UICollectionView doesn't scroll after setting contentInset - ios

Question

I have a collection view that scrolls horizontally and spans its parent view's full width. My cheap way to achieve paging on it is to set the cell widths to be equal to 1/3 of the collection view width, and to set that same amount of width as left and right content insets.
I disable scrolling in IB and replace with left and right swipe recognizers. My code almost works without setting contentInset, but setting the contentInset seems prevent any scrolling from happening
- (void)viewDidLayoutSubviews {
[super viewDidLayoutSubviews];
CGFloat itemWidth = self.collectionView.bounds.size.width/3.0;
NSInteger count = [self collectionView:self.collectionView numberOfItemsInSection:0];
self.collectionView.contentSize = (CGSize){ .width=itemWidth*count, .height=self.collectionView.bounds.size.height };
// uncomment this line, and the scroll code in the swipes below fails to work
//self.collectionView.contentInset = UIEdgeInsetsMake(0, itemWidth, 0, itemWidth);
self.collectionView.contentOffset = (CGPoint){ .x=self.collectionView.contentSize.width/2.0, .y=0 };
}
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
CGFloat width = self.view.bounds.size.width/3.0;
return (CGSize){ .width=width, .height=collectionView.bounds.size.height };
}
This code handles the swipes...
- (NSIndexPath *)centerIndexPath {
CGRect visibleRect = (CGRect){.origin = self.collectionView.contentOffset, .size = self.collectionView.bounds.size};
CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
return [self.collectionView indexPathForItemAtPoint:visiblePoint];
}
- (void)swipeLeft:(UISwipeGestureRecognizer *)gr {
NSIndexPath *centerIndexPath = [self centerIndexPath];
NSLog(#"at %#", centerIndexPath);
if (centerIndexPath.row < [self collectionView:self.collectionView numberOfItemsInSection:0]-1) {
[self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionLeft animated:YES];
}
}
- (void)swipeRight:(UISwipeGestureRecognizer *)gr {
NSIndexPath *centerIndexPath = [self centerIndexPath];
NSLog(#"at %#", centerIndexPath);
if (centerIndexPath.row > 0) {
[self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionRight animated:YES];
}
}
All of this works, except when I set the contentInsets in the setup above. Then, even though I reach the scrollToItemAtIndexPath: code in the debugger, no scrolling occurs.
It's important to have those insets, because I want user to understand that center item is the selected item.
Can somebody explain why contentInset spoils scrolling and how to fix?

Answer 1

It looks like UICollectionView has its own built-in way to handle insets:
https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/UsingtheFlowLayout/UsingtheFlowLayout.html#//apple_ref/doc/uid/TP40012334-CH3-SW1
Using Section Insets to Tweak the Margins of Your Content
Section insets are a way to adjust the space available for laying out cells. You can use insets to insert space after a section’s header view and before its footer view. You can also use insets to insert space around the sides of the content. Figure 3-5 demonstrates how insets affect some content in a vertically scrolling flow layout.
Figure 3-5 Section insets change the available space for laying out cells