一、使用方法。

Looper用来处理消息循环，在创建过程中会初始化MessageQueue。

Handler在其它线程发消息给当前线程

MessageQueue用来存放消息

Looper对象在哪个线程创建，Handler的handleMessage方法就在哪个线程执行

在创建activity时，android系统本身会为activity创建Looper。

final Handler mainHandler = new Handler(getMainLooper())
{
	@Override
	public void handleMessage(Message msg)
	{
		String content = "当前线程：" + "msg:" + msg.what;
		Toast.makeText(MainActivity.this, content, Toast.LENGTH_SHORT).show();
	}
};
mainHandler.sendEmptyMessage(0x1);

HandlerThread mThread = new HandlerThread("MyThread");
mThread.start();
Handler mHandle = new Handler(mThread.getLooper())
{
	@Override
	public void handleMessage(Message msg)
	{
		String content = "当前线程：" + Thread.currentThread().getName() + "msg:" + msg.what;
		System.out.println(content);
	}
};
mHandle.sendEmptyMessage(0x2);

二、Looper解析

构造函数：

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mRun = true;
    mThread = Thread.currentThread();
}

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));
}

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
        msg.target.dispatchMessage(msg);

        // Make sure that during the course of dispatching the
        // identity of the thread wasn't corrupted.
        final long newIdent = Binder.clearCallingIdentity();

        msg.recycle();
    }
}

在loop()中，Looper不断的通过queue.next()从MessageQueue取消息，然后调用语句msg.target.dispatchMessage(msg) 来执行。这里target为msg消息的发送者Handler，在分析Handler时再来分析，dispatchMessage在一般情况下会调用Handler类的handleMessage来处理消息，也就是上面例子中我们重载的这个handleMessage。

三、Handler解析

public Handler(Looper looper) {
    this(looper, null, false);
}

public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

public final boolean sendEmptyMessage(int what)
{
    return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}
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);
}

sendEmptyMessage将message加到了Looper的MessageQueue中，之后Looper在loop()中调用queue.next()

public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

如果没有设置Callback的话，就会运行handleMessage了。

四、HandlerThread解析

public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
}

public Looper getLooper() {
    if (!isAlive()) {
        return null;
    }

    // If the thread has been started, wait until the looper has been created.
    synchronized (this) {
        while (isAlive() && mLooper == null) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
    }
    return mLooper;
}