Android MessageQueue

01.MessageQueue作用介绍

• MessageQueue，主要包含2个操作：插入和读取。
  • 读取操作会伴随着删除操作，插入和读取对应的方法分别为enqueueMessage和next，其中enqueueMessage的作用是往消息队列中插入一条消息，而next的作用是从消息队列中取出一条消息并将其从消息队列中移除。
• 虽然MessageQueue叫消息队列，但是它的内部实现并不是用的队列。
  • 实际上它是通过一个单链表的数据结构来维护消息列表，单链表在插入和删除上比较有优势。

02.MessageQueue的定义

• 通过源码我们可以知道，MessageQueue维护了一个消息列表。Messgae并不是直接添加到MessageQueue中，而是通过和Looper相关联的Handler来添加的。在当前线程中可以通过调用Looper.myQueue()方法来获取当前线程的MessageQueue。

  /**
   * Low-level class holding the list of messages to be dispatched by a
   * {@link Looper}.  Messages are not added directly to a MessageQueue,
   * but rather through {@link Handler} objects associated with the Looper.
   * 
   * <p>You can retrieve the MessageQueue for the current thread with
   * {@link Looper#myQueue() Looper.myQueue()}.
   */
  public final class MessageQueue

03.enqueueMessage()源码

• 源码如下所示

  • 在Message的源码中定义了一个成员属性target，其类型为Handler。由上面enqueuMessage的源码，我们可以看到，当Message没有处理其的Handler或该Message正在被处理的时候，都不能正常进入MessageQueue，这一点也是很容易理解的。当线程处于死亡状态的时候，Message会被回收掉，而不再进入该线程对应的MessageQueue中。否则，一切正常，enqueMessage就执行单链表的插入操作，将Message插入到MessageQueue中。

  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;
  }

04.next()方法源码分析

• next方法源码如下所示

  • 在 MessageQueue 中消息的读取其实是通过内部的 next() 方法进行的，next() 方法是一个无限循环的方法。
  • 如果消息队列中没有消息，则该方法会一直阻塞，
  • 当有新消息来的时候 next() 方法会返回这条消息并将其从单链表中删除。

  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;
      }
  }