Android消息机制之三Handler分析

Handler的简单使用

在项目中，Handler的声明和初始化一般如下，使用默认的构造函数，当然Handler中也带参数的构造函数。

	// 使用默认构造函数
	private Handler mHandler = new Handler();
	// 使用默认构造函数，并重写handleMessage方法
	private Handler myHandler = new Handler() {
		public void handleMessage(android.os.Message msg) {
		};
	};

Handler的构造函数

下面是Handler.java所有的构造函数(好多个呢)。

Handler类在构造方法中，可指定Looper，Callback回调方法以及消息的处理方式(同步或异步)，对于无参的handler，默认是当前线程的Looper。

在初始化时就会把当前线程的Looper和MessageQueue赋值到Handler中的对应变量中。然而Looper和MessageQueue的初始化是在Looper.prepare()中进行的。

同时在Handler初始化时有提示“If this thread does not have a looper, this handler won’t be able to receive messages”，因此必须要有一个Looper.loop()进行循环查询消息队列。又因为loop()中是一个死循环，除非异常或者自动退出，才会执行loop后面代码，所以Handler的使用和初始化必须在Looper.loop()之前。（好像扯远了^_^）

继续看代码

biumall.com

    /**
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     *
     * If this thread does not have a looper, this handler won't be able to receive messages
     * so an exception is thrown.
     */
    public Handler() {
        this(null, false);
    }
    public Handler(Callback callback) {
        this(callback, false);
    }
    public Handler(Looper looper) {
        this(looper, null, false);
    }
    public Handler(Looper looper, Callback callback) {
        this(looper, callback, false);
    }
    public Handler(boolean async) {
        this(null, async);
    }
    public Handler(Callback callback, boolean async) {
        //匿名类、内部类或本地类都必须申明为static，否则会警告可能出现内存泄露
        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());
            }
        }
        //获取当前线程的Looper，所以在Handler使用前Looper.prepare()
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        //获取初始化中初始化的消息队列
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
    public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

Handler的消息发送

直接看图，Handler可以发送的参数类型有what,Runnable,message，然后一步一步进行封装成message,最终发送的是message的

1、发送what

在项目中，发送what时是使用sendEmptyMessage、sendEmptyMessageDelayed, sendEmptyMessageAtTime的，其实这三个的作用差不多，就是延迟时间不同，其他的都是进一步把what封装

biumall.com

    //delayMillis延迟时间为0
    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }
     //delayMillis延迟时间，如果为0，和上面sendEmptyMessage的方法一样
     public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
    }
    //uptimeMillis 是当前系统时间+延迟的时间（SystemClock.uptimeMillis() + delayMillis）
    public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
    }

2、发送Message

我们会发现sendMessage()调用sendMessageDelayed()再调用sendMessageAtTime()，再调用enqueueMessage(),最终调用消息队列的enqueueMessage()，也就把message发送到MessageQueue中了。

biumall.com

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

3、发送Runnable

biumall.com

    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
    //uptimeMillis= 是当前系统时间+延迟的时间（SystemClock.uptimeMillis() + delayMillis）
    public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
    }
    //delayMillis 延迟
    public final boolean postDelayed(Runnable r, long delayMillis)
    {
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }

Handler.sendEmptyMessage()等一系列方法最终调用MessageQueue.enqueueMessage(msg, uptimeMillis)，将消息添加到消息队列中，其中uptimeMillis为系统当前的运行时间，不包括休眠时间。

Handler的消息派发

Handler消息的分发是在Looper.loop()查询消息，然后调用msg.target.dispatchMessage(msg);进行派发，最终到了Handler中。

biumall.com

    /**
     * Callback interface you can use when instantiating a Handler to avoid
     * having to implement your own subclass of Handler.
     *
     * @param msg A {@link android.os.Message Message} object
     * @return True if no further handling is desired
     */
    public interface Callback {
        public boolean handleMessage(Message msg);
    }
 /**
     * Subclasses must implement this to receive messages. 子类必须实现此接口
     */
    public void handleMessage(Message msg) {
    }
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        //如果msg中有callback，直接调用message.callback.run()
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                 //如果有mCallback,就走这里
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            //这个方法要我们实现，然后处理发送来的message
            handleMessage(msg);
        }
    }