<> Chapter 10

Android的消息机制

Android消息机制概述

Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程.

• 一个Thread包含一个Looper
• 一个Looper包含一个MessageQueue
• 一个Handler包含一个Looper和一个Messagequeue(和Looper中的是同一个)

Handler通过sendMessage()或post()方法将Message放到MessageQueue中， 然后Looper.loop()方法不停的循环从MessageQueue中取Message，成功取出后，通过在loop()中调用message.target.dispatchMessage()方法(message.target其实就是发送Message的Handler)来执行Handler.handleMessage方法.

总结:跨线程的关键是Handler中的Looper是定义Handler时所在Thread的Looper, MessageQueue也是这个Thread的Looper中的MessageQueue，所以之后我们不管在哪个线程调用Handler.sendMessage()方法，Message都会被发送到定义Handler时的Looper的MessageQueue中, 因此Handler.dispatchMessage()都是在定义Handler时的那个线程中执行的。

ThreadLocal

ThreadLocal是一个线程内部的数据存储类， 它可以保证， 同一个变量， 在不同的线程中， 使用的都是不同的副本。
Looper就是利用ThreadLocal来保证它在每个Thread中都是独立存在的。

private ThreadLocal mBooleanThreadLocal = new ThreadLocal();
mBooleanThreadLocal.set(true);
Log.d(TAG, "MainThread mBooleanThreadLocal=" + mBooleanThreadLocal.get());

new Thread("Thread1"){
    @Override
    public void run(){
      mBooleanThreadLocal.set(false);
      Log.d(TAG, "Thread1 mBooleanThreadLocal=" + mBooleanThreadLocal.get());
    }
  
  new Thread("Thread1"){
      @Override
      public void run(){
        Log.d(TAG, "Thread2 mBooleanThreadLocal=" + mBooleanThreadLocal.get());
    }
}

输出结果为:

MainThread mBooleanThreadLocal=true
Thread1 mBooleanThreadLocal=false
Thread2 mBooleanThreadLocal=null

从上面的结果中可以发现， 虽然看起来主线程和Thread1、Thread2使用的都是同一个ThreadLocal对象， 但是他们其实操作的都是不同的对象。
具体的实现原理可以参考这篇文章

MessageQueue工作原理

MessageQueue主要包含两种操作：插入(enqueueMessage())和读取(next()).
MessageQueue内部实现并不是一个队列， 而是一个单链表.

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

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

        msg.markInUse();
        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;
}

从enqueueMessage()的实现可以看出它的主要操作就是单链表的插入。

Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }
            
            //省略...
        }
    }
}

next()方法其实是一个无限循环的方法，如果消息队列没有消息，那么next()方法会一直堵塞在那里。当有新消息来时，next()方法会返回这条消息并将其从单链表中移除。

Looper工作原理

• 我们在自定义的Thread中通过Looper.prepare()来进行初始化Thread本身的Looper，通过调用Looper.loop()方法来循环取出MessageQueue中的Message
Handler在构造方法中从通过Looper.myLooper()方法取出当前Thread的Looper(在Looper.prepare()中set到sThreadLocal中的Looper)

  new Thread("Thread#1") {
      @Override
      public void run(){
          Looper.prepare();
          Handler handler = new Handler();
          Looper.loop();
      }
  }
  
  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 Handler(Callback callback, boolean async) {
      //省略...
      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 static void loop() {
      final Looper me = myLooper();
      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;
          }
  
          try {
              msg.target.dispatchMessage(msg);
              end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
          } finally {
              if (traceTag != 0) {
                  Trace.traceEnd(traceTag);
              }
          }
          //省略...
      }
  }
  

• Looper会在它的构造方法中初始化属于它的MessageQueue

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

• Looper提供了quit()和quitSafely()两个方法来退出, 两者的区别是quit()会直接退出， quitSafely()会把MessageQueue()已有的消息处理完后再退出
• 在子线程中使用完Looper后需要调用quit()方法来退出， 否则那个子线程就会一直处于等待状态

  public void quit() {
      mQueue.quit(false);
  }
  

Handler工作原理

Handler的主要工作包含发送和接收过程。消息的发送可以通过post系列方法以及sendMessage方法实现，但是post的一系列方法最终是通过sendMessage系列方法实现的。
sendMessage系列方法发送一条消息的典型过程如下所示：

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);
}

可以发现Handler发送消息的过程仅仅是向MessageQueue插入了一条消息，MessageQueue通过next方法就会将这条Message返回给Looper，最终消息由Looper交由Handler处理，即Handler的dispatchMessage方法会被调用，这时Handler就进入了消息处理的阶段。dispatchMessage()如下所示：

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

首先，检查msg.callback是否为null， 不为null就通过handleCallback()来处理消息。
其次，如果mCallback 不为null，就通过mCallback.handleMessage(msg)来处理消息。
最后，调用最常见的handleMessage(msg)来处理消息。

• msg.callback就是我们调用Handler的post系列方法所传递的Runnable参数。
• mCallback是一个接口，Handler提供了一个构造方法，使我们可以传递一个Callback作为参数，这样我们就不用重写Handler了

  public interface Callback {
      public boolean handleMessage(Message msg);
  }
  
  Handler handler = new Handler(callback);