Android: several gesture detectors on the same view

Question

I have a TextView wrapped with ScrollView. The thing I need to do is to detect scale and common gestures (like tap, double tab and others) on my TextView. Also, I want some gestures to handle with standard handlers (scroll, for example). I just can't get the basics, I don't know what do I code for this. I made two detectors (ScaleGestureDetector and GestureDetector) and listeners for them. Also, I set onTouchListener for my TextView like this:

tw.setOnTouchListener(new OnTouchListener() {
    public boolean onTouch(View view, MotionEvent motionEvent) {
        if (!gestureDetector.onTouchEvent(motionEvent)) {
            scaleGestureDetector.onTouchEvent(motionEvent);
        };
        return true;
    }
});

No gestures are being detected. Every example which I googled operates with one detector. Could somebody help me out with the concept, how do I detect scale (or pinch zoom), double tap, single tap etc. on the same view?

Answer 1

One solution is to call onTouchEvent for both of your Detectors:

tw.setOnTouchListener(new OnTouchListener() {
    public boolean onTouch(View view, MotionEvent motionEvent) {
        gestureDetector.onTouchEvent(motionEvent);
        scaleGestureDetector.onTouchEvent(motionEvent);
        return true;
    }
});

This may, however, lead to undesirable behavior as both Detectors may simultaneously trigger; for example, one gesture may be identified as both a scale gesture and a scroll gesture.

To solve this, you can introduce a new variable scaleOngoing to keep track of whether or not a scale event is ongoing:

scaleGestureDetector = new ScaleGestureDetector(context, new ScaleGestureDetector.OnScaleGestureListener() {
    public void onScaleEnd(ScaleGestureDetector detector) {
        scaleOngoing = false;
    }
    public boolean onScaleBegin(ScaleGestureDetector detector) {
        scaleOngoing = true;
        return true;
    }
    public boolean onScale(ScaleGestureDetector detector) {
        // do scaling here
        return false;
    }
});

Then have your GestureDetector's listener check whether a scale gesture is occurring:

mGestureDetector = new GestureDetector(context, new GestureDetector.SimpleOnGestureListener() {
    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
        if(!scaleOngoing) {
            // do scrolling here
         }
         return true;
    }
});