android高手进阶教程（一）之--android中的消息机制

1.1 异步神器Handler

Handler允许我们在一个线程的消息队列(MessageQueue)里面发送、处理Message和Runnable,每一个Handler只能在一个线程和一个消息队列(MessageQueue)，当我们创建Handler时，这个Handler与当前线程和当前线程里面的消息队列(MessageQueue)关联，当线程执行操作结束后，Handler可以将消息分发出去。

Handler中我们使用频率较高的是post(Runnable r)和sendMessage(Message msg)，通过源码我们可以看到都调用了将当前Message加入消息队列，然后当前线程的Looper通过获取当前的MessageQueue获取消息再发送个Handler，由Handler分发给拥有这个Handler的线程（必须是拥有Handler的线程）。

    /**
     * Causes the Runnable r to be added to the message queue.
     * The runnable will be run on the thread to which this handler is 
     * attached. 
     *  
     * @param r The Runnable that will be executed.
     * 
     * @return Returns true if the Runnable 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 post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }



   /**
     * 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 the absolute time (in milliseconds) uptimeMillis.
     * The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * 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.
     *
     */
        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);
    }

1.2 Handler 使用实例

public class MainActivity extends Activity {
     


    private ImageView image;
    private Handler mHandler = new Handler(){

        /**
         * 获取子线程中消息队列发送的Message
         * @param msg
         */
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            Bitmap bm = (Bitmap) msg.obj;
            image.setImageBitmap(bm);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        image = (ImageView) findViewById(R.id.image);
        new MyThread(mHandler).start();


    }

    private class MyThread extends Thread{
     
        private Handler mHandler;
        public MyThread(Handler handler){
            this.mHandler = handler;
        }
        @Override
        public void run() {
               //1.耗时操作，比如请求网络
            try {
                HttpURLConnection connection = (HttpURLConnection) new URL("http://h.hiphotos.baidu.com/image/pic/item/e7cd7b899e510fb300679675da33c895d0430cd1.jpg").openConnection();
                connection.setConnectTimeout(5000);
                connection.setReadTimeout(20000);
                InputStream is = connection.getInputStream();
                Bitmap bm = BitmapFactory.decodeStream(is);
                //2.结果包装到Message
                Message msg = new Message();
                msg.obj = bm;
                //3.handler将消息发送给主线程（一般式UI线程）
                mHandler.sendMessage(msg);
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
    }


}

1.3 HandlerThread

HandlerThread是一个拥有looper的线程，这个looper可以用来创建handler，启动这个线程需要调用start().

    public class HandlerThread extends Thread {
     
        int mPriority;
        int mTid = -1;
        Looper mLooper;

        public HandlerThread(String name) {
            super(name);
            mPriority = android.os.Process.THREAD_PRIORITY_DEFAULT;
        }

        /**
         * 构造方法
         * @param name  thread的name
         * @param priority 优先级
         */
        public HandlerThread(String name, int priority) {
            super(name);
            mPriority = priority;
        }


        /**
         * 回调方法，可以被复写
         */
        protected void onLooperPrepared() {
        }


        @Override
        public void run() {
            //获取线程id
            mTid = Process.myTid();

            //将当前线程初始化为Looper线程

            Looper.prepare();
            synchronized (this) {
                mLooper = Looper.myLooper();
                notifyAll();
            }
            Process.setThreadPriority(mPriority);
            onLooperPrepared();
            //开始循环处理消息队列
            Looper.loop();
            mTid = -1;
        }

        /**
         * 这个方法返回和此线程相关的Looper，如果这个线程没有启动或者isAlive()返回false，Looper将返回null，
         * 如果这个线程已经启动，这个方法将会阻塞，一直到looper实例化后。
         * @return The looper.
         */
        public Looper getLooper() {
            if (!isAlive()) {
                return null;
            }

            // If the thread has been started, wait until the looper has been created.
            synchronized (this) {
                while (isAlive() && mLooper == null) {
                    try {
                        wait();
                    } catch (InterruptedException e) {
                    }
                }
            }
            return mLooper;
        }


        public boolean quit() {
            Looper looper = getLooper();
            if (looper != null) {
                looper.quit();
                return true;
            }
            return false;
        }

        /**
         * 停止
         * @return
         */
        public boolean quitSafely() {
            Looper looper = getLooper();
            if (looper != null) {
                looper.quitSafely();
                return true;
            }
            return false;
        }

        /**
         * 获取线程id
         */
        public int getThreadId() {
            return mTid;
        }
    }

2.1 单曲循环Looper

Looper的字面意思就是“循环者”，一个普通线程通过它可以变成一个循环线程，上文中的HandlerThread就是典型的例子，在平时开发中，我们经常需要一个循环线程，一旦有新任务则执行，执行完毕后必须等待下一个任务，这就是Looper线程。使用Looper创建线程也很简单。

    class LooperThread extends Thread {
     
        public Handler mHandler;

        public void run() {
            Looper.prepare();

            mHandler = new Handler() {
                public void handleMessage(Message msg) {
                    // 处理消息                }
            };
            Looper.loop();
        }
    }

2.2 Looper源码解析

public class Looper {
     
    // 每个线程中的Looper对象其实是一个ThreadLocal，即线程本地存储(TLS)对象
    private static final ThreadLocal sThreadLocal = new ThreadLocal();
    // Looper内的消息队列
    final MessageQueue mQueue;
    // 当前线程
    Thread mThread;
    //其他属性

    // 每个Looper对象中有它的消息队列，和它所属的线程
    private Looper() {
        mQueue = new MessageQueue();
        mRun = true;
        mThread = Thread.currentThread();
    }

    // 我们调用该方法会在调用线程的TLS中创建Looper对象
    public static final void prepare() {
        if (sThreadLocal.get() != null) {
            // 试图在有Looper的线程中再次创建Looper将抛出异常
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
    }
  public static final void loop() {
        Looper me = myLooper();  //得到当前线程Looper
        MessageQueue queue = me.mQueue;  //得到当前looper的MQ

        // 保证拥有此Looper的线程是本地线程
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
        // 开始循环
        while (true) {
            Message msg = queue.next(); // 取出message
            if (msg != null) {
                if (msg.target == null) {
                    // message没有target为结束信号，退出循环
                    return;
                }
                // 日志
                if (me.mLogging!= null) me.mLogging.println(
                        ">>>>> Dispatching to " + msg.target + " "
                        + msg.callback + ": " + msg.what
                        );
                // 将Message分发给target-->Handler
                msg.target.dispatchMessage(msg);
                // 还是日志。。。
                if (me.mLogging!= null) me.mLogging.println(
                        "<<<<< Finished to    " + msg.target + " "
                        + msg.callback);


                final long newIdent = Binder.clearCallingIdentity();
                if (ident != newIdent) {
                    Log.wtf("Looper", "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);
                }
                // 回收message资源
                msg.recycle();
            }
        }
    }

}