onBackPressed to kill a handler within an activity

As you can see, I have this PlayLesson_01 activity which displays images and audio at the same time. This activity has two handlers, one to control images and audio, while the other one will finish the lesson and goes back to previous activity (Lessons Menu).

Goal: Click on hard Back button to go back to previous activity and stop playing images and audio.

Problem: When I click on hard "Back" I'm able to go back to the previous activity, but the audio is still playing.I'm not able to stop or kill the handler that launched within the activity.

Question: How can I kill or stop those handlers completely?

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.media.MediaPlayer;
import android.media.MediaPlayer.OnCompletionListener;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.view.View;
import android.view.ViewGroup;
import android.view.Window;
import android.view.animation.AnimationUtils;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.BaseAdapter;
import android.widget.Gallery;
import android.widget.ImageSwitcher;
import android.widget.ImageView;
import android.widget.RelativeLayout.LayoutParams;
import android.widget.ViewSwitcher.ViewFactory;

public class PlayLesson_01 extends Activity implements OnItemSelectedListener,
    ViewFactory, Runnable {

// Setting up images
// Keeping all Images in array references to our images
public Integer[] mThumbIds = { R.drawable.nouraniyah,
        R.drawable.back_angle, R.drawable.back_fox, R.drawable.back_apple,
        R.drawable.back_twitter, R.drawable.back_thunderbird };
// setting up music
int[] myMusic = { R.raw.button_3, R.raw.button_3, R.raw.button_3,
        R.raw.button_3, R.raw.button_3, R.raw.button_3 };
int mNext;
private ImageSwitcher mSwitcher;
MediaPlayer mp;
Handler mHandlerWholeLesson = new Handler();
Runnable mRunnableWholelesson = new Runnable() {

    @Override
    public void run() {
        // TODO Auto-generated method stub
        startActivity(new Intent("com.example.AmazingGame.LESSONONE"));
    }
};

Handler mHandlerNextfile = new Handler();
Runnable mRunnableNextFile = new Runnable() {

    @Override
    public void run() {
        // TODO Auto-generated method stub
        mp.setOnCompletionListener(mListener);
        mp.start();
    }

};

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.image_switcher); // it could be lesson one
                                                // layout.

    mSwitcher = (ImageSwitcher) findViewById(R.id.imgswitcher);
    mSwitcher.setFactory(this);
    mSwitcher.setInAnimation(AnimationUtils.loadAnimation(this,
            android.R.anim.fade_in));
    mSwitcher.setOutAnimation(AnimationUtils.loadAnimation(this,
            android.R.anim.fade_out));

    Gallery g = (Gallery) findViewById(R.id.gallery);
    g.setAdapter(new ImageAdapter(this));
    g.setOnItemSelectedListener(this);
    // usual onCreate stuff here, then...
    // either here or whenever you want to start the sequence
    mNext = 0;
    startNextFile();

    // Is this good !! Yes it is. count the time for all audio.
    mHandlerWholeLesson.postDelayed(mRunnableWholelesson, 25000);

}

@Override
public void onBackPressed() {
    super.onBackPressed();
    startActivity(new Intent("com.example.AmazingGame.LESSONONE"));
    System.exit(0);
    mHandlerNextfile
            .removeCallbacks(mRunnableNextFile, PlayLesson_01.class);
    mHandlerWholeLesson.removeCallbacks(mRunnableWholelesson,
            PlayLesson_01.class);

    PlayLesson_01.this.finish();
}

@Override
protected void onDestroy() {
    // TODO Auto-generated method stub
    super.onDestroy();
    // mHandlerNextFile.removeCallbacksAndMesssages(PlayLesson_01.class);
}

public void onItemSelected(AdapterView<?> parent, View v,
        final int position, long id) {

}

@Override
public void onNothingSelected(AdapterView<?> arg0) {
    // TODO Auto-generated method stub

}

@Override
public View makeView() {
    ImageView i = new ImageView(this);
    i.setBackgroundColor(0xFF000000);
    i.setScaleType(ImageView.ScaleType.FIT_CENTER);
    i.setLayoutParams(new ImageSwitcher.LayoutParams(
            LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
    return i;
}

public class ImageAdapter extends BaseAdapter {
    public ImageAdapter(Context c) {
        mContext = c;
    }

    public int getCount() {
        return mThumbIds.length;
    }

    public Object getItem(int position) {
        return position;
    }

    public long getItemId(int position) {
        return position;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        ImageView i = new ImageView(mContext);

        i.setImageResource(mThumbIds[position]);
        i.setAdjustViewBounds(true);
        i.setLayoutParams(new Gallery.LayoutParams(
                LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT));
        i.setBackgroundResource(R.drawable.picture_frame);
        return i;
    }

    private Context mContext;

}

OnCompletionListener mListener = new OnCompletionListener() {

    @Override
    public void onCompletion(MediaPlayer mp) {
        mp.release();
        startNextFile();
    }
};

@Override
protected void onPause() {
    // TODO Auto-generated method stub
    super.onPause();
    this.finish();

}

public void startNextFile() {

    if (mNext < myMusic.length) {

        mp = MediaPlayer.create(this, myMusic[mNext]);
        mSwitcher.setImageResource(mThumbIds[mNext++]);
        mHandlerNextfile.postDelayed(mRunnableNextFile, 3000);
    }

}

@Override
public void run() {
    // TODO Auto-generated method stub

}

}

回答1:

Now there are couple of wrong things in your code:

If you want to go back to your previous activity, you do not relaunch it using an startActivity as the default behaviour of android is that it maintains a stack of previous activities. On press of back it will by default go back to the previous activity (unless you have launched the child activity using some flags.)
Why are you using a System.exit(0); on back press? You just have to call the finish() to finish the current activity.
The best way to remove callbacks from a handler is using a null as a parameter. You may try the following code:
```
mHandlerNextfile.removeCallbacksAndMessages(null);
mHandlerWholeLesson.removeCallbacksAndMessages(null);
```
since this would remove all callbacks. Check this link to know more: http://developer.android.com/reference/android/os/Handler.html#removeCallbacksAndMessages(java.lang.Object)

回答2: