Android Handler机制(四)---Handler源码解析

Handler的主要用途有两个:(1)、在将来的某个时刻执行消息或一个runnable,(2)把消息发送到消息队列。

   主要依靠post(Runnable)、postAtTime(Runnable, long)、postDelayed(Runnable, long)、sendEmptyMessage(int)、sendMessage(Message)、sendMessageAtTime(Message)、sendMessageDelayed(Message, long)这些方法来来完成消息调度。post方法是当到Runable对象到达就被插入到消息队列;sendMessage方法允许你把一个包含有信息的Message插入队列,而且它会Handler的handlerMessage(Message)方法中执行(该方法要求在Handler的子类中实现)。

1.构造方法

 /**   默认的构造方法,handler是和当前线程的队列关联在一起,如果队列不存在,那么handler就不能接受消息。
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     * 如果线程没有looper,就会抛出异常
     * 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);
    }
//传入一个callback接口用于处理handler传递的Message。
 public Handler(Callback callback) {
        this(callback, false);
    }

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

  方法都差不多,主要是完成了赋值的过程,还有几个没粘贴,但是都差不多。。。

2.变量

/*
     设置这个标记为true来检测不是静态的匿名,本地或成员类继承Handler类。这些类型的类可以带来潜在的泄漏。在Handler的构造方法里面使用到了这个参数,目的就如上所述。
     */
    private static final boolean FIND_POTENTIAL_LEAKS = false;
    final MessageQueue mQueue;
    final Looper mLooper;
    final Callback mCallback;  //回调接口
    final boolean mAsynchronous;
    IMessenger mMessenger;

  接着下面就是Callback接口:

 /**
     * Callback interface you can use when instantiating(实例化) a Handler to avoid
     * having to implement(实现) 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);
    }

callback接口你可以在实例化的时候用,避免去实现你自己的handler子类

这个Callback接口里只有一个handleMessage方法返回boolean值,在后面Handler的ctor会用到,一般情况下都是null。这个接口的存在

没什么特殊的含义,只是为了让你不extends Handler就能处理消息而已(正如此方法的doc所说),类似Thread和Runnable接口的关系。

3.handleMessage

/**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

子类需要实现这个方法,因为这是个空方法。

4.dispatchMessage​

/**
     * 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);
        }
    }
private static void handleCallback(Message message) {
        message.callback.run();
    }

它的处理是如果message自身设置了callback,则

直接调用callback.run()方法,否则Callback接口的作用就显现了;如果我们传递了Callback接口的实现,即mCallback非空,则调用它处理

message,如果处理了,返回true,则直接返回,否则接着调用Handler自己的handleMessage方法,其默认实现是do nothing,如果你

是extends Handler,那么你应该在你的子类中为handleMessage提供自己的实现。

5.一系列obtainMessage

public final Message obtainMessage()
    {
        return Message.obtain(this);
    }
public final Message obtainMessage(int what)
    {
        return Message.obtain(this, what);
    }
public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
    {
        return Message.obtain(this, what, arg1, arg2, obj);
    }

从上面可以看出来还是调用的Message的obtain方法,来构造message。

6.一系列postXXX方法:

 /**
     * 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);
    }
public final boolean postAtTime(Runnable r, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
    } 
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
    }
public final boolean postDelayed(Runnable r, long delayMillis)
    {
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }
 public final boolean postAtFrontOfQueue(Runnable r)
    {
        return sendMessageAtFrontOfQueue(getPostMessage(r));
    }

上面方法的作用就是:把Runnable发送到消息队列,执行的时候实行Runnable的run方法。。

下面看一系列sendXXX方法,和上边对应的。。。

//把消息入队
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);
    }
 public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);

  上面所有的postXXX,sendXXX方法最后都会调用这个方法:enqueueMessage

 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

 最后调用queue的enqueueMessage方法把msg入队,对这个方法不了解的,可以看前边的MessageQueue源码解析。

SystemClock.uptimeMillis(),就是表示系统开机到当前的时间总数,如果有延迟,就加上延迟时间,分析到现在,我们也能发现,postDelayed不是延迟多少秒发送消息,这个消息是直接发送给队列的,不过在MessaegQueue中,消息按时间排放的,不到时间不会把它取出来,所以应该说延迟多少秒取出消息更合适。。

至于上边的getPostMessage(r)就是把r设置给callback。

private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

removeCallbacks:

  调用MQ的removeMessages方法。就是移出messageQueue中所有满足条件的message,当然前提是消息还没取走。

  /**
     * Remove any pending posts of Runnable r that are in the message queue.
     */
    public final void removeCallbacks(Runnable r)
    {
        mQueue.removeMessages(this, r, null);
    }
    /**
     * Remove any pending posts of Runnable <var>r</var> with Object
     * <var>token</var> that are in the message queue.  If <var>token</var> is null,
     * all callbacks will be removed.
     */
    public final void removeCallbacks(Runnable r, Object token)
    {
        mQueue.removeMessages(this, r, token);
    }

   分析结束,鼓励自己下。。哈哈哈。。。。

前三篇传送门: Looper源码解析: http://www.cnblogs.com/jycboy/p/5787443.html

                    MessageQueue源码解析: http://www.cnblogs.com/jycboy/p/5786682.html

                    Message源码解析:http://www.cnblogs.com/jycboy/p/5786551.html

 

posted @ 2016-08-20 22:56 超超boy 阅读(...) 评论(...) 编辑 收藏