Android中的Looper，MessageQueue，Handler的理解

Looper中的ThreadLocal对象sThreadLocal保存有Looper对象，在其构造方法中会获取当前的线程并且创建一个消息队列mQueue

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
 }

prepare方法（一个静态方法）中会创建一个Looper实例保存到sThreadLocal中，这个实例里有一个mQueue

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));
    }

因为Looper.prepare()在一个线程中只能调用一次，所以MessageQueue在一个线程中只会存在一个。否则就会报RuntimeException异常。

然后Looper.loop()会让当前线程进入一个无限循环，不端从MessageQueue的实例中读取消息，然后回调msg.target.dispatchMessage(msg)方法。在Activity的启动代码中，已经在当前UI线程调用了Looper.prepare()和Looper.loop()方法。

public static void loop() {
        final Looper me = myLooper();//myLooper方法获得Looper实例
        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 (;;) {//循环获取队列中的消息
            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);//<span style="color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px;">msg.target.dispatchMessage(msg)最终调用</span><span style="font-family: Arial;">Handler实例的handleMessage方法</span>

            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();
        }
    }

问题：Looper如何绑定当前线程？Looper里有ThreadLocal变量，ThreadLocal为每一个线程维护变量的副本，在ThreadLocal类中有一个Map，用于存储每一个线程的变量副本

2.Handler‘

可以看到msg.target是一个Handler实例，下面看看他的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);
        }
    }

如果不设置Callback就会调用我们自己重写的handleMessage方法

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();//得到当前线程的Looper
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;<span style="font-family: Arial, Helvetica, sans-serif;">//得到Looper实例中保存的消息队列</span>
        mCallback = callback;
        mAsynchronous = async;
    }

通过得到Looper和MessageQueue，就保证了handler的实例与我们Looper实例中MessageQueue关联上了。

我们常用的Message的sendToTarget会调用handler的sendMessage方法。

public void sendToTarget() {
        target.sendMessage(this);
    }

下面看看Handler的sendMessage方法和

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

再看 sendEmptyMessageDelayed

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);  
   }

Handler发送消息的时候最后会调用dispatchMessage方法，下图可以看出如果没有给msg设置callback就会调用我们自己写的handleMessage。

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

总结：Looper.prepare()创建一个Looper，并且放到这个线程的ThreadLocal里保存。Looper创建一个MessageQueue。Looper.loop();进入一个循环不断的从MessageQueue里面读取Message。Handler类中有MessageQueue和Looper对象作为其成员变量，在其构造方法中通过Looper的myLooper()方法获得Looper对象，从而获得MessageQueue。即Handler中有Looper，Looper中有MessageQueue，MessageQueue中有Handler。