Question

Background:

I have a UICollectionViewController that shows items in one of two modes, which the user can toggle between. Each mode uses a different class of UICollectionViewCell. Let’s call these modes “list view” and “grid view”.

When I switch modes, I call .reloadData on the UICollectionView, which redraws the collection view using the correct cell classes. Everything works fine here.

Now: Inside the UICollectionViewCell subclass for one type of cells, I want to be notified when the collection view that contains it switches modes. Visually, a cell which was on-screen vanishes; the collection view is drawn fully with the other type of cell. When switching back, the cell is re-displayed.

Question:

How can I be notified when a UICollectionViewCell is “removed” (i.e., no longer shown; I’m not sure what’s happening under the hood yet) from its parent collection view?

Notes:

• prepareForReuse is not called on the cell when the collection view’s updateData causes the cell to no longer be included.
• willTransitionFromLayout:toLayout: (an empty layout?) is not called.
• Overriding didMoveToSuperview is of no help; it is not called.
• Observing .hidden or .alpha on the cell does not work.
• The cell’s dealloc is not called; it sticks around in the reuse queue.

Something in the cell must be changing that I can observe or hook into, what is it?

Answer 1

Update: UICollectionViewDelegate has this method, which from the documentation seems like it does what I am asking:

• collectionView:didEndDisplayingCell:forItemAtIndexPath:

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Original answer:

I got this working as desired by having the UICollectionViewController manually notify visible cells of impending doom with this method when I’m about to toggle and call reloadData:

- (void)notifyCellsWillBeHidden {
    for (UICollectionViewCell *cell in self.collectionView.visibleCells) {
        if ([cell respondsToSelector:@selector(willBeRemovedFromCollectionView)]) {
            [cell performSelector:@selector(willBeRemovedFromCollectionView)];
        }        
    }
}

These cells can then do what they need to do if they implement the above method.

Calling prepareForReuse may not a good idea because it will be called again before the cells are re-displayed, if this is a problem.

I’m still curious whether there is a way for a cell to receive notification that it is going to be made non-visible without an explicit call.