【Android】从源码中探讨Handler机制

Thread thread = new Thread() {
	public void run() {
		//子线程中发送消息给主线程
		Message msg = new Message();
		msg.what = 200;
		msg.obj = param;
		msg.arg1 = 3;
		handler.sendMessage(msg);
	};
};

Handler handler = new Handler() {
	public void handleMessage(Message msg) {
		//主线程接收到消息，更新UI
	};
};

public Handler() {
    this(null, false);
}
public Handler(Callback callback, boolean async) {
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can‘t create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

final MessageQueue mQueue;	//消息队列（链表结构，下面会分析到）
final Looper mLooper;	//可理解为消息处理器

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {}
    return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
     //注意这一行，将Handler自已赋值给了Message的target属性，下面的析中会用到
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

boolean enqueueMessage(Message msg, long when) {
    synchronized (this) {
        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;
}

class LooperThread extends Thread {
  public Handler mHandler;
  public void run() {
      Looper.prepare();
      mHandler = new Handler() {
          public void handleMessage(Message msg) {
              // process incoming messages here
          }
      };
      Looper.loop();
  }
}

Looper.prepare();
Looper.loop()；

public static void prepare() {
    prepare(true);
}
//设置当前线程私有的Looper对象
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));
}
//定义当前线程私有的Looper对象
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
//获取当前线程私有的Looper对象
public static Looper myLooper() {
    return sThreadLocal.get();
}

/**
 * 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;
    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        msg.target.dispatchMessage(msg);
        msg.recycle();
    }
}

/**
 * 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);
    }
}

在多线程的环境中，主线程和子线程之间交互是通过一个链表结构的消息队列(MessageQueue)，子线程只管往里面放入消息(Message),消息是按时间的先后顺序排列的，主线程用一个消息处理器（Looper）不断地逐个逐个地处理掉消息。

@容新华技术博客 - http://blog.csdn.net/rongxinhua - 原创文章，转载请注明出处

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【Android】从源码中探讨Handler机制

原文：http://blog.csdn.net/rongxinhua/article/details/20576185