UICollectionView performBatchUpdates: animates all

Question

I'm writing a custom UICollectionViewFlowLayout and I've noticed that initialLayoutAttributesForAppearingItemAtIndexPath: and initialLayoutAttributesForAppearingDecorationElementOfKind:atIndexPath: will be called for all sections when I invoke performBatchUpdates:completion: on the collection view. The result is that all sections have the animation applied to them instead of just the newly added section.

[collectionView performBatchUpdates:^{
    currentModelArrayIndex++;
    [collectionView insertSections:[NSIndexSet indexSetWithIndex:currentModelArrayIndex]];
    [collectionView reloadSections:[NSIndexSet indexSetWithIndex:currentModelArrayIndex-1]];
} completion:^(BOOL finished) {
    [collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:currentModelArrayIndex] atScrollPosition:UICollectionViewScrollPositionTop animated:YES];
}];

What I've tried so far is removing the call to performBatchUpdates:completion: in lieu of simple updates, but the already existing sections (all of them) are animated in anyway. I've come up with a solution of checking to make sure that I'm changing the layout attributes of only the last section, but it feels hacky.

if (decorationIndexPath.section == [(id<UICollectionViewDataSource>)self.collectionView.delegate numberOfSectionsInCollectionView:self.collectionView] - 1)
{
   layoutAttributes.alpha = 0.0f;
   layoutAttributes.transform3D = CATransform3DMakeTranslation(-CGRectGetWidth(layoutAttributes.frame), 0, 0);
}

Is this the proper way to go about animating only some sections?

Answer 1

OK, I've got an answer; it's not a lot prettier than the previous one, but it keeps the layout from touching the datasource, so it's cleaner.

Basically, we need to override prepareForCollectionViewUpdates: and finalizeCollectionViewUpdates to keep track of the sections that we're inserting. I have a mutable set that contains NSNumber instances of the sections that we're inserting.

-(void)prepareForCollectionViewUpdates:(NSArray *)updateItems
{
    [super prepareForCollectionViewUpdates:updateItems];

    [updateItems enumerateObjectsUsingBlock:^(UICollectionViewUpdateItem *updateItem, NSUInteger idx, BOOL *stop) {
        if (updateItem.updateAction == UICollectionUpdateActionInsert)
        {
            [insertedSectionSet addObject:@(updateItem.indexPathAfterUpdate.section)];
        }
    }];
}

-(void)finalizeCollectionViewUpdates
{
    [super finalizeCollectionViewUpdates];

    [insertedSectionSet removeAllObjects];
}

Next, I check to see if the index path's section is included in that set when setting the initial layout attributes for items and decoration views.

-(UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingDecorationElementOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)decorationIndexPath
{
    UICollectionViewLayoutAttributes *layoutAttributes;

    if ([elementKind isEqualToString:AFCollectionViewFlowLayoutBackgroundDecoration])
    {
        if ([insertedSectionSet containsObject:@(decorationIndexPath.section)])
        {
            layoutAttributes = [self layoutAttributesForDecorationViewOfKind:elementKind atIndexPath:decorationIndexPath];
            layoutAttributes.alpha = 0.0f;
            layoutAttributes.transform3D = CATransform3DMakeTranslation(-CGRectGetWidth(layoutAttributes.frame), 0, 0);
        }
    }

    return layoutAttributes;
}

-(UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    UICollectionViewLayoutAttributes *layoutAttributes;

    if ([insertedSectionSet containsObject:@(itemIndexPath.section)])
    {
        layoutAttributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
        layoutAttributes.transform3D = CATransform3DMakeTranslation([self collectionViewContentSize].width, 0, 0);
    }

    return layoutAttributes;
}

I'm returning nil from these methods otherwise because nil is the default value.

This has the added benefit of having nicer rotation animations, as well.