Handler消息机制小结

最近复习了下Handler消息机制，跟了一下源码，准备做个小总结作为自身知识归纳，也可以跟博友们一起进行讨论讨论。

一、Handler是什么？它是一种消息机制，是进行线程间通信的。实际上Android的消息机制就是Handler的运行机制。

二、Handler的组成：handler、looper，masseage，messagequene

运行原理：handler通过发消息进入到messagequene中，message携带消息，looper轮询messagequene，将消息发出来，handler接受消息，处理逻辑。

上述运行原理是个非常浅显的，谁都知道的东西。但是深入底层，看底层代码运行原理的时候，我们先需要将每个成员都需要弄懂。

handler：handler的作用就是通过post和send的一系列方法将消息发送到messagequene中。messagequenes收到消息之后，通过looper轮询，将消息处理。具体的细节通过看源码（以sendMessage为例）：

 

    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);
    }

这几段代码说明了通过handler.sendMessage走的handler底层的一系列方法，最终将消息放入到了messagequene中，然后looper通过轮询messagequene，然后将消息交给handler，handler处理消息。那这个过程我们看源码：

 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
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            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();
        }
    }

looper.loop()这个方法就是一直来轮询messagequene的，for(；；)这是个死循环。也就是说handler通过send将消息发送到messagequene中，looper通过轮询messagequene，调用msg.target.dispatchMessage(msg);msg.target看Message源码发现，其实就是Handler对象，其实调用的就是handler..dispatchMessage（）方法。

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

最后调用的就是handleMessage方法，我们会重写这个方法，然后处理逻辑。

总结Handler:Handler将消息发送到Messagequene中，Looper通过死循环Messagequene，获取消息，然后将消息交给Handler，Handler通过handleMessage方法处理掉消息。