为所有可见单元格触发的initialLayoutAttributesForAppearingItemAtIndexPath，而不仅仅是插入的单元格

有没有人看到这个问题的合适答案？

initialLayoutAttributesForAppearingItemAtIndexPath似乎是所有可见的单元格都被调用，而不仅仅是被插入的单元格。根据苹果自己的文档 https://developer.apple.com/library/ios/#documentation/UIKit/Reference/UICollectionViewLayout_class/Reference/Reference.html:

对于移动的项目，集合视图使用标准方法来检索项目的更新布局属性。对于插入或删除的项目，集合视图会调用一些不同的方法，您应该重写这些方法以提供适当的布局信息

这听起来不像正在发生的事情......其他单元格没有被插入，它们正在被移动，但它正在调用initialLayoutAttributesForAppearingItemAtIndexPath为了那些被感动 too.

我已经看到使用的解决方法prepareForCollectionViewUpdates:跟踪哪些索引路径正在更新并且只更改它们，但这似乎有点奇怪，因为它违背了他们自己的文档。还有其他人找到更好的方法来解决这个问题吗？

I found 马克·波斯佩塞尔 (Mark Pospesel) 的这篇博文 https://mpospese.com/2012/10/25/fixing-circlelayout/提供帮助。
作者还修复了WWDCCircleLayout样品和将其发布到 Github 上 https://github.com/mpospese/CircleLayout/blob/master/CircleLayout/CircleLayout.m.

感兴趣的方法：

- (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;
}