android(七)Looper Handler分析

一。总结

Looper有一个MessageQueue,用于封装消息循环。   Handler封装了消息投递，消息处理等的辅助类

二。分析

1.从Looper的用法开始分析

 class LooperThread extends Thread {
      public Handler mHandler;
      public void run() {
          Looper.prepare();          //(1)调用prepare
          mHandler = new Handler() {
              public void handleMessage(Message msg) {
                  // process incoming messages here
              }
          };
          Looper.loop();          //(2)进入消息循环
      }
  }

（11）prepare做了什么？

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();    
public static void prepare() {

　　　　if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
　　　　sThreadLocal.set(new Looper());                            //(12)构造一个Looper,设置到线程本地变量中 

 } 

 （12）构造Looper对象

    private Looper() {
        mQueue = new MessageQueue(quitAllowed);  
        mRun = true;
        mThread = Thread.currentThread();        //得到当前线程
    }

所以由（12）可知，prepare把Looper对象保存在线程本地变量中。巧妙地把Looper和调用线程关联起来。

2.Handler-MessageQueue-Looper，三者构成了死循环+消息通信的模型

    public static void loop() {
        final Looper me = myLooper();        // （21）取得线程本地变量Looper
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;         // 取出Looper中的queue

        // 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);              //Handler target; 参照下面的dispatchMessage方法

            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();
        }
    }
    public static Looper myLooper() {               //(22)返回线程本地变量Looper
        return sThreadLocal.get();
    }

 所以由（21）、（22）可看出loop()处理本线程消息队列中的消息。

三。Looper, Message,Handler之间的关系

1.Message中有个Handler,指定这个Message由哪个Handler处理

Handler target;    

2.分析Handler成员变量

    final MessageQueue mQueue;
    final Looper mLooper;
    final Callback mCallback;

3.Handler默认构造函数

  public Handler() {
        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();  //(31)获得调用线程的Looper
        if (mLooper == null) {
            throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;　　　　//(32)获得Looper中的消息队列
        mCallback = null;
    }

4.Handler的另一个构造函数

public Handler(Looper looper) {          //（33）传入某个线程的Looper
        mLooper = looper;
        mQueue = looper.mQueue;　　　　　　//(34)获得Looper中的消息队列

        mCallback = null;
 }

由上可知，Handler中的MessageQueue都会指向Looper中的消息队列。

5.There are two main uses for a Handler:

　　 (1) to schedule messages and runnables to be executed as some point in the future;

 　　(2) to enqueue an action to be performed on a different thread than your own.

　 　Handler只有一个消息队列，即MessageQueue。通过post()传进去的Runnable将会被封装成消息对象后传入MessageQueue。当Looper轮询到该线程时，并不会单独开启一个新线程，

而Runnable仍然在当前Looper绑定的线程中执行，Handler只是调用了该线程对象的run()而已。通过post()将Runnable提交到主线程的Looper中可以实现UI的更新。

51.sendMessage

    public boolean sendMessageAtTime(Message msg, long uptimeMillis){
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;　　　　　　　　//（41）把处理这个message的Handler设为自己
            sent = queue.enqueueMessage(msg, uptimeMillis);
        }
        else {
            RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }

 2.post

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

    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;　　　　　　　　　　　　　　　　　　　　　　　　　　///*package */ Runnable callback;    将Runnable封装成消息对象
        return m;
    }
    public final boolean sendMessageDelayed(Message msg, long delayMillis){
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

6.消息处理

    public void handleMessage(Message msg) {   //子类要重写的方法
    }
 
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {　　　　　　//如果msg本身有callback
            handleCallback(msg);
        } else {
            if (mCallback != null) {　　　　　　　　//如果handler本身有callback
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);　　　　　　　　　　　　//最后交给子类处理
        }
    }

7.msg的Callback

    private static void handleCallback(Message message) {
        message.callback.run();                                //直接调用Runnable的run方法，也就是说不是新建线程来执行
    }