Android异步之Thread+Handler传递消息机制原理

   1.主线程和子线程通过Thread+Handler通讯的一般使用模式(当然在主线程中Thread+Handler
        实现异步的过程并没有完全展示出来，因为在主线程启动的时候，程序就自动帮我们完成了一些操作)    
 

        Handler mHandler;
        mHandler = new Handler(){
            //主线程用来接收数据或更新UI
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
            }
        };
        //线程用作处理耗时操作
        new Thread(new Runnable() {
            @Override
            public void run() {
                 /*
                与下面效果相同
                Message msg = mHandler.obtainMessage();
                msg.obj = bitmap;
                msg.arg1 = SUCCESS_MSG;
                mHandler.sendMessage(msg);
                */
                //mHandler主线程的定义的Handler
                mHandler.obtainMessage( what,obj).sendToTarget();
            }
        }).start();   

   
   2.下面我们看一个在子线程中使用Handler的案例，看他怎么做的(只是放了重要代码)    

        
        private Handler handler;         
        new Thread(new Runnable() {  
            @Override  
            public void run() {
                Looper.prepare();            
                mHandler = new Handler() {  
                    public void handleMessage(Message msg) {  
                        // process incoming messages here  
                    }  
                };  
                Looper.loop();
            }  
        }).start();   

   
    哎呀，怎么多了两步Looper.prepare()和Looper.loop()，嗯这就是UI线程中隐藏的操作，伙伴们可以试试去掉会报啥错
我就不啰嗦了。接下来我们就跟踪一下Handler和Thread是怎么实现在子线程mHandler.obtainMessage( what,obj).sendToTarget();
/mHandler.sendMessage(msg);发送消息，UI线程public void handleMessage(Message msg){}接收到消息的。
    
    1.work线程调用：mHandler.obtainMessage(SUCCESS_MSG,bitmap).sendToTarget();
    
    2. 调用Message.java类sendToTarget方法   

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

    3. 调用Handler.java类的sendMessage方法  

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

  
    4.调用Handler.java类的sendMessageDelayed方法

        public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

   
    5.调用Handler.java类的sendMessageAtTime方法

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        //这里如果queue为空也会报错，也就是没有创建的looper实例
        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);
    }
    

    伙伴们发现return中的方法多了一个queue，他是从哪儿来的呢？
    看到MessageQueue queue = mQueue;嗯？mQueue又从哪儿来的呢？再看

    
    public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

    可以看出mQueue = looper.mQueue原来是Looper类的实例looper调用了Looper类的成员变量mQueue
    初始化的，looper实例有哪儿来的？这个时候Looper.prepare();出来了

    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));
    }    
    

    看looper实例是sThreadLocal.set(new Looper(quitAllowed));这么出来的，原来一开始在初始化Hanler之前就
    通过Looper.prepare()实例化并放到了sThreadLocal中了，而在Looper的构造方法中实例化了mQueue，这个时候
    mQueue = loopery.mQueue中looper实例和mQueue成员变量都有了吧(先不要管sThreadLocal是啥，就当他是一个Looper
    的容器,由上可以看出每个线程对应一个looper实例，每个looper实例对应一个它自己的MessageQueue)
     

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

    再看Looper.java类中myQueue方法显示通过myLooper().mQueue实现

    public static MessageQueue myQueue() {
        return myLooper().mQueue;
    }

    而myLooper().mQueue刚好就是sThreadLocal.get().mQueue也就是通过Looper.prepare()实例化的looper实例对应的MessageQueue

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

  
    回到sendMessageAtTime方法继续往下看

    
    6.调用MessageQueue.java类的enqueueMessage方法
    

    boolean enqueueMessage(Message msg, long when) {
        //判断目标handler是否存在
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }   

  
    这里实现了消息队列的排队与队序管理，哎，伙伴们肯定疑惑，这也没有实现传递数据啊，为啥handleMessage(Message msg) {}
就能接收到数据了呢？好吧别急，我们还没看Looper.loop()呢。
    
    7.调用Looper.java中的loop()方法
    从代码中可以看出他依然获取了之前Looper.prepare()中的looper实例和对应的MessageQueue，当然数据信息也在里面，在看到
msg.target.dispatchMessage(msg);中msg.target当然就是目标的Handler了也就是发送数据消息的mHandler,接下来我们看看dispatchMessage(msg)
    

    public static void loop() {
        //获取到Looper.prepare()中的looper实例
        final Looper me = myLooper();
        //如果为空就会报No Looper; Looper.prepare() wasn't called on this thread.的错
        //一开始叫伙伴们试试不写Looper.prepare()会报啥错，看看是不是这个，哈哈
        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 (;;) {
            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);

            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();
        }
    }

  
    8.调用Handler.java中的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);
        }
    }

   
    Ok，看到了吧，这里调用了Handler.java中的handleMessage(msg)方法，并把数据信息msg传递了过来，当然在UI线程中就可以
通过当前Handler实例对象mHandler重写handleMessage(msg)方法获取数据信息更新UI咯。

    ps:该篇文章是本人对Handler+Thread的学习理解，若有不对的地方，希望能指出来，共同进步。

    推荐文章： http://blog.youkuaiyun.com/guolin_blog/article/details/9991569