如何使UICollectionView在将项目添加到顶部时不滚动

Question

I finally got my collection view to not scroll when updated. I think it was when I added this:

self.collectionView.performBatchAnimated(false, updates: {() -> Void in
          self.collectionView.reloadData()
          }, completion: { _ in 
    })

Then I realized the newest posts were going to the bottom, and I wanted them at the top. To do this I just used added them to the array in the opposite order. Now the only problem is that when a post is added and goes to the top, the collectionView moves down by one post, since a post is being added at the top. This is the entire thing:

DataService.ds.REF_POSTS.child("\(self.loggedInUser!.uid)").observeEventType(.ChildAdded, withBlock: { postDictionary in
            if postDictionary.exists() {
                self.posts.insert(postDictionary, atIndex: 0)
            }
            self.collectionView.performBatchAnimated(false, updates: {() -> Void in
                self.collectionView.reloadData()
                }, completion: { _ in
            })
})

Since this is Firebase .ChildAdded, it runs several times on load to fill the collectionView with all the posts, then runs again every time a post is added. How can I stop that autoscroll (by 1 screenheight)? I tried using setContentOffset to counterscroll, but the problem is that any code I add will be performed on load for every single post. So it crashed the app due to using too much memory, from scrolling up and down repeatedly like 30+ times on load. I just need the user to keep their scroll position when a post is added to the top of the collection view.

Answer 1

Ok so this is the best I could do. It still does a little quick screen flash, similar to that of taking a screenshot, when reloading the table.

        self.posts.insert(postDictionary, atIndex: 0)

        self.collectionView.performBatchAnimated(false, updates: {() -> Void in
            let currentOffset = self.collectionView.contentOffset.y
            let newOffset = currentOffset + self.screenHeight
            self.collectionView.setContentOffset(CGPointMake(0, newOffset), animated: false)
            self.collectionView.reloadDataImmediately()
            }, completion: { _ in
        })

This inserts an item at the top, then counter scrolls so the user keeps the position. I tried probably 30+ other things to get rid of the flash, and the only way I could do it is if I somehow reload all items at different times, to kind of dodge around the user's current scroll positions. If I do this, they will end up seeing some odd behavior like repeating posts and stuff, but if they go to double check it will have refreshed...so it wouldn't really be all that efficient. If anyone knows how to get rid of the quick "flash" on reload please let me know!

Some things I tried: disabling animation, running different things on different threads, reloading the rows one by one, reloading current row one way and the rest a different way, bunch of other stuff I don't remember lol. If anyone can get rid of that flash that is caused with my current code then I will select their answer as correct!