Android消息机制（3）- Handler和Looper

Looper

Looper主要的作用是，不断的从MessageQueue中取出Message，交给Message的target（Handler）处理。
打开Looper的源码，发现一段注释的代码：

  * <pre>
  *  class LooperThread extends Thread {
  *      public Handler mHandler;
  *      public void run() {
  *          // 初始化Looper
  *          Looper.prepare();
  *          // 初始化Handler
  *          mHandler = new Handler() {
  *              public void handleMessage(Message msg) {
  *                  // process incoming messages here
  *              }
  *          };
  *          // Looper不断取Message
  *          Looper.loop();
  *      }
  *  }</pre>

这是怎么在子线程中，正确的使用消息机制的代码。
先看看Looper.prepare();做了什么：

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}

注意 ThreadLocal ，按我自己的理解，他内部保存一个Map，Key为线程，Value是你要保存的值。使用ThreadLocal，可以保证一个线程，只有一个副本。sThreadLocal被声明为ThreadLocal，也就说明一个线程只有一个Looper。
接着我们看看Looper.loop();做了什么：

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();
    // 死循环，一直取消息
    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);
        }
        // 交给Handler处理消息
        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Message
        msg.recycle();
    }
}
public static Looper myLooper() {
   return sThreadLocal.get();
}

Looper.loop()其实就是一直从MessageQueue取出消息，有就交给target（Handler）处理，没有就阻塞。代码很简单，原理也不难，但是Looper非常重要。
额外还要知道之前说过一个东西，叫“同步分割栏”，在Looper中，代码为：

// 设置同步分割栏
public int postSyncBarrier() {
    return mQueue.enqueueSyncBarrier(SystemClock.uptimeMillis());
}
// 取消同步分割栏
public void removeSyncBarrier(int token) {
    mQueue.removeSyncBarrier(token);
}

Handler

Handler是消息的产生者和消费者。Handler会将Message加入MessageQueue中，Looper不断地从MessageQueue中取出Message，交给target的Handler处理。
先看看Handler的构造函数，主要的有2个：

public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }
    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;
}
public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

如果我们new一个Handler实例时，没有指定Looper的话，默认使用当前线程Looper。但是如果当前线程的Looper还没有初始化，就会报错。所以我们在示例中，要先调用Looper.prepare();初始化。mCallback是一个接口，代码：

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

再看看Handler处理消息的代码：

public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        // 如果mCallback不为null，消息直接交给mCallback处理
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

所以我们new一个Handler不一定要重写handleMessage(Message)方法，还可以设置mCallback来处理消息。
再从构造函数有一个mAsynchronous，这是就是设置同步/异步，暂时先不用太在意。
我们再来看看Handler怎么“发送”Message到MessageQueue，代码：

public final boolean post(Runnable r) {
   return  sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(Runnable r, long uptimeMillis) {
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) {
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(Runnable r, long delayMillis) {
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postAtFrontOfQueue(Runnable r) {
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}
public final boolean sendMessage(Message msg) {
    return sendMessageDelayed(msg, 0);
}
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 sendEmptyMessageAtTime(int what, long uptimeMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageAtTime(msg, uptimeMillis);
}

从代码可以看出，postXXX和sendXXX最终都调用了sendMessageDelayed和sendMessageAtTime这两个方法。我们先看看sendMessageDelayed方法：

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

sendMessageDelayed方法最终调用了sendMessageAtTime，所以正真的大Boss是sendMessageAtTime，看看：

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) {
    // 把目标设置为自己
    msg.target = this;
    // 设置同步/异步
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    // 加入到MessageQueue中
    return queue.enqueueMessage(msg, uptimeMillis);
}

然后我们看看Looper取出之后，Handler处理消息的代码（其实上面已经有了）：

// Looper代码片段: 取出Message，调用target的dispatchMessage方法处理消息
msg.target.dispatchMessage(msg);

// Handler的dispatchMessage方法
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        // post(Runnable)在这里处理
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            // return true就不执行重写的handleMessage
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        // 我们继承Handler的子类重写的handleMessage方法
        handleMessage(msg);
    }
}
private static void handleCallback(Message message) {
    message.callback.run();
}

dispatchMessage方法内，首先看看Message的Callback有没有，那么Message的Callback什么时候才不为null呢？答案在这里：

public final boolean post(Runnable r) {
   return  sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

当我们使用Handler.post(Runnable)时，此时Message的callback就是Runnable，这下理解了吧。虽然使用了很多次，这下终于知道原理了。
接着看dispatchMessage，之前也说了mCallback，如果mCallback没有处理，那么我们声明Handler时重写的handleMessage方法就会执行。Handler处理消息就这么简单。