Android Handler原理解析

我们都知道Android的主线程不能处理耗时的任务，否者会导致ANR的出现，但是界面的更新又必须在主线程中进行，这样我们就必须在子线程中处理耗时的任务，在主线程中进行UI更新。
Android的消息处理有三个核心类：
Looper,Handler和Message
MessageQueue（消息队列）封装到Looper里面了,我们不会直接与MessageQueue打交道。

1、Looper
Looper 在 Android 的消息机制中扮演着消息循环的角色，它会不停地从 MessageQueue 通过 next() 获取是否有新消息，如果有新消息就处理，否则就 MessageQueue 阻塞在那里。
Looper主要是prepare()、loop()这个两个方法

public static void prepare() {
        prepare(true);
    }

private static void prepare(boolean quitAllowed) {
		
		//sThreadLocal是否为null，否则抛出异常，Looper.prepare()方法只能被调用1次
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        //将一个Looper的实例放入了ThreadLocal
        sThreadLocal.set(new Looper(quitAllowed));
        
    private Looper(boolean quitAllowed) {
	    //构造方法中创建MessageQueue(消息队列)
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

接着看下loop()做了什么?

public static void loop() {
		//获取sThreadLocal存储的Looper实例，me为null则抛出异常，也就是说looper方法在prepare方法之后调用
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        //拿到当前线程Looper的MessageQueue
        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();
		
		//开始消息开始循环
        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是Handler类型对象
			//把消息交给msg的target的dispatchMessage方法去处理
            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);
            }
			//释放资源
            msg.recycleUnchecked();
        }
    }

public static Looper myLooper() {
return sThreadLocal.get();
}

Looper用于封装了android线程中的消息循环，默认情况下一个线程是没有消息循环的，需要调用Looper.prepare()来给线程创建一个消息循环，调用Looper.loop()来使消息循环，从消息队列里取消息，处理消息。

Looper作用：
1、Looper使一个线程变成Looper线程
2、 每个线程只会有一个Looper实例，保证Looper实例也只有一个MessageQueue。
3、 Looper内部有一个消息队列，loop()方法调用后线程开始不断从MessageQueue中去取消息，交给消息的target属性的dispatchMessage去处理。

那么，我们如何往MessageQueue上添加消息呢？
那么该Handler登场了~

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

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());
            }
        }
		//Looper.myLooper()获取了当前线程保存的Looper对象
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        //获取了Looper实例中保存的MessageQueue（消息队列），这样就保证了handler的实例与我们Looper实例中MessageQueue关联上了
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

有了handler之后，我们就可以使用 post(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long)和 sendMessageDelayed(Message, long)这些方法向MessageQueue上发送消息了。光看这些API你可能会觉得handler能发两种消息，一种是Runnable对象，一种是message对象，其实post发出的Runnable对象最后都被封装成message对象：

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

public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
 //最终调用了sendMessageAtTime方法发送消息
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);
    }

看完了消息的发送，再来看下handler如何处理消息。消息的处理的核心方法dispatchMessage(Message msg)

最后调用了handleMessage方法

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

为什么是一个空方法呢
消息最终回调是由我们控制的，在创建handler的时候都是重写handleMessage方法，然后根据msg.what进行消息处理!