1. Handler类

Handler类被用于异步消息处理线程，常被用来更新UI线程。

一般来说我们会在主线程中创建一个Handler的匿名内部类，然后重写它的handleMessage方法来处理我们的UI操作。

  private final static int UPDATE_TIME = 1;

  private TextView mTv;
  private Handler mHandler = new Handler() {
      @Override
      public void handleMessage(Message msg) {
          switch (msg.what) {
              // 根据what的值处理不同操作
              case UPDATE_TIME:
                  mTv.setText(getCurrentTime());
                  break;
          }
      }
  };

调用Handler的sendMessage方法来发送消息。

  mHandler.sendEmptyMessage(UPDATE_TIME);

其他类似方法

  sendEmptyMessage(int what)
  sendEmptyMessageAtTime(int what, long uptimeMillis)
  sendEmptyMessageDelayed(int what, long delayMillis)
  sendMessage(Message msg)
  sendMessageAtFrontOfQueue(Message msg)
  sendMessageAtTime(Message msg, long uptimeMillis)
  sendMessageDelayed(Message msg, long delayMillis)

在线程中调用post方法

  mHandler.post(new Runnable() {
      @Override
      public void run() {
          mTv.setText(getCurrentTime());
      }
  });

其他类似方法

  post(Runnable r)
  postAtFrontOfQueue(Runnable r)
  postAtTime(Runnable r, long uptimeMillis)
  postAtTime(Runnable r, Object token, long uptimeMillis)
  postDelayed(Runnable r, long delayMillis)

2. Handler类解析

Android的消息机制中主要是由Handler，Looper，MessageQueue，Message等组成。

Looper主要是prepare()和loop()两个方法。

首先是prepare方法，sThreadLocal是一个ThreadLocal对象，可以在一个线程中存储变量。prepare方法不能被调用两次，这保证了一个线程中只有一个Looper实例。

  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));
  }

Looper的构造函数，创建一个消息队列。

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

loop方法，先获得当前线程的Looper，调用消息队列的next方法获取消息，随后回调Handler的dispatchMessage来处理消息。

  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();
      }
  }

  public static @Nullable Looper myLooper() {
      return sThreadLocal.get();
  }

MessageQueue是消息队列

在enqueueMessage中加入消息。如果当前没有消息，或者when小于队列头的话，作为新的队列头。否则插入到队伍中。

  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;
  }

在next方法中获取消息。

  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;
      }
  }

最后是Handler类

sendEmptyMessage方法会调用sendMessageDelayed方法，指定target是Handler，最终调用MessageQueue.enqueueMessage放入消息列表中。

  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 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);
  }

post方法，同样会调用sendMessageDelayed方法，只是会指定Message.callback属性。

  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;
  }

dispatchMessage方法处理消息回调。

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

  private static void handleCallback(Message message) {
      message.callback.run();
  }

  public void handleMessage(Message msg) {
  }

Android Handler类

1. Handler类

2. Handler类解析

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