Open Radar

 
Summary:
When a UIScrollView's delegate adjusts its deceleration-target contentOffset, the scroll view can get into a strange state where it decelerates most of the way, then slides linearly.

One way this can happen is with very small scroll gestures (velocity < ~0.27).

We have also seen cases of this linear movement behavior with very fast scroll gestures, but the reproduction steps are currently unclear.

Steps to Reproduce:

Add the following code to an empty view controller; try scrolling with a very small velocity.

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(375/2.0-200/2, 180, 200, 300)];
    _scrollView.delegate = self;
    _scrollView.backgroundColor = [UIColor colorWithWhite:0.9 alpha:1];
    _scrollView.alwaysBounceVertical = YES;
    _scrollView.clipsToBounds = NO;
    
    UIView *top = [[UIView alloc] initWithFrame:CGRectMake(5, 5, 190, 250)];
    top.backgroundColor = [UIColor colorWithWhite:0.7 alpha:0.8];
    UILabel *label = [UILabel new];
    label.text = @"Hello";
    [label sizeToFit];
    label.center = [top.layer convertPoint:top.layer.position fromLayer:top.layer.superlayer];
    [top addSubview:label];
    [_scrollView addSubview:top];
    
    UIView *boundary = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 1000)];
    boundary.layer.borderColor = [UIColor blackColor].CGColor;
    boundary.layer.borderWidth = 1;
    [_scrollView addSubview:boundary];
    
    _scrollView.contentSize = boundary.bounds.size;
    
    [self.view addSubview:_scrollView];
    
}


- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                     withVelocity:(CGPoint)velocity
              targetContentOffset:(inout CGPoint *)targetContentOffset
{
    if (scrollView == _scrollView) {
        CGFloat maxY = (_scrollView.contentSize.height - _scrollView.bounds.size.height);
        targetContentOffset->y = MAX(0, MIN(1, round(targetContentOffset->y / maxY))) * maxY;
        NSLog(@"Target %g,  velocity %g", targetContentOffset->y, velocity.y);
    }
}


Expected Results:
Scroll-deceleration should always be exponential decay, never linear.

Actual Results:
Scroll deceleration is linear in certain cases.