、子線程是如何切換到主線程的？

通過Handler來切換線程。在子線程中將消息發送到MessageQueue，在Looper輪循消息時，調用了msg.target.dispatchMessage(msg);讓Handler來分發處理消息，此時已從子線程切換到主線程了。

3、Looper的loop方法為什麼不會卡死主線程？
loop方法會阻塞主線程但不會卡死主線程。在源碼中我們知道，在 Looper.loop();之後又這麼一句

throw new RuntimeException("Main thread loop unexpectedly exited");

我們可以知道如果loop結束運行則會拋出此異常。因此loop方法正常是不會結束的。它配合ActivityThread內部Handler（即mH），通過不停的消息輪循與事件處理來完成與用戶的交互。
4、為什麼在子線程中創建Handler時會出錯？
由上面我們分析源碼可知，在構造Handler時會初始化Looper對象，而子線程通常是沒有Looper對象的（如果我們並沒有自己創建Looper對象的話），會拋出如下異常

09-07 19:55:44.050: E/AndroidRuntime(1870): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

為了解決這個錯誤，我們可以手動創建Looper對象，並開啟消息輪循

new Thread(){
    public void run() {
        Looper.prepare();
        Handler h=new Handler(){
            public void handleMessage(android.os.Message msg) {};
        };
        Looper.loop();
    };
}.start();

這樣就解決了問題了，但同時要注意，當我們不需要使用此子線程的Handler時，我們應該手動停止Looper以回收資源，通過Looper的quit(立即停止)或quitSafely(當消息輪循完時)即可停止消息輪循。