UICollectionView automatically scroll to bottom when screen loads - ios

Question

I'm trying to figure out how to scroll all the way to the bottom of a UICollectionView when the screen first loads. I'm able to scroll to the bottom when the status bar is touched, but I'd like to be able to do that automatically when the view loads as well. The below works fine if I want to scroll to the bottom when the status bar is touched.
- (BOOL)scrollViewShouldScrollToTop:(UITableView *)tableView
{
NSLog(#"Detect status bar is touched.");
[self scrollToBottom];
return NO;
}
-(void)scrollToBottom
{//Scrolls to bottom of scroller
CGPoint bottomOffset = CGPointMake(0, collectionViewReload.contentSize.height - collectionViewReload.bounds.size.height);
[collectionViewReload setContentOffset:bottomOffset animated:NO];
}
I've tried calling [self scrollToBottom] in the viewDidLoad. This isn't working. Any ideas on how I can scroll to the bottom when the view loads?

Answer 1

I found that nothing would work in viewWillAppear. I can only get it to work in viewDidLayoutSubviews:
- (void)viewDidLayoutSubviews
{
[super viewDidLayoutSubviews];
[self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:endOfModel inSection:0] atScrollPosition:UICollectionViewScrollPositionNone animated:NO];
}

Answer 2

Just to elaborate on my comment.
viewDidLoad is called before elements are visual so certain UI elements cannot be manipulated very well. Things like moving buttons around work but dealing with subviews often does not (like scrolling a CollectionView).
Most of these actions will work best when called in viewWillAppear or viewDidAppear. Here is an except from the Apple docs that points out an important thing to do when overriding either of these methods:
You can override this method to perform additional tasks associated
with presenting the view. If you override this method, you must call
super at some point in your implementation.
The super call is generally called before custom implementations. (so the first line of code inside of the overridden methods).

Answer 3

So had a similar issue and here is another way to come at it without using scrollToItemAtIndexPath
This will scroll to the bottom only if the content is larger than the view frame.
It's probably better to use scrollToItemAtIndexPath but this is just another way to do it.
CGFloat collectionViewContentHeight = myCollectionView.contentSize.height;
CGFloat collectionViewFrameHeightAfterInserts = myCollectionView.frame.size.height - (myCollectionView.contentInset.top + myCollectionView.contentInset.bottom);
if(collectionViewContentHeight > collectionViewFrameHeightAfterInserts) {
[myCollectionView setContentOffset:CGPointMake(0, myCollectionView.contentSize.height - myCollectionView.frame.size.height) animated:NO];
}

Answer 4

Swift 3 example
let sectionNumber = 0
self.collectionView?.scrollToItem(at: //scroll collection view to indexpath
NSIndexPath.init(row:(self.collectionView?.numberOfItems(inSection: sectionNumber))!-1, //get last item of self collectionview (number of items -1)
section: sectionNumber) as IndexPath //scroll to bottom of current section
, at: UICollectionViewScrollPosition.bottom, //right, left, top, bottom, centeredHorizontally, centeredVertically
animated: true)

Answer 5

Get indexpath for last item. Then...
- (void)scrollToItemAtIndexPath:(NSIndexPath *)indexPath atScrollPosition:(UICollectionViewScrollPosition)scrollPosition animated:(BOOL)animated

Answer 6

For me, i found next solution:
call reloadData in CollectionView, and make dcg on main to scroll.
__weak typeof(self) wSelf = self;
[wSelf.cv reloadData];
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"HeightGCD:%#", #(wSelf.cv.contentSize.height));
[wSelf.cv scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:50 inSection:0] atScrollPosition:UICollectionViewScrollPositionBottom animated:YES];
});

Answer 7

none of these were working so well for me, I ended up with this which will work on any scroll view
extension UIScrollView {
func scrollToBottom(animated: Bool) {
let y = contentSize.height - 1
let rect = CGRect(x: 0, y: y + safeAreaInsets.bottom, width: 1, height: 1)
scrollRectToVisible(rect, animated: animated)
}
}

Answer 8

The issue is likely that even if your collection view is on screen, it might not have the actual contentSize.
If you scroll in viewDidAppear, you will have a contentSize, but your scollectionview will briefly show content before scrolling.
And the problem with viewDidLayoutSubviews is that it is called multiple times, so you then need to add an ugly boolean to limit scrolling.
The best solution i've found is to force layout in view will appear.
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
// force layout before scrolling to most recent
collectionView.layoutIfNeeded()
// now you can scroll however you want
// e.g. scroll to the right
let offset = collectionView.contentSize.width - collectionView.bounds.size.width
collectionView.setContentOffSet(CGPoint(x: offset, y: 0), animated: animated)
}

Answer 9

Consider if you can use performBatchUpdates like this:
private func reloadAndScrollToItem(at index: Int, animated: Bool) {
collectionView.reloadData()
collectionView.performBatchUpdates({
self.collectionView.scrollToItem(at: IndexPath(item: index, section: 0),
at: .bottom,
animated: animated)
}, completion: nil)
}
If index is the index of the last item in the collection's view data source it'll scroll all the way to the bottom.