ScrollView willScrollToEnd simulation #ios #scrolling #physics

scrollview_decelerate_end.m

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView
{
    const CGFloat DECEL = 500;     // pixels/second decrease in velocity per second.  need to guess this
    CGPoint vel = [self.panGestureRecognizer velocityInView:self.superview];
    
    if (vel.y > 0) return;
    
    CGFloat bottomEdge = scrollView.contentOffset.y + scrollView.frame.size.height;
    CGFloat distanceToBottom = scrollView.contentSize.height -bottomEdge;

    // v1 = v + at
    // 0 = v + at
    // t = -v / a
    // d = d0 + vt + 0.5 * at^2
    //   = -v^2 / a + 0.5 * v^2 / a
    CGFloat a = -DECEL;
    CGFloat v = -vel.y;
    CGFloat decelDist = -v*v/a + 0.5*v*v/a;
    if (distanceToBottom < decelDist) {
        DLOG(@"WILL scroll to end!")
    } else {
        DLOG(@"WONT scroll to end!");
    }
}