private boolean shouldStartDoubleTapScroll(PointF viewPoint) {
    double dist =
        Math.hypot(viewPoint.x - mDoubleTapViewPoint.x, viewPoint.y - mDoubleTapViewPoint.y);
    return dist > DOUBLE_TAP_SCROLL_THRESHOLD;
  }
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