initialLayoutAttributesForAppearingItemAtIndexPath

问题:

Has anyone seen a decent answer to this problem?

initialLayoutAttributesForAppearingItemAtIndexPath seems to be being called for all visible cells, not just the cell being inserted. According to Apple's own docs:

For moved items, the collection view uses the standard methods to retrieve the item’s updated layout attributes. For items being inserted or deleted, the collection view calls some different methods, which you should override to provide the appropriate layout information

Which doesn't sound like what is happening... the other cells aren't being inserted, they are being moved, but it's calling initialLayoutAttributesForAppearingItemAtIndexPath for the ones being moved too.

I have seen work arounds using prepareForCollectionViewUpdates: to trace which indexPaths are being updated and only changing those, but this seems a bit odd that it's going agains their own docs. Has anyone else found a better way around this?

回答1:

I found this blog post by Mark Pospesel to be helpful.
The author also fixed WWDC CircleLayout sample and posted it on Github.

Methods of interest:

- (void)prepareForCollectionViewUpdates:(NSArray *)updateItems
{
    // Keep track of insert and delete index paths
    [super prepareForCollectionViewUpdates:updateItems];

    self.deleteIndexPaths = [NSMutableArray array];
    self.insertIndexPaths = [NSMutableArray array];

    for (UICollectionViewUpdateItem *update in updateItems)
    {
        if (update.updateAction == UICollectionUpdateActionDelete)
        {
            [self.deleteIndexPaths addObject:update.indexPathBeforeUpdate];
        }
        else if (update.updateAction == UICollectionUpdateActionInsert)
        {
            [self.insertIndexPaths addObject:update.indexPathAfterUpdate];
        }
    }
}

- (void)finalizeCollectionViewUpdates
{
    [super finalizeCollectionViewUpdates];
    // release the insert and delete index paths
    self.deleteIndexPaths = nil;
    self.insertIndexPaths = nil;
}

// Note: name of method changed
// Also this gets called for all visible cells (not just the inserted ones) and
// even gets called when deleting cells!
- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    // Must call super
    UICollectionViewLayoutAttributes *attributes = [super initialLayoutAttributesForAppearingItemAtIndexPath:itemIndexPath];

    if ([self.insertIndexPaths containsObject:itemIndexPath])
    {
        // only change attributes on inserted cells
        if (!attributes)
            attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];

        // Configure attributes ...
        attributes.alpha = 0.0;
        attributes.center = CGPointMake(_center.x, _center.y);
    }

    return attributes;
}

// Note: name of method changed
// Also this gets called for all visible cells (not just the deleted ones) and
// even gets called when inserting cells!
- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    // So far, calling super hasn't been strictly necessary here, but leaving it in
    // for good measure
    UICollectionViewLayoutAttributes *attributes = [super finalLayoutAttributesForDisappearingItemAtIndexPath:itemIndexPath];

    if ([self.deleteIndexPaths containsObject:itemIndexPath])
    {
        // only change attributes on deleted cells
        if (!attributes)
            attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];

        // Configure attributes ...
        attributes.alpha = 0.0;
        attributes.center = CGPointMake(_center.x, _center.y);
        attributes.transform3D = CATransform3DMakeScale(0.1, 0.1, 1.0);
    }

    return attributes;
}

回答2:

You're not alone. The UICollectionViewLayout header file comments make things a little clearer.

For each element on screen before the invalidation, finalLayoutAttributesForDisappearingXXX will be called and an animation setup from what is on screen to those final attributes.

For each element on screen after the invalidation, initialLayoutAttributesForAppearingXXX will be called an an animation setup from those initial attributes to what ends up on screen.

Basically finalLayoutAttributesForDisappearingItemAtIndexPath is called for each item on screen before the animation block starts, and initialLayoutAttributesForAppearingItemAtIndexPath is called for each item after the animation block ends. It's up to you to cache the array of UICollectionViewUpdateItem objects sent in prepareForCollectionViewUpdates so you know how to setup the initial and final attributes. In my case I cached the previous layout rectangles in prepareLayout so I knew the correct initial positions to use.

One thing that stumped me for a while is you should use super's implementation of initialLayoutAttributesForAppearingItemAtIndexPath and modify the attributes it returns. I was just calling layoutAttributesForItemAtIndexPath in my implementation, and animations weren't working because the layout positions were different.

回答3:

If you've subclassed UICollectionViewFlowLayout, you can call the super implementation. Once you've got the default initial layout, you can check for an .alpha of 0. If alpha is anything other than 0, the cell is being moved, if it's 0 it's being inserted.

Bit of a hack, I know, but it works