Android Handler原理介绍

Andriod提供了Handler 和 Looper 来满足线程间的通信。Handler先进先出原则。Looper类用来管理特定线程内对象之间的消息交换(MessageExchange)。UIthread 通常就是main thread，而Android启动程序时会替Looper实例建立一个MessageQueue消息队列。

1 Looper

Activity启动的时候(OnCreate之前)就会创建一个Looper线程，由它来管理Looper线程里的MessageQueue(消息队列)。 UIThread线程通过Looper.prepare()创建唯一的Looper实例，然后创建该Looper实例中会同时创建一个MessageQueue对象；

对于Looper主要是prepare()和loop()两个方法。

1.1prepare() 

因为Looper.prepare()在一个UiThread线程中只能调用一次，所以MessageQueue在一个UIThread线程中只会存在一个。

在Looper.java中

     /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    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));
    }

可以看出prepare()方法会首先判断主线程中是否存在Looper线程（对象），如果有，则抛出异常，如果没有则创建一个新的Looper对象（线程）.

1.2loop()

接下来UIThread主线程中会调用Looper.loop()会让当前Looper线程进入一个无限循环，不断的从MessageQueue的队列中读取消息，然后调用msg.target.dispatchMessage(msg)方法分发消息。

在Looper.java文件代码如下

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    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;//1.此处是获取MessageQueue对象.
		                             //其实就是和MessageQueue产生一个关联，或者关系，
					     //为后续操作MessageQueue提供一个对象。

        // 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 (;;) {                      //2.此处for无限循环很重要，也就是此处说明了Looper对象是一直无限的在操作MessageQueue
            Message msg = queue.next(); // might block 每次循环都先从消息队列中取出一个消息
            if (msg == null) {          //如果取出的消息为空，则表明当前MessageQueue没有消息。如果不为就将消息分发出去，进行处理。
                // No message indicates that the message queue is quitting.
                return;                //消息为空返回，进行下一次取消息判断消息，然后处理消息
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);//3.此处是将消息分发出去进行处理，可能你会想msg.target是谁呢？接下来的代码会展示。
				                //其实就是你在UIThread中创建的handler对象本身。
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

            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();
        }
    }

这段代码太长了，看起来真是很麻烦，所以我觉着只要抓住我文中的1、2、3三处代码就足够了。总的来说Looper中的loop()方法就是无限的从Message中去取消息，然后判断消息是否为null，如果为空则继续去取消息，再判断是否为null。如果消息不为空就通过dispatchMessage方法将消息分发出去处理。

接下来我们看一下msg.target是个什么东西，首先从msg的定义我们可以看出是Message的一个实例，我们就看Message类

在Message.java中有如下的代码

/**
 *
 * Defines a message containing a description and arbitrary data object that can be
 * sent to a {@link Handler}.  This object contains two extra int fields and an
 * extra object field that allow you to not do allocations in many cases.
 *
 * <p class="note">While the constructor of Message is public, the best way to get
 * one of these is to call {@link #obtain Message.obtain()} or one of the
 * {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
 * them from a pool of recycled objects.</p>
 */
public final class Message implements Parcelable {
.....
/*package*/ Handler target;
.....
}

由此我们可以看出，原来msg.target就是Handler，也就是你在UIThread中创建的handler对象本身.至于这个msg.target在什么时候被赋值为handler对象本身的呢？后续讲解会解答这个问题.接下来我们继续看dispatchMessage。从上知道msg.target.dispatchMessage(msg)自然就是Handler.dispatchMessage(msg)。那么我们看一下Handler中的dispatchMessage方法

在Handler.java中

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);//有没有觉着此方法很熟悉？
			       //其实这就是你在创建Handler对象的时候所重写的handleMessage方法
			       //到此处我们知道了，原来是消息被取出来之后，
			       //就是调用我们的重写的handleMessage方法
        }
    }

2 MessageQueue

其实MessageQueue没有啥可说的，就是在创建Looper对象实例的时候，创建了该消息队列。他就是类似于水管的东西，只不过水管中装的是水，但是MessageQueue装的是一个一个的Message对象罢了。

Looper.java中

    /**
     * Return the {@link MessageQueue} object associated with the current
     * thread.  This must be called from a thread running a Looper, or a
     * NullPointerException will be thrown.
     */
    public static @NonNull MessageQueue myQueue() {
        return myLooper().mQueue;
    }

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

在Looper的构造方法中，可以看出在创建Looper对象的时候，创建了MessageQueue对象。

Looper主要作用：
1、Looper与UIThread线程绑定，并且一个UIThread线程只有一个Looper实例。创建一个Looper实例过程中会创建一个MessageQueue对象。所以一个Looper实例也保证了只有一个MessageQueue实例。

2、Looper的loop()方法，不断从MessageQueue中取消息，交给消息的target属性的dispatchMessage去处理。

好了，我们的异步消息处理线程已经有了消息队列（MessageQueue），也有了在无限循环体中取出消息的Looper，现在缺的就是发送消息的对象了。接下来就是Handler的创建了.

3 Handler

3.1 创建Handler的实例的时候，在Handler构造方法，会首先得到当前线程中保存的Looper实例，进而与Looper实例中的MessageQueue关联。

在Handler.java中

    /**
     * Use the {@link Looper} for the current thread with the specified callback interface
     * and set whether the handler should be asynchronous.
     *
     * Handlers are synchronous by default unless this constructor is used to make
     * one that is strictly asynchronous.
     *
     * Asynchronous messages represent interrupts or events that do not require global ordering
     * with respect to synchronous messages.  Asynchronous messages are not subject to
     * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
     *
     * @param callback The callback interface in which to handle messages, or null.
     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
     *
     * @hide
     */
    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;
    }

3.2 Handler的sendMessage方法，会给msg的target赋值为handler自身，然后加入MessageQueue中。

在Handler.java中

    /**
     * Pushes a message onto the end of the message queue after all pending messages
     * before the current time. It will be received in {@link #handleMessage},
     * in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
	
	
    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
	
    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
     * Time spent in deep sleep will add an additional delay to execution.
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    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;  //赋值为本身
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

走了这么大一圈，代码这么多，但是最终就是将消息加到了消息队列中。此处也刚好回到了刚才我们所说的msg.target是什么时候赋值为Handler对象本身的，就是在你调用sendMessage的时候，将msg.target赋值为handler的对象.

3.3 在构造Handler实例时，我们会重写handleMessage方法，也就是msg.target.dispatchMessage(msg)最终调用的方法。此部分代码就是我们要实现的handlermessage内容了，不同的需求实现的代码不同，所以接下来的就看各位自己怎么实现了。

总结一下

1、首先Looper.prepare()在UIThread线程中创建一个Looper实例，然后该实例中创建一个MessageQueue对象；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。

2、Looper.loop()会让当前Looper线程进入一个无限循环，不停地从MessageQueue的实例中取消息msg，然后调msg.target.dispatchMessage(msg)方法将消息分发出去。

3、Handler的构造方法，会首先关联当前UIThread线程中保存的Looper实例，进而与Looper实例中的MessageQueue关联。

4、Handler的sendMessage方法，会给msg的target赋值为handler自身，然后将消息msg加入MessageQueue中。

5、创建对象Handler时，我们会重写handleMessage方法，也就是msg.target.dispatchMessage(msg)最终调用的方法。

好了，总结完成，大家可能还会问，那么在Activity中，我们并没有显示的调用Looper.prepare()和Looper.loop()方法，为啥Handler可以成功创建呢，这是因为在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。