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Handler消息机制

Handler消息机制主要涉及Looper、Handler、MessageQueue、Message。其中，Looper主要负责获取消息，Handler负责发送消息及处理消息，MessageQueue是消息队列，Message是消息类。

Looper循环获取消息

1、ActivityThread的main()方法:

  public static void main(String[] args) {

      ...

//准备looper
      Looper.prepareMainLooper();

      ...

//进入无限循环
      Looper.loop();

//如果loop()循环退出，则抛出异常，整个应用退出
      throw new RuntimeException("Main thread loop unexpectedly exited");
  }

2、prepareMainLooper()方法:

  /**
   * Initialize the current thread as a looper, marking it as an
   * application's main looper. The main looper for your application
   * is created by the Android environment, so you should never need
   * to call this function yourself.  See also: {@link #prepare()}
   */
  public static void prepareMainLooper() {
//初始化looper
      prepare(false);
      synchronized (Looper.class) {
	//如果已经设置过sMainLooper，则抛出异常。每个线程中只允许存在一个looper。
          if (sMainLooper != null) {
              throw new IllegalStateException("The main Looper has already been prepared.");
          }
	//设置sMainLooper
          sMainLooper = myLooper();
      }
  }

3、在prepareMainLooper()方法中，首先调用prepare(false)方法:

  private static void prepare(boolean quitAllowed) {
//如果ThreadLocal中已经存在looper,则抛出异常
      if (sThreadLocal.get() != null) {
          throw new RuntimeException("Only one Looper may be created per thread");
      }
//如果没有初始化looper，则将looper保存到ThradLocal中。
      sThreadLocal.set(new Looper(quitAllowed));
  }

4、在prepare()方法中调用Looper的构造方法初始化MessageQueue:

  private Looper(boolean quitAllowed) {
//初始化MessageQueue
      mQueue = new MessageQueue(quitAllowed);
//设置当前线程给mThread变量
      mThread = Thread.currentThread();
  }

5、在prepareMainLooper()方法调用prepare(false)方法后，会调用myLooper()方法:

  /**
   * Return the Looper object associated with the current thread.  Returns
   * null if the calling thread is not associated with a Looper.
   */
  public static @Nullable Looper myLooper() {
//将保存在ThreadLocal中的looper返回
      return sThreadLocal.get();
  }

5、到这里，prepareMainLooper()方法执行完毕。然后执行Looper.loop()方法:

/**
    * Run the message queue in this thread. Be sure to call
    * {@link #quit()} to end the loop.
    */
   public static void loop() {

	//获取looper
       final Looper me = myLooper();
	//如果looper为null,则抛出异常
       if (me == null) {
           throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
       }
	//获取MessageQueue
       final MessageQueue queue = me.mQueue;

	...

	//开启无限循环
       for (;;) {
		//从消息队列中取消息，如果没有消息，则会阻塞
           Message msg = queue.next(); // might block
		//如果消息为null，则表示退出循环
           if (msg == null) {
               // No message indicates that the message queue is quitting.
               return;
           }
		
		...
         
           try {
			//回调target，即Handler的dispatchMessage方法
               msg.target.dispatchMessage(msg);
               ...
           } finally {
               if (traceTag != 0) {
                   Trace.traceEnd(traceTag);
               }
           }
           
		...

           msg.recycleUnchecked();
       }
   }

首先，获取looper,如果没有设置过looper，则抛出异常。然后，开启无限循环，通过looper的MessageQueue，不停获取消息，如果没有消息，则阻塞。如果获取到了消息，则会回调Handler的dispatchMessage方法，方法执行会切换到Handler的线程。

6、MessageQueue的next()方法:

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;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler

            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }

        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;

        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

在next()方法中，通过for(;;)开启无限循环去获取消息，如果获取到消息则返回。

Handler发送消息

1、sendMessage()方法:

/**
 * Pushes a message onto the end of the message queue after all pending messages
 * before the current time. It will be received in {@link #handleMessage},
 * in the thread attached to this handler.
 *  
 * @return Returns true if the message was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */
public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

2、sendMessage()方法会调用sendMessageDelayed()方法:

/**
 * Enqueue a message into the message queue after all pending messages
 * before (current time + delayMillis). You will receive it in
 * {@link #handleMessage}, in the thread attached to this handler.
 *  
 * @return Returns true if the message was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.  Note that a
 *         result of true does not mean the message will be processed -- if
 *         the looper is quit before the delivery time of the message
 *         occurs then the message will be dropped.
 */
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

3、sendMessageDelayed()方法会调用sendMessageAtTime():

/**
 * Enqueue a message into the message queue after all pending messages
 * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
 * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
 * Time spent in deep sleep will add an additional delay to execution.
 * You will receive it in {@link #handleMessage}, in the thread attached
 * to this handler.
 * 
 * @param uptimeMillis The absolute time at which the message should be
 *         delivered, using the
 *         {@link android.os.SystemClock#uptimeMillis} time-base.
 *         
 * @return Returns true if the message was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.  Note that a
 *         result of true does not mean the message will be processed -- if
 *         the looper is quit before the delivery time of the message
 *         occurs then the message will be dropped.
 */
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);
}

4、在这个方法中，会通过MessageQueue的enqueueMessage()方法，将消息发送到消息队列中。

5、enqueueMessage()方法:

  private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//设置Message的target为当前的Handler，以便获取到消息后能回调dispatchMessage方法。
      msg.target = this;
      if (mAsynchronous) {
          msg.setAsynchronous(true);
      }
      return queue.enqueueMessage(msg, uptimeMillis);
  }

6、MessageQueue的enqueueMessage()方法:

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;
}

sendMessageAtTime()方法中的mQueue是在Handler的构造方法中赋值的:

  public Handler(Callback callback, boolean async) {
      if (FIND_POTENTIAL_LEAKS) {
	//检测是否会有泄漏
          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());
          }
      }

//获取looper
      mLooper = Looper.myLooper();
//如果looper为null,则抛出异常
      if (mLooper == null) {
          throw new RuntimeException(
              "Can't create handler inside thread that has not called Looper.prepare()");
      }
//设置mQueue
      mQueue = mLooper.mQueue;
      mCallback = callback;
      mAsynchronous = async;
  }

如果没有传入looper，则会通过Looper.myLooper()获取looper,如果没有在线程中设置过looper，则会抛出异常

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

如果传入了looper，则直接设置mQueue。

Handler处理消息

dispatchMessage()方法,如果Message设置了callBack,则会回调callBack的run()方法；如果Message没有设置callBack,在这种情况下，如果Handler的callBack不为null，则会回调handleMessage()方法;如果Handler没有设置callBack或者Handler的callBack处理了消息，并没有返回true,则会回调Handler的handleMessage()方法:

  public void dispatchMessage(Message msg) {
//如果callBack不为null,则传给callBack处理。
      if (msg.callback != null) {
          handleCallback(msg);
      } else {
	//如果Handler的callBack不为空，则传给callBack处理
          if (mCallback != null) {
              if (mCallback.handleMessage(msg)) {
                  return;
              }
          }
          handleMessage(msg);
      }
  }

handleCallback()方法:

  private static void handleCallback(Message message) {
//回调Runnable的run()方法
      message.callback.run();
  }