Handler Looper MessageQueue解析

先分别介绍一下Handler，Looper，MessageQueue：

1、Handler，Handler封装了消息的发送

2、Looper，（1）内部包含一个消息队列（即messageQueue）,所有的Handler发送的消息都要走向这个队列

（2）Looper.loop()是一个死循环，不断的从MessageQueue中取出消息，如果MessageQueue中有消息就处理消息，没有消息就阻塞。

3、MessageQueue，就是一个消息的队列（可以理解成一个数组）。

分析一下：

当我们new一个Handler的时候，看一下构造函数

 <span style="font-size:14px;">public Handler(Callback callback, boolean async) {        if (FIND_POTENTIAL_LEAKS) {            final Class<? extends Handler> klass = getClass();            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&                    (klass.getModifiers() & Modifier.STATIC) == 0) {                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +                    klass.getCanonicalName());            }        }        mLooper = Looper.myLooper();        if (mLooper == null) {            throw new RuntimeException(                "Can't create handler inside thread that has not called Looper.prepare()");        }        mQueue = mLooper.mQueue;        mCallback = callback;        mAsynchronous = async;    }</span>

我们看到已经拿到一个Looper对象

看下Looper 的构造函数

<span style="font-size:14px;">  public static Looper myLooper() {        return sThreadLocal.get();    }</span>

调用了sThreadLocal.get()返回一个Looper对象 ，sThreadLocal是一个ThreadLocal的对象，这也就是为什么每个activity都会有一个Looper的原因。

再看下Looper.loop()做了什么

<span style="font-size:14px;">    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.recycle();        }    }</span>

可以看到 实际上就是不断的从MessageQueue中取出消息。

总结一下三者的关系：Handler负责发送消息，Looper负责接收Handler发送的消息，并把消息回传给Handler自己，MessageQueue是一个存储消息的容器。