Android Handler消息机制

Android 的消息机制主要是指 Handler 的运行机制及MessageQueue 和 Looper 的工作过程，这三个实际上是一体的，只不过是我们开发中比较多地接触到 Handler 而已。 Handler 的主要作用就是将一个任务切换到某个指定的线程去执行。

• MessageQueue

  保存由Looper 调度的消息列表。消息不是直接添加到MessageQueue，而是通过与Looper关联的 Handler 对象添加。可以通过 Looper.myQueue() 获取当前线程的MessageQueue。

  主要使用到 enqueueMessage(Message msg, long when) 、next() 两个方法，分别是插入、取出。从源码实现可以看出MessageQueue 其实是单链表的结构实现的，单链表在插入和删除上具有优势。

  • enqueueMessage(Message msg, long when)

  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;
  }
  

  以上为enqueueMessage 源码，其实就是单链表的插入操作，具体逻辑就不赘述了。

  • next()

  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;
      }
  }
  

  可以发现next 方法是一个死循环方法，如果消息队列中没有消息就会阻塞，当有新消息加入时next 会返回这条消息并从列表中移除。

• Looper

  用于为线程运行消息循环的类。主要就是循环MessangerQueue 中的消息，有就拿出来处理，没就阻塞在那里。

  默认情况下，线程没有与之关联的消息循环，可以在线程中调用prepare() 方法创建一个，并且调用loop() 开启消息循环，可以通过调用quit() 可以进行退出。

  • prepare()

   /** Initialize the current thread as a looper.
    * This gives you a chance to create handlers that then reference
    * this looper, before actually starting the loop. Be sure to call
    * {@link #loop()} after calling this method, and end it by calling
    * {@link #quit()}.
    */
  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));
  }
  
  /**
   * 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() {
      prepare(false);
      synchronized (Looper.class) {
          if (sMainLooper != null) {
              throw new IllegalStateException("The main Looper has already been prepared.");
          }
          sMainLooper = myLooper();
      }
  }
  
      /**
   * Returns the application's main looper, which lives in the main thread of the application.
   */
  public static Looper getMainLooper() {
      synchronized (Looper.class) {
          return sMainLooper;
      }
  }
  

  可以看到Looper 不仅提供了prepare() 方法，还提供了prepareMainLooper() 方法，这个方法主要是提供给ActivityThread 线程使用，而且还提供getMainLooper() 方法，我们可以通过它获取主线程的Looper。

  • loop()

   /**
   * Run the message queue in this thread. Be sure to call
   * {@link #quit()} to end the loop.
   */
  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
          final Printer logging = me.mLogging;
          if (logging != null) {
              logging.println(">>>>> Dispatching to " + msg.target + " " +
                      msg.callback + ": " + msg.what);
          }
  
          final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
  
          final long traceTag = me.mTraceTag;
          if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
              Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
          }
          final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
          final long end;
          try {
              msg.target.dispatchMessage(msg);
              end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
          } finally {
              if (traceTag != 0) {
                  Trace.traceEnd(traceTag);
              }
          }
          if (slowDispatchThresholdMs > 0) {
              final long time = end - start;
              if (time > slowDispatchThresholdMs) {
                  Slog.w(TAG, "Dispatch took " + time + "ms on "
                          + Thread.currentThread().getName() + ", h=" +
                          msg.target + " cb=" + msg.callback + " msg=" + msg.what);
              }
          }
  
          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);
          }
  
          msg.recycleUnchecked();
      }
  }
  

  可以看到loop() 方法是一个死循环去获取MessageQueue 中的消息并进行处理，只有当next 返回null 时才会退出。

  • quit()

  /**
   * Quits the looper.
   * <p>
   * Causes the {@link #loop} method to terminate without processing any
   * more messages in the message queue.
   * </p><p>
   * Any attempt to post messages to the queue after the looper is asked to quit will fail.
   * For example, the {@link Handler#sendMessage(Message)} method will return false.
   * </p><p class="note">
   * Using this method may be unsafe because some messages may not be delivered
   * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
   * that all pending work is completed in an orderly manner.
   * </p>
   *
   * @see #quitSafely
   */
  public void quit() {
      mQueue.quit(false);
  }
  
  /**
   * Quits the looper safely.
   * <p>
   * Causes the {@link #loop} method to terminate as soon as all remaining messages
   * in the message queue that are already due to be delivered have been handled.
   * However pending delayed messages with due times in the future will not be
   * delivered before the loop terminates.
   * </p><p>
   * Any attempt to post messages to the queue after the looper is asked to quit will fail.
   * For example, the {@link Handler#sendMessage(Message)} method will return false.
   * </p>
   */
  public void quitSafely() {
      mQueue.quit(true);
  }
  

  可以看到还有一个quitSafely() 二者的区别是，quit() 会直接退出，quitSafely() 会把已有的消息处理完后再退出。 它们都是调用MessageQueue 的quit(),当该方法被调用是会标记退出队列，此时next方法将会返回null 这样就终止了。

• Handler

  Handler 的工作主要包含消息的发送和接收过程。消息的发送可以通过post 一系列方法和send 一系列方法来实现,最终都是调用sendMessageAtTime(Message msg, long uptimeMillis) 方法实现的。

      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);
  }
  

  可以看到，Handler 发送消息其实就是往MessageQueue 里面插入一条消息，然后通过next 方法把这条消息给Looper，最终又由Looper 交给Handler 的dispatchMessage 处理。

   /**
   * Handle system messages here.
   */
  public void dispatchMessage(Message msg) {
      if (msg.callback != null) {
          handleCallback(msg);
      } else {
          if (mCallback != null) {
              if (mCallback.handleMessage(msg)) {
                  return;
              }
          }
          handleMessage(msg);
      }
  }
  

  可以看到dispatchMessage 的实现其实很简单，如果Message 的callback != null 就交给handleCallback 处理。这里其实就是执行post 传进来的Runable，然后检查 mCallback，不为空就交给mCallback 处理，否则交给子类处理。

图片来源于《Android 开发艺术探索》

本文主要参考《Android 开发艺术探索》，为了加深印象所写，也希望能够帮助到其他的小伙伴。