项目中使用Collection时，需要添加一些动画。
当 Collection 插入或移除 item 时，函数执行的顺序：
空数据插入Item
1.[UIApplication sendAction:to:from:forEvent] _IOHIDEvent
2.类insert函数
3.[UICollectionView _updateRowsAtIndexPaths:updateAction:]
4.[UICollectionViewData _prepareToLoadData]
5.-prepareLayout
6.-prepareForCollectionViewUpdates:
7.[UICollectionView _viewAnimationsForCurrentUpdate]
8.-initialLayoutAttributesForAppearingItemAtIndexPath:
9.[UICollectionView _updateWithItems:tentativelyForRecording:animator]
10.-finalizeCollectionViewUpdates
有数据插入Item
1. ~ 8.
9.[UICollectionView _doubleSidedAnimateionsForView:withStartingLayoutAttributes:startingLayout:endingLayoutAttributes:endingLayout:withAnimationSetup:animationCompletion:enableCustomAnimations:customAnimationsType:]
10.-finalLayoutAttributesForDisappearingItemAtIndexPath:
11.8/10步骤循环（循环次数取决于剩下的Item个数）
12.-finalizeCollectionViewUpdates
空数据移除Item
1.[UIApplication sendAction:to:from:forEvent:] _IOHIDEvent
2.类remove函数
3.[UICollectionView _updateRowsAtIndexPaths:updateAction:]
4.[UICollectionViewData _prepareToLoadData]
5.-prepareLayout
6.-prepareForCollectionViewUpdates:
7.[UICollectionView _viewAnimationsForCurrentUpdate]
8.-finalLayoutAttributesForDisappearingItemAtIndexPath:
9.[UICollectionView _updateWithItems:tentativelyForRecording:animator]
4.finalizeCollectionViewUpdates
有数据移除Item
类似有数据插入Item
参考博客中对这样的描述：
In general, layout attributes are linearly interpolated from the initial state to the final state to compute the collection view animations. However, for the newly inserted or removed items, there are no initial and final attributes to interpolate from. To compute the animations for such cells, the collection view will ask its layout object to provide the initial and final attributes through the initialLayoutAttributesForAppearingItemAtIndexPath: and finalLayoutAttributesForDisappearingItemAtIndexPath: methods.
....
During the animation of the bounds change, the collection view acts as if the currently displayed items are removed and inserted again in the new bounds, resulting in a series of finalLayoutAttributesForDisappearingItemAtIndexPath: and initialLayoutAttributesForAppearingItemAtIndexPath: calls for each index path.
> 参考：[Animating Collection Views](https://www.objc.io/issues/12-animations/collectionview-animations/)