UIScrollView Adjust Bounciness - ios

Question

Is it possible to adjust the amount of bounce in a UIScrollView?
For example, if dragging the scroll view 100pt past the end of the content moves the scroll view by 50pt, I'd like to be able to reduce the distance travelled by the same drag to 20pt.
I have experimented with a few things without success.
Adjusting the contentOffset in the scrollViewDidScroll: delegate method does not seem possible because you lose the original scrolling position when the value is set and the content offset approaches zero.
Adding a transform which translates the scroll view by some fraction of the content offset in reverse seems like a good idea. This breaks all the touch tracking and causes a mess of problems. Additionally, in many other situations, moving the scroll view would break the layout of a view.

Answer 1

One thing you can do is to make your own scroll view with UIView + UIPanGestureRecognizer.
(Kind of an over-kill, depending on how bad you want this custom effect.)
Here's a good explanation of Apple's rubber band algorithm.
The core of it is:
b = (1.0 – (1.0 / ((x * c / d) + 1.0))) * d
where:
x = distance from the edge
c = constant value, UIScrollView uses 0.55
d = dimension, either width or height
You can then play with the value of c and x to get the effect you desired.
Some code snippet to get you started:
- (IBAction)handlePan:(UIPanGestureRecognizer *)sender
{
static CGRect beginRect;
CGPoint translation = [sender translationInView:sender.view.superview];
CGPoint velocity = [sender velocityInView:sender.view.superview];
CGRect currentRect = CGRectOffset(beginRect, 0, translation.y);
if (sender.state == UIGestureRecognizerStateBegan) {
beginRect = sender.view.frame;
}
else if (sender.state == UIGestureRecognizerStateChanged) {
if (currentRect.origin.y > self.naturalHidePosition) {
CGFloat distanceFromEdge = currentRect.origin.y - self.naturalHidePosition;
CGFloat height = CGRectGetHeight(sender.view.frame);
CGFloat b = [self rubberBandDistance:distanceFromEdge dimension:height];
currentRect.origin.y = self.naturalHidePosition + b;
}
else if (currentRect.origin.y < self.naturalShowPosition) {
CGFloat distanceFromEdge = self.naturalShowPosition - currentRect.origin.y;
CGFloat height = CGRectGetHeight(sender.view.frame);
CGFloat b = [self rubberBandDistance:distanceFromEdge dimension:height];
currentRect.origin.y = self.naturalShowPosition - b;
}
sender.view.frame = currentRect;
}
else if (sender.state == UIGestureRecognizerStateEnded) {
if (velocity.y < 0) {
currentRect.origin.y = self.naturalShowPosition;
}
else if (velocity.y > 0) {
currentRect.origin.y = self.naturalHidePosition;
}
else if (currentRect.origin.y > (0.5 * (self.naturalShowPosition + self.naturalHidePosition))) {
currentRect.origin.y = self.naturalHidePosition;
}
else {
currentRect.origin.y = self.naturalShowPosition;
}
[UIView animateWithDuration:0.25 delay:0 options:UIViewAnimationOptionCurveEaseOut animations:^{
sender.view.frame = currentRect;
} completion:nil];
}
}
- (CGFloat)rubberBandDistance:(CGFloat)distanceFromEdge dimension:(CGFloat)d
{
CGFloat c = 0.55;
CGFloat b = (1.0 - (1.0 / ((distanceFromEdge * c / d) + 1.0))) * d;
return b;
}