How to erase image with finger in Android

Question

I'm developing an app that need the feature in object.

I have an Image A that cover an Image B. With finger I need to erase the Image A to show the Image B. Erase must follow your finger flowing image A

I'm trying some code but still I couldn't erase the image A. this is the code that i'm using to draw a line on image (_imageToErase is Image A):

Canvas canvas;
Paint paint;
float downx = 0, downy = 0, upx = 0, upy = 0;
ImageView _imageToErase;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.g_layout);

    _imageToErase = (ImageView) findViewById(R.id.image_to_erase);

    _imageToErase.setOnTouchListener(this);
}

@Override
public void onWindowFocusChanged(boolean hasFocus){

    int width = _imageToErase.getWidth();
    int height = _imageToErase.getHeight();

    Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    canvas = new Canvas(bitmap);
    paint = new Paint();
    paint.setColor(Color.WHITE);
    paint.setStrokeWidth(25);
    paint.setAntiAlias(true);
    _imageToErase.setImageBitmap(bitmap);
}

public boolean onTouch(View v, MotionEvent event) {
    int action = event.getAction();

    switch (action){
        case MotionEvent.ACTION_DOWN:
            downx = event.getX();
            downy = event.getY();
            break;
        case MotionEvent.ACTION_MOVE:
            upx = event.getX();
            upy = event.getY();
            canvas.drawLine(downx, downy, upx, upy, paint);
            _imageToErase.invalidate();
            downx = upx;
            downy = upy;
            break;
        case MotionEvent.ACTION_UP:
            break;
        case MotionEvent.ACTION_CANCEL:
            break;
        default:
            break;
    }

    return true;
}

This code produce only a line that follow the finger but not erase the image.

How to modify this code to erase the image? Thanks

EDIT

The link suggested in comments not solved my problem. Simply add this line:

 paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));

not work for me.