private float calcScale(PointF currentViewPoint) {
    float dy = (currentViewPoint.y - mDoubleTapViewPoint.y);
    float t = 1 + Math.abs(dy) * 0.001f;
    return (dy < 0) ? mDoubleTapScale / t : mDoubleTapScale * t;
  }
Tap gesture listener for double tap to zoom / unzoom and double-tap-and-drag to zoom.

@see ZoomableDraweeView#setTapListener(GestureDetector.SimpleOnGestureListener)
samples/zoomable/src/main/java/com/facebook/samples/zoomable/DoubleTapGestureListener.java