Handler,MessageQueue,Looper的关系

• Looper的作用是在线程中处理消息的

• MessageQueue用来存储消息的队列

• Handler使用来定义具体处理消息的方式，以及发送消息；Android主线程中持有一个可以更新UI的Handler；对其他的线程，如果需要处理消息，可以自己new一个Handler在线程的 run 方法中。

• ThreadLocal：MessageQueue对象，和Looper对象在每个线程中都只会有一个对象，怎么能保证它只有一个对象，就通过ThreadLocal来保存。Thread Local是一个线程内部的数据存储类，通过它可以在指定线程中存储数据，数据存储以后，只有在指定线程中可以获取到存储到数据，对于其他线程来说则无法获取到数据。

一.Looper

源码分析： Looper.prepare()方法;创建一个Looper的实例，但是该实例在一个线程中只能有一个；保存在ThreadLocal中，如果发现已经存在了Looper对象，会抛出一个异常。

    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.loop()方法；loop()方法中有一个无限的循环。for(;;)那一段；线程执行到loop()方法之后会一直留在loop()方法中，直到被打断终止 （源码可以看出，发送空消息就可以打断该循环，终止该线程）

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();//死循环for (;;) {Message msg = queue.next(); // might blockif (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 loggerfinal Printer logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}final long traceTag = me.mTraceTag;if (traceTag != 0) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}try {msg.target.dispatchMessage(msg);} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}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();}}

二.Handler

重要的构造方法：

• 传入某个线程thread的Looper对象；这样这个Handler就是该thread的Handler了(和在该thread的run方法中直接new一个没有参数的Handler的效果是一样的；但是在外部定义可以使我们更好使用)

   /***  Use the provided {@link Looper} instead of the default one.**  @param looper The looper, must not be null.*/public Handler(Looper looper) {this(looper, null, false);}/*** Default constructor associates this handler with the {@link Looper} for the* current thread.** If this thread does not have a looper, this handler won't be able to receive messages* so an exception is thrown.*/public Handler() {this(null, false);}

三.MessageQueue

MessagerQueue是一个单向链表

HandlerThread

HandlerThread是Android系统自己实现的一个可以实现 处理异步消息 的线程。简单来说就是和UI线程是一样的原理，通过Looper和Message通信，让该线程为我们服务。

具体可以看源码就能知道他的原理（我们自己也可以写这样的线程，但是系统给我们的总是要好一点的）

    @Overridepublic void run() {mTid = Process.myTid();Looper.prepare();synchronized (this) {mLooper = Looper.myLooper();notifyAll();}Process.setThreadPriority(mPriority);onLooperPrepared();Looper.loop();mTid = -1;}

HandlerThread的使用

HandlerThread workThread = new HandlerThread();
workThread.start();
Handler handler = new Handler(workThread.getLooper());