Make UIPanGestureRecognizer stretch when pulling past certain threshold - iphone

Question

I'm trying to implement something similar to how Mac/iOS web pages are implemented, where if you pull past a certain threshold, the main view becomes "stretchy" and moves based on the velocity of the pull.
The difference, though, is I'm trying to do this horizontally for UITableViewCells with a UIPanGestureRecognizer.
I've got a handlePan method:
- (void)handlePan:(UIPanGestureRecognizer *)recognizer
{
[recognizer setMaximumNumberOfTouches:1];
CGPoint velocity = [recognizer velocityInView:self];
int panDelta = ([recognizer locationInView:self].x - [recognizer locationInView:self.superview].x);
if (recognizer.state == UIGestureRecognizerStateBegan) {
_originalCenter = self.center;
}
if (recognizer.state == UIGestureRecognizerStateChanged) {
CGPoint translation = [recognizer translationInView:self];
float xVal = 0;
if (panDelta <= 38) {
xVal = _originalCenter.x + translation.x;
} /*else {
// this is where I struggle.
xVal = _originalCenter.x;
}*/
self.center = CGPointMake(xVal, _originalCenter.y);
}
if (recognizer.state == UIGestureRecognizerStateEnded) {
CGRect newFrame;
int xOffset = 0;
newFrame = CGRectMake(xOffset, self.frame.origin.y,
self.bounds.size.width, self.bounds.size.height);
[UIView animateWithDuration:0.2 animations:^{
self.frame = newFrame;
}];
}
}
I don't have any specific algorithm to create the "stretchiness"; all I'm trying to do is make it so the sliding doesn't go as wide as the user's interaction is, and is fluid.
Ideas?

Answer 1

I think you are looking for a simple function with a horizontal asymptote indicating a maximum offset:
// this is where I struggle.
CGFloat maxOffset = 50.0; // change as you like
CGFloat elementWidth = ?; // fill in the width of your view animated, eg: self.frame.size.width
CGFloat absPannedOffset = fabsf(translation.x);
CGFloat rico = powf(elementWidth, 2) / maxOffset;
CGFloat absOffset = (((- 1 / rico) * powf(absPannedOffset - elementWidth, 2)) + maxOffset);
if (pannedOffset < 0) {
xVal = -absOffset;
} else {
xVal = absOffset;
}