How can I test if the scroll view is bouncing?

Question

How can I test if the scroll view is bouncing? Is there a notification or something when the bounce ends?

Answer 1

Here is how I detected if the scroll view is bouncing horizontally:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {

  if (scrollView.contentOffset.x < 0) {
    NSLog(@"bouncing left");
  }

  if (scrollView.contentOffset.x > (scrollView.contentSize.width - scrollView.frame.size.width)) {
    NSLog(@"bouncing right");
  }
}

Answer 2

I've implemented extension for UIScrollView to handle this for vertical and horizontal scrolling. This will work even with non-zero contentInsets and in case when content is not big enough for covering scrollView insets:

Objective-C

@interface UIScrollView (Bouncing)

@property (nonatomic, readonly) BOOL isBouncing;
@property (nonatomic, readonly) BOOL isBouncingTop;
@property (nonatomic, readonly) BOOL isBouncingLeft;
@property (nonatomic, readonly) BOOL isBouncingBottom;
@property (nonatomic, readonly) BOOL isBouncingRight;

@end

@implementation UIScrollView (Bouncing)

- (BOOL)isBouncing
{
    return self.isBouncingTop || self.isBouncingLeft || self.isBouncingBottom || self.isBouncingRight;
}

- (BOOL)isBouncingTop
{
    return self.contentOffset.y < - self.contentInset.top;
}

- (BOOL)isBouncingLeft
{
    return self.contentOffset.x < - self.contentInset.left;
}

- (BOOL)isBouncingBottom
{
    BOOL contentFillsScrollEdges = self.contentSize.height + self.contentInset.top + self.contentInset.bottom >= CGRectGetHeight(self.bounds);
    return contentFillsScrollEdges && self.contentOffset.y > self.contentSize.height - CGRectGetHeight(self.bounds) + self.contentInset.bottom;
}

- (BOOL)isBouncingRight
{
    BOOL contentFillsScrollEdges = self.contentSize.width + self.contentInset.left + self.contentInset.right >= CGRectGetWidth(self.bounds);
    return contentFillsScrollEdges && self.contentOffset.x > self.contentSize.width - CGRectGetWidth(self.bounds) + self.contentInset.right;
}

@end

Swift 3.0+

extension UIScrollView {
  var isBouncing: Bool {
    return isBouncingTop || isBouncingLeft || isBouncingBottom || isBouncingRight
  }
  var isBouncingTop: Bool {
    return contentOffset.y < -contentInset.top
  }
  var isBouncingLeft: Bool {
    return contentOffset.x < -contentInset.left
  }
  var isBouncingBottom: Bool {
    let contentFillsScrollEdges = contentSize.height + contentInset.top + contentInset.bottom >= bounds.height
    return contentFillsScrollEdges && contentOffset.y > contentSize.height - bounds.height + contentInset.bottom
  }
  var isBouncingRight: Bool {
    let contentFillsScrollEdges = contentSize.width + contentInset.left + contentInset.right >= bounds.width
    return contentFillsScrollEdges && contentOffset.x > contentSize.width - bounds.width + contentInset.right
  }
}

For RxSwift you can just map scrollViewDidScroll with isBouncing and filter with distinctUntilChanged.

Answer 3

A minor amendment to Justin's method, allowing for contentInset:

if( scrollView.contentOffset.x < -scrollView.contentInset.left )
{
    NSLog( @"bounce left" );
}
if( scrollView.contentOffset.x > scrollView.contentSize.width - scrollView.frame.size.width + scrollView.contentInset.right )
{
    NSLog( @"bounce right" );
}