Handler消息处理机制详解

Handler封装了消息的发送，也负责接收消。通过post方法和sendMessage发送消息。内部会跟Looper关联。

public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

public Handler(Looper looper) {
        this(looper, null, false);
    }

Looper 消息封装的载体，内部包含了MessageQueue，负责从MessageQueue取出消息，然后交给Handler处理 ，循环，默认线程是没有消息循环队列的，需要执行prepare或者prepareMainLooper方法创建Looper对象，调用loop开启消息循环。

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));
    }

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;
............

MessageQueue 就是一个消息队列，负责存储消息，有消息过来就存储起来，Looper会循环的从MessageQueue读取消息。MessageQueue包含了需要Looper处理的Message，MessageQueue不能直接添加Message而是通过Handler。 MessageQueue是由Looper初始化的时候创建的：

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;

源码地址：http://androidxref.com/9.0.0_r3/xref/frameworks/base/core/java/android/os/Handler.java
http://androidxref.com/9.0.0_r3/xref/frameworks/base/core/java/android/os/Looper.java