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Android: detecting fling gestures

Posted by: admin May 14, 2020 Leave a comment

Questions:

In my app I try to capture a fling event on a view (a ScrollView to be exact, but tried with LinearLayout as well).

By setting breakpoints I can see the that the MotionEvents happen correctly up to the point where the onFling() should fire.

The event flow is as follows:

  1. The view’s onTouchEvent
  2. The OnGestureListener's onDown
  3. The OnGestureListener's onShowPress
  4. The OnGestureListener's onLongPress

I’m at a loss on how to go on trying to debug something like that and Google search does not turn up much. Any pointers ?

I also attach the relevant view that should handle the gestures:

import android.content.Context;
import android.content.Intent;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.widget.ScrollView;

public class GestureScrollView extends ScrollView {
    private static int MAJOR_MOVE = 60;
    private GestureDetector gd;

    public GestureScrollView(final Context context) {
        super(context);
        gd = new GestureDetector(context,
                new GestureDetector.OnGestureListener() {

                    @Override
                    public boolean onSingleTapUp(MotionEvent e) {
                        // TODO Auto-generated method stub
                        return false;
                    }

                    @Override
                    public void onShowPress(MotionEvent e) {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX,
                            float distanceY) {
                        // TODO Auto-generated method stub
                        return false;
                    }

                    @Override
                    public void onLongPress(MotionEvent e) {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
                            float velocityY) {
                        int dx = (int) (e2.getX() - e1.getX());
                        if (Math.abs(dx) > MAJOR_MOVE
                                && Math.abs(velocityX) > Math.abs(velocityY)) {
                            if (velocityX < 0) {
                                Intent intent = new Intent(context, Main.class);
                                context.startActivity(intent);
                            }
                        }
                        return false;
                    }

                    @Override
                    public boolean onDown(MotionEvent e) {
                        // TODO Auto-generated method stub
                        return false;
                    }
                });
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        return gd.onTouchEvent(event);
    }
}
How to&Answers:

Take the MotionEvent from Activity not from your View. When your Gesture detector is based on MotionEvent that comes from Activity you will get the onFling event. So you have to move onTouchEvent() from your GestureScrollView to your Activity class.
Regards!