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[操作系统]Handler机制(四)


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。。

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