UICollectionView contentOffset changes with custom

问题:

I have a UICollectionView with a custom UICollectionViewLayout (actually, I'm using this nice layout).

I set contentOffset = CGPointZero in viewDidLoad. After viewDidLoad, however, the offset is -20, and the content gets pushed down like so:

---

(It should be flush with the line). I'm loading the collection view layout in interface builder. It seems that my problem is very similar to this one, however the solutions there don't work for me.

I tried modifying collectionViewContentSize in my layout implementation to ensure it was always greater than the size of the collectionView. Although this means I can scroll my content down (it's shorter than the height of the collectionView) and hide the extra space, I can also scroll back up to see it.

Nothing seems to work!

回答1:

I found the reason why this happens. Check the accepted answer from this question: Status bar and navigation bar appear over my view's bounds in iOS 7

Indeed, we could just set edgesForExtendedLayout or automaticallyAdjustsScrollViewInsets properties of UIViewController in IB (if you use Storyboard) of in viewDidLoad and it will fix our issue.

Just don't forget check for this property is available, because in iOS6 or prior it will cause crash:

if ([self respondsToSelector:@selector(setAutomaticallyAdjustsScrollViewInsets:)]) {
    self.automaticallyAdjustsScrollViewInsets = NO;
}

Regards!

回答2:

The only solution I could come up with that had barely-acceptable behavior:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    if (self.collectionView.contentOffset.y < 0) {
        self.collectionView.contentOffset = CGPointMake(self.collectionView.contentOffset.x, 0.0);
    }
}

As well as setting the height of the content to fmax(self.collectionView.frame.size.height + 20, [self stackedSectionHeight]) in collectionViewContentSize

This removes the space above the section header, but it removes the "bounce" from the top. A pretty sub-optimal solution, but fairly acceptable.

I'll accept a better answer if anyone has one, or if I find one I'll update this answer.

回答3:

I started to find this issue appearing on iOS 11 where I had a scrollview within a scroll view. Took me almost a day to figure it out.

Try setting the content inset adjustment behaviour:

if #available(iOS 11.0, *) {
    scrollView.contentInsetAdjustmentBehavior = .never
}

This fixed my collectionview adjusting it's content inset down a small amount as the collectionview frame would move past the safe area of the enclosing scrollview.

回答4:

let oldContentSizeHeight: CGFloat = self.colName.contentSize.height
        self.colName.reloadData()
        self.colName.layoutIfNeeded()
        self.view.layoutIfNeeded()
        self.colName.contentOffset.y = self.colName.contentOffset.y + (self.colName.contentSize.height - oldContentSizeHeight)

回答5:

This is the swift 4.2 version of the correct answer but to make this workable you have to add the protocol UIScrollViewDelegate:

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    if self.collectionView.contentOffset.y < 0 {
        self.collectionView.contentOffset = CGPoint(x: self.collectionView.contentOffset.x, y: 0.0)
    }
}