[java] How to remove all callbacks from a Handler?

I have a Handler from my sub-Activity that was called by the main Activity. This Handler is used by sub-classes to postDelay some Runnables, and I can't manage them. Now, in the onStop event, I need to remove them before finishing the Activity (somehow I called finish(), but it still call again and again). Is there anyway to remove all callbacks from a Handler?

For any specific Runnable instance, call Handler.removeCallbacks(). Note that it uses the Runnable instance itself to determine which callbacks to unregister, so if you are creating a new instance each time a post is made, you need to make sure you have references to the exact Runnable to cancel. Example:

Handler myHandler = new Handler();
Runnable myRunnable = new Runnable() {
    public void run() {
        //Some interesting task
    }
};

You can call myHandler.postDelayed(myRunnable, x) to post another callback to the message queue at other places in your code, and remove all pending callbacks with myHandler.removeCallbacks(myRunnable)

Unfortunately, you cannot simply "clear" the entire MessageQueue for a Handler, even if you make a request for the MessageQueue object associated with it because the methods for adding and removing items are package protected (only classes within the android.os package can call them). You may have to create a thin Handler subclass to manage a list of Runnables as they are posted/executed...or look at another paradigm for passing your messages between each Activity

Hope that Helps!

As josh527 said, handler.removeCallbacksAndMessages(null); can work.
But why?
If you have a look at the source code, you can understand it more clearly. There are 3 type of method to remove callbacks/messages from handler(the MessageQueue):

1. remove by callback (and token)
2. remove by message.what (and token)
3. remove by token

Handler.java (leave some overload method)

/**
 * 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);
}

/**
 * Remove any pending posts of messages with code 'what' and whose obj is
 * 'object' that are in the message queue.  If <var>object</var> is null,
 * all messages will be removed.
 */
public final void removeMessages(int what, Object object) {
    mQueue.removeMessages(this, what, object);
}

/**
 * Remove any pending posts of callbacks and sent messages whose
 * <var>obj</var> is <var>token</var>.  If <var>token</var> is null,
 * all callbacks and messages will be removed.
 */
public final void removeCallbacksAndMessages(Object token) {
    mQueue.removeCallbacksAndMessages(this, token);
}

MessageQueue.java do the real work:

void removeMessages(Handler h, int what, Object object) {
    if (h == null) {
        return;
    }

    synchronized (this) {
        Message p = mMessages;

        // Remove all messages at front.
        while (p != null && p.target == h && p.what == what
               && (object == null || p.obj == object)) {
            Message n = p.next;
            mMessages = n;
            p.recycleUnchecked();
            p = n;
        }

        // Remove all messages after front.
        while (p != null) {
            Message n = p.next;
            if (n != null) {
                if (n.target == h && n.what == what
                    && (object == null || n.obj == object)) {
                    Message nn = n.next;
                    n.recycleUnchecked();
                    p.next = nn;
                    continue;
                }
            }
            p = n;
        }
    }
}

void removeMessages(Handler h, Runnable r, Object object) {
    if (h == null || r == null) {
        return;
    }

    synchronized (this) {
        Message p = mMessages;

        // Remove all messages at front.
        while (p != null && p.target == h && p.callback == r
               && (object == null || p.obj == object)) {
            Message n = p.next;
            mMessages = n;
            p.recycleUnchecked();
            p = n;
        }

        // Remove all messages after front.
        while (p != null) {
            Message n = p.next;
            if (n != null) {
                if (n.target == h && n.callback == r
                    && (object == null || n.obj == object)) {
                    Message nn = n.next;
                    n.recycleUnchecked();
                    p.next = nn;
                    continue;
                }
            }
            p = n;
        }
    }
}

void removeCallbacksAndMessages(Handler h, Object object) {
    if (h == null) {
        return;
    }

    synchronized (this) {
        Message p = mMessages;

        // Remove all messages at front.
        while (p != null && p.target == h
                && (object == null || p.obj == object)) {
            Message n = p.next;
            mMessages = n;
            p.recycleUnchecked();
            p = n;
        }

        // Remove all messages after front.
        while (p != null) {
            Message n = p.next;
            if (n != null) {
                if (n.target == h && (object == null || n.obj == object)) {
                    Message nn = n.next;
                    n.recycleUnchecked();
                    p.next = nn;
                    continue;
                }
            }
            p = n;
        }
    }
}

If you don't have the Runnable references, on the first callback, get the obj of the message, and use removeCallbacksAndMessages() to remove all related callbacks.

Define a new handler and runnable:

private Handler handler = new Handler(Looper.getMainLooper());
private Runnable runnable = new Runnable() {
        @Override
        public void run() {
            // Do what ever you want
        }
    };

Call post delayed:

handler.postDelayed(runnable, sleep_time);

Remove your callback from your handler:

handler.removeCallbacks(runnable);

Please note that one should define a Handler and a Runnable in class scope, so that it is created once.removeCallbacks(Runnable) works correctly unless one defines them multiple times. Please look at following examples for better understanding:

Incorrect way :

    public class FooActivity extends Activity {
           private void handleSomething(){
                Handler handler = new Handler();
                Runnable runnable = new Runnable() {
                   @Override
                   public void run() {
                      doIt();
                  }
               };
              if(shouldIDoIt){
                  //doIt() works after 3 seconds.
                  handler.postDelayed(runnable, 3000);
              } else {
                  handler.removeCallbacks(runnable);
              }
           }

          public void onClick(View v){
              handleSomething();
          }
    } 

If you call onClick(..) method, you never stop doIt() method calling before it call. Because each time creates new Handler and new Runnable instances. In this way, you lost necessary references which belong to handler and runnable instances.

Correct way :

 public class FooActivity extends Activity {
        Handler handler = new Handler();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                doIt();
            }
        };
        private void handleSomething(){
            if(shouldIDoIt){
                //doIt() works after 3 seconds.
                handler.postDelayed(runnable, 3000);
            } else {
                handler.removeCallbacks(runnable);
            }
       }

       public void onClick(View v){
           handleSomething();
       }
 } 

In this way, you don't lost actual references and removeCallbacks(runnable) works successfully.

Key sentence is that 'define them as global in your Activity or Fragment what you use'.