Creating slow scrolling to indexPath in UICollectionView

Question

I'm working on a project where I'm using a UICollectionView to create an 'image ticker' where I'm advertising a series of logos. The collectionView is one item high and twelve items long, and shows two to three items at a time (depending on size of the logos visible).

I would like to make a slow automatic scrolling animation from the first item to the last, and then repeat.

Has anyone been able to make this work? I can get the scrolling working using

[myCollection scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:(myImages.count -1) inSection:0] atScrollPosition:UICollectionViewScrollPositionRight animated:YES];

But this is way too fast!

[UIView animateWithDuration:10 delay:2 options:(UIViewAnimationOptionAutoreverse + UIViewAnimationOptionRepeat) animations:^{
    [myCollection scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:(myImages.count -1) inSection:0] atScrollPosition:UICollectionViewScrollPositionRight animated:NO];
} completion:nil];

This yields the desired scrolling speed, but only the last few cells are visible in the series. I suspect they (and even the starting visible cells) are being dequeued immediately.

Any thoughts?

Answer 1

You can try this approach:

@property (nonatomic, assign) CGPoint scrollingPoint, endPoint;
@property (nonatomic, strong) NSTimer *scrollingTimer;
@synthesize scrollingPoint, endPoint;
@synthesize scrollingTimer;

- (void)scrollSlowly {
    // Set the point where the scrolling stops.
    self.endPoint = CGPointMake(0, 300);
    // Assuming that you are starting at {0, 0} and scrolling along the x-axis.
    self.scrollingPoint = CGPointMake(0, 0);
    // Change the timer interval for speed regulation. 
    self.scrollingTimer = [NSTimer scheduledTimerWithTimeInterval:0.015 target:self selector:@selector(scrollSlowlyToPoint) userInfo:nil repeats:YES];
}

- (void)scrollSlowlyToPoint {
    self.collectionView.contentOffset = self.scrollingPoint;
    // Here you have to respond to user interactions or else the scrolling will not stop until it reaches the endPoint.
    if (CGPointEqualToPoint(self.scrollingPoint, self.endPoint)) {
        [self.scrollingTimer invalidate];
    }
    // Going one pixel to the right.
    self.scrollingPoint = CGPointMake(self.scrollingPoint.x, self.scrollingPoint.y+1);
}

Answer 2

I use a "1 pixel offset" trick. When scrolling programmatically, just set the end contentOffset.x to 1 pixel more/less than it should. This should be unnoticeable. And in completion block set it to actual value. That way you can avoid the premature cell dequeuing problem and get smooth scrolling animation ;)

Here is an example of scrolling to the right page (e.g. from page 1 to 2). Notice that in the animations: block I actually scrolls one pixel less (pageWidth * nextPage - 1). I then restore the correct value in the completion: block.

CGFloat pageWidth = self.collectionView.frame.size.width;
int currentPage = self.collectionView.contentOffset.x / pageWidth;
int nextPage = currentPage + 1;

[UIView animateWithDuration:1
                      delay:0
                    options:UIViewAnimationOptionCurveEaseOut
                 animations:^{
                     [self.collectionView setContentOffset:CGPointMake(pageWidth * nextPage - 1, 0)];
                 } completion:^(BOOL finished) {
                     [self.collectionView setContentOffset:CGPointMake(pageWidth * nextPage, 0)];
                 }];