- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section{
return UIEdgeInsetsMake(0, (kScreenWidth - _itemWidth)/2, 0, (kScreenWidth - _itemWidth)/2);
}

-(void)collectionViewDidScrollWithScrollBlock:(DidScrollBlock)didScrollBlcok;
-(void)collectionViewDidScrollWithScrollBlock:(IndexScrollBlock)didScrollBlcok;

IndexFlow *flowLayout = [[IndexFlow alloc] init];
self = [super initWithFrame:frame collectionViewLayout:flowLayout];

- (void)collectionViewDidScrollWithScrollBlock:(DidScrollBlock)didScrollBlcok{

self.didScrollBlcok = didScrollBlcok;
}
- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer{

self.shouldDid = YES;

return YES;
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
self.shouldDid = NO;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
 if (self.shouldDid) {
  if (self.didScrollBlcok) {
    self.didScrollBlcok(self);
  }
 }
}

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section{
return UIEdgeInsetsMake(0, (kScreenWidth - _itemWidth)/2, 0, (kScreenWidth - _itemWidth)/2);
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{
return CGSizeMake(_itemWidth, self.height);
}

- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section{
return _minLineSpacing;
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)oldBounds
{
return YES;
}

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

/**
 * 设置collectionView停止滚动那一刻的位置
 *
 * @param proposedContentOffset 原本collectionView停止滚动那一刻的位置
 * @param velocity       速度
 *
 * @return 最终的位置
 */
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
CGFloat offsetAdjustment = MAXFLOAT;
CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);
// 停止时刻的可视区域
CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
NSArray* array = [super layoutAttributesForElementsInRect:targetRect];
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);
}