UICollectionView动画可重复使用的单元格？

Question

I've got a UICollectionView that is formatted as a single, horizontally scrollable row across the screen. It represents the next-up entries in a queue, so whenever items are added and deleted, it always happens at Index 0 (the far left of the View) and then the whole thing moves forward or back by one space. It's super-simple in the code, just:

[_collectionviewNextUp insertItemsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:0 inSection:0]]];

and

[_collectionviewNextUp deleteItemsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:0 inSection:0]]];

Everything is working properly, except the animations for insertItemsAtIndexPaths: and deleteItemsAtIndexPaths: , specifically when the CollectionView items extend beyond the edge of the screen. As long as there are only 6 items in the queue (the number that can be seen at once), it animates perfectly; but 7 items or more and the animation gets weird. Delete an item (from the left) and it also animates one being added to the right; add an item (on the left) and it also animates one being deleted from the right. Even if the queue is 50 items long, it always animates item #6 (the last visible one onscreen) disappearing and reappearing. There are no errors or warnings or anything, and I'm left with the correct number of items in the View after the animation is over, it just animates a cell being added/deleted at the edge of the screen.

My theory is that the reusable cells are being animated. Does that sound like it could be a thing? Scrolling left and right works perfectly, but inserting and deleting seems to only animate in pairs, and they always "fly in" and "fly out" in the same direction, as though one is taking the other's place. It looks (to my inexperienced eye) like the cell on the end is being flown over and added to the front, or the one on the front is being moved over to the end.

I've tried to do research on this but all I can find are animation problems that throw errors and exceptions, which mine doesn't do. Has anyone else experienced this? And can anyone offer a fix, wherein the Index 0 item gets animated and the rest just smoothly slide onto and off of the screen?

Answer 1

I assume you're using flow layout. Flow layout is very buggy animating items across the layoutAttributesForElementsInRect: boundary (ie items moving in and out of view). In my experience, you can fix some of these issues by overriding the 'initialLayoutAttributesForAppearingItem' & finalLayoutAttributesForDisappearingItemAtIndexPath: methods and correcting the frames when the super class returns incorrect values. Here is a successful answer to a very similar issue .

I open sourced a simple uniform grid layout VCollectionViewGridLayout that greatly improves animations over what UICollectionViewFlowLayout provides. However, it currently only does vertical scrolling. But if you went in there and transposed all of the horizontal and vertical calculations, I'm sure it would solve your issue.