说到Handler想必大家都经常用到,在非UI线程更新UI那可是利器,用起来也非常容易上手
从使用上来说,我们只需要关注sendMessage和handleMessage即可
所以我们先从Handler和Message来说起,先看一小段代码

    public static final int UPDATE_TEXT_VIEW = 0;
    public TextView mResultTextView = null;

    // new 一个 Handler 对象, 以内部类的方式重写 handleMessage 这个函数
    public Handler mMyHandler = new Handler() {
        // ③ 处理消息
        public void handleMessage(android.os.Message msg) {
            switch (msg.what) {
                case UPDATE_TEXT_VIEW:
                    /* 这里可更新 ui */
                    mResultTextView.setText(String.valueOf(msg.arg1));
                    break;

                default:
                    /* do nothing */
                    break;
            }
        };
    };

    // ==================================================================
    // 函数名: calc
    // 日期: 2015-08-30
    // 功能: 计算入参并显示在 UI 上,然后后续以每秒 +1 在 UI 上更新
    // 输入参数: int a
    // int b
    // 返回值: 无
    // 修改记录:千里草新增函数
    // ==================================================================
    public void calc(int a, int b) {
        int c = a + b;
        final Message msg = new Message();
        msg.what = UPDATE_TEXT_VIEW;
        msg.arg1 = c;
        // ① 发送消息
        mMyHandler.sendMessage(msg);
        // 这里更新 UI 线程的TextView OK
        // mResultTextView.setText(String.valueOf(msg.arg1));

        // 启动一个新的线程来每秒刷新 TextView
        new Thread() {
            public void run() {
                while (true) {
                    msg.arg1++;
                    // ② 发送消息
                    mMyHandler.sendMessage(msg);

                    // mResultTextView.setText(String.valueOf(msg.arg1)); 这里会报错
                    try {
                        sleep(1000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }

            };
        }.start();
    }

代码中有①,②两处发送消息,. 分别是从UI线程和从子线程发送的消息, 在③处两个消息都可以被准确的接收到.
在接受消息函数handleMessage 里我们可以做任何想做的事情,包括更新UI…
从使用来说,大家根据这个例子,想必已经可以简单使用Handler来处理多线程之间的交互了…

此处需要注意的是在使用Handler时,必须需要重写handleMessage 才能达到我们想达到的目的…(从逻辑上来说,没有接受消息的地方,发送的消息有何意义呢,是吧….从原理上..我们接下来会谈到……)

谈到Handler,我们还可以用它执行一个Runnable多线程…例如下面的代码..

mMyHandler.post(new Runnable() {

            @Override
            public void run() {
                // 这里需要注意的是,calc 是在子线程里被调用,所以calc 就无法操作 UI 了.
                calc(1, 2);
            }
});

Handler可真神奇,既可以发消息也可以执行某个任务线程. 为了更深层次的了解它,只能去看看它的源码了.let’s go! 先看看是如何post一个Runnable线程的

public final boolean post(Runnable r)
    {
       //这里实际上是发送了一个Message
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

    //组装一个只带Runnable的Message
    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

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

哎哟喂,这Handler小丫头片子,还假装两幅面孔呢, 经查看源码,发现它实际上是发送了一个Message消息,然而这个消息仅仅携带了一个Runnable对象.

现在来看只需要分析Handler和Message了..我们来看看最终Handler是如何发送消息的..我们继续看源码,非得把他们扒个精光不可

public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
    boolean sent = false;
    //消息队列,这里存储Handler发送的消息
    MessageQueue queue = mQueue;
    if (queue != null) {
        //Message与Handler绑定
        msg.target = this;
        //消息进队列
        sent = queue.enqueueMessage(msg, uptimeMillis);
    }
    else {
        RuntimeException e = new RuntimeException(
            this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
    }
    return sent;
}

从消息最终被发出的函数来看,这里只做了两个事情:

1.Message与Handler绑定

2.将Message放进消息队列里

消息进队列之后,Handler发送消息的任务算是完美的完成了,接下来我们该介绍介绍Looper了,如果没有Looper,Handler和Message之间的爱情可以说是不完整的.

我们先来看看一个Looper是如何使用的吧

class LooperThread extends Thread {
      public Handler mHandler;

      public void run() {
          // 为当前线程准备一个 Looper
          Looper.prepare();

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

//继续看源码,prepare执行之后sThreadLocal绑定一个Looper对象
//sThreadLocal 能且只能绑定一个Looper对象
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));
}

/**
 * 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;

    // 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();

    //无条件的for循环,来遍历Messagequeue
    for (;;) {
        //获取下一条消息
          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);
        }

        //Message绑定的Handler来开始处理消息.. 下面贴上dispatchMessage,函数,大家继续往下看
          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.recycle();
    }
}
/**
 * Callback interface you can use when instantiating a Handler to avoid
 * having to implement your own subclass of Handler.
 */
public interface Callback {
    public boolean handleMessage(Message msg);
}

/**
 * Subclasses must implement this to receive messages.
 * 子类必须完成handleMessage这个函数来接收消息
 */
public void handleMessage(Message msg) {
}

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    //收到消息后,先处理Runnable 对象callback.
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        //如果这里有回调接口,那么就直接调用该接口,不再继续调用Handler的handleMessage函数来处理消息
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

从上述加了中文注释的代码中能看出,Handler发送的Message通过Looper分发给了各自的Handler.

所以上面说的在使用Handler时,必须需要重写handleMessage 才能达到我们想达到的目的…也不完全正确,

我们也可以实现一个CallBack接口来处理消息,用Google的原话是:

/**
 * Callback interface you can use when instantiating a Handler to avoid
 * having to implement your own subclass of Handler.
 */
public interface Callback {
    public boolean handleMessage(Message msg);
}

再贴最后一段代码来彻彻底底说明Handler Looper之间的关系, 那就是Handler的 构造函数

/**
 * Default constructor associates this handler with the queue for the
 * current thread.
 *
 * If there isn't one, this handler won't be able to receive messages.
 */
public Handler() {
    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());
        }
    }
    //绑定当前线程的Looper
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    //与当前线程的消息队列绑定
    mQueue = mLooper.mQueue;
    mCallback = null;
}

一句话总结来说,Handler初始化时绑定了线程的MessageQueue,当前线程的Looper来依次分发MessageQueue里的消息.

MessageQueue的消息会根据Message的target绑定的Handler被Looper分发到各自的Handler里去处理.

例如Activity里有两个HandlerA和HandlerB, HandlerA发送的消息绝对不可能被HandlerB处理..

上述只是个人的很片面的理解,希望大家补充和指出不足的地方…