- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path
{
    UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];
    attributes.size = CGSizeMake(ITEM_SIZE, ITEM_SIZE);
    attributes.center = CGPointMake(_center.x + _radius * cosf(2 * path.item * M_PI / _cellCount),
                                    _center.y + _radius * sinf(2 * path.item * M_PI / _cellCount));
    return attributes;
}

- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForInsertedItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
    attributes.alpha = 0.0;
    attributes.center = CGPointMake(_center.x, _center.y);
    return attributes;
}

- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDeletedItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
    attributes.alpha = 0.0;
    attributes.center = CGPointMake(_center.x, _center.y);
    attributes.transform3D = CATransform3DMakeScale(0.1, 0.1, 1.0);
    return attributes;
}

- (void)performBatchUpdates:(void (^)(void))updates completion:(void (^)(BOOL finished))completion;该方法调用的时候会触发上边两个动画方法

注意，现在这两个动画方法已经改掉了api名称，由下边两个替代

- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath{
}
- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath{
}

前面一个xx表明了这个方法被引入时的iOS版本，后面一个表明它被废弃时的iOS版本。被废弃并不是指这个方法就不存在了，只是意味着我们应当开始考虑将相关代码迁移到新的API上去了。

    -(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray* array = [super layoutAttributesForElementsInRect:rect];
    CGRect visibleRect;
    visibleRect.origin = self.collectionView.contentOffset;
    visibleRect.size = self.collectionView.bounds.size;
    
    for (UICollectionViewLayoutAttributes* attributes in array) {
        if (CGRectIntersectsRect(attributes.frame, rect)) {
            CGFloat distance = CGRectGetMidX(visibleRect) - attributes.center.x;
            CGFloat normalizedDistance = distance / ACTIVE_DISTANCE;
            if (ABS(distance) < ACTIVE_DISTANCE) {
                CGFloat zoom = 1 + ZOOM_FACTOR*(1 - ABS(normalizedDistance));
                attributes.transform3D = CATransform3DMakeScale(zoom, zoom, 1.0);
                attributes.zIndex = 1;
            }
        }
    }
    return array;
}

方法调用，通过判断当前item和collectionview可视区域两个的中心间距同item的宽度来进行对比，以决定是否进行transform3D变换。变幻的时候添加了一个比例因子ZOOM_FACTOR，使缩放不会太夸张。

- (CGPoint)targetContentOffsetForProposedContentOffset: (CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    
    //proposedContentOffset是没有对齐到网格时本来应该停下的位置
    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);
    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    NSArray* array = [super layoutAttributesForElementsInRect:targetRect];
    
    //对当前屏幕中的UICollectionViewLayoutAttributes逐个与屏幕中心进行比较，找出最接近中心的一个
    for (UICollectionViewLayoutAttributes* layoutAttributes in array) {
        CGFloat itemHorizontalCenter = layoutAttributes.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
        }
    }
    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

与上个方法一样，需要求得可是区域的cgrect，通过该参数求出所有的UICollectionViewLayoutAttributes，然后用每个参数里的center和中心点求差比较，比较的算法就是如果当前差数比上一个差的绝对值小就记录下来。
最后返回的应该停下来的点，y不动，x加的当前记录的差值。

UICollectionViewLayoutAttributes记录了一个元素所有与布局相关联的参数,仅此而已。