UICollectionView: Animate cell size change on selection

Animating the size of a cell works the same way for collection views as it does for table views. If you want to animate the size of a cell, have a data model that reflects the state of the cell (in my case, I just use a flag (CollectionViewCellExpanded, CollectionViewCellCollapsed) and return a CGSize accordingly.

When you call and empty performBathUpdates call, the view animates automatically.

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    [collectionView performBatchUpdates:^{
        // append your data model to return a larger size for
        // cell at this index path
    } completion:^(BOOL finished) {

    }];
}

With the help of Chase Roberts, I now found the solution to my problem. My code basically looks like this:

// Prepare for animation

[self.collectionView.collectionViewLayout invalidateLayout];
UICollectionViewCell *__weak cell = [self.collectionView cellForItemAtIndexPath:indexPath]; // Avoid retain cycles
void (^animateChangeWidth)() = ^()
{
    CGRect frame = cell.frame;
    frame.size = cell.intrinsicContentSize;
    cell.frame = frame;
};

// Animate

[UIView transitionWithView:cellToChangeSize duration:0.1f options: UIViewAnimationOptionCurveLinear animations:animateChangeWidth completion:nil];

For further explanation:

1. One of the most important step is the invalidateLayout call on the UICollectionView.collectionViewLayout you are using. If you forget it, it is likely to happen that cells will grow over the borders of your collection - that's something you most probably don't want to happen. Also consider that your code should at some point present the new size of your cell to your layout. In my case (I'm using an UICollectionViewFlowLayout), I adjusted the collectionView:layout:sizeForItemAtIndexPath method to reflect the new size of the modified cell. Forgetting that part, nothing will happen to your cell sizes at all if you first call invalidateLayout and then try to animate the change in size.

2. Most strange (at least to me) but important is that you call invalidateLayout not inside the animation block. If you do so, the animation won't display smooth but glitchy and flickering.

3. You have to call UIViews transitionWithView:duration:options:animations:completion: method with the cell as argument for transitionWithView, not the whole collection view, because it's the cell you want to get animated, not the entire collection.

4. I used the intrinsicContentSize method of my cell to return the size I want - in my case, I grow and shrink the width of the cell to either display a button or hide it, depending on the selection state of the cell.

I hope this will help some people. For me it took several hours to figure out how to make that animation work correctly, so I try to free others from that time-consuming burden.

[UIView transitionWithView:collectionView 
                  duration:.5 
                   options:UIViewAnimationOptionTransitionCurlUp 
                animations:^{

    //any animatable attribute here.
    cell.frame = CGRectMake(3, 14, 100, 100);

} completion:^(BOOL finished) {

    //whatever you want to do upon completion

}];

Play around with something along those lines inside your didselectItemAtIndexPath method.

I've had success animating changes by using -setCollectionViewLayout:animated: to create a new instance of the same collection view layout.

For example, if you're using a UICollectionViewFlowLayout, then:

// Make sure that your datasource/delegate are up-to-date first

// Then animate the layout changes
[collectionView setCollectionViewLayout:[[UICollectionViewFlowLayout alloc] init] animated:YES];