Use ScaleGestureDetector with GestureDetector?

Question

In my Android app I have an ImageView where I'd like the user to be able to fling it left/right/up/down to change the image (static maps) to the adjacent one. But in addition, I'd like pinch-zoom abilities and a map itself.

I can get either flinging OR pinch-zooming to work, but not together. I'm using GestureDetector (with a SimpleOnGestureListener) for the flinging. And I'm using ScaleGestureDetector (from Making Sense of Multitouch) for the scaling.

The difficulty is to determine which gesture listener to invoke upon a touch action. This is less a coding issue, but logic issue. Upon a single finger touch action, is it a fling or scale? Even when a pinch-zoom is used, the initial MotionEvent is ACTION_DOWN. I've been trying to use the image size (intrinsic or scaled?) as a decision point. But the initial scaling operation (when image size is intrinsic and I want to zoom on it) with ACTION_DOWN seems to escape me.

Has anyone tackled this successfully previously?

Answer 1

You can pass the events on to both gesture detectors.

Check http://developer.android.com/training/gestures/scale.html under "More complex scaling example":

public boolean onTouchEvent(MotionEvent event) {
    boolean retVal = mScaleGestureDetector.onTouchEvent(event);
    retVal = mGestureDetector.onTouchEvent(event) || retVal;
    return retVal || super.onTouchEvent(event);
}

Of course given the bug Ratatat is referencing, super.onTouchEvent will never be called in the above example, which may or may not be fine, depending on your use case.

Answer 2

The idea of Ratatat's answer is OK but we should still pass events to the gestureDetector even if we don't want to scroll, or it will be messed up.

I ended up with something like this:

scaleDetector = new ScaleGestureDetector( ... );

gestureDetector = new GestureDetector(context, new GestureDetector.SimpleOnGestureListener() {
    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
        if (scaleDetector.isInProgress()) {
            // don't allow scrolling while scaling
            return false;
        }

        // handle scrolling

        return true;
    }
}

And then onTouchEvent's implementation should be like in aij's answer:

boolean result = scaleDetector.onTouchEvent(event);
result = gestureDetector.onTouchEvent(event) || result;
return result || super.onTouchEvent(event);

Answer 3

Finally found the answer on a link: http://code.google.com/p/android/issues/detail?id=42591

@Override
public boolean onTouchEvent(MotionEvent event) {
    boolean result = mScaleGestureDetector.onTouchEvent(event);

    // result is always true here, so I need another way to check for a detected scaling gesture
    boolean isScaling = result = mScaleGestureDetector.isInProgress();
    if (!isScaling) {
        // if no scaling is performed check for other gestures (fling, long tab, etc.)
        result = mCommonGestureDetector.onTouchEvent(event);
    }

    // some irrelevant checks...

    return result ? result : super.onTouchEvent(event);
}