原文地址：http://blog.csdn.net/u013282507/article/details/54136812
原文讲的特别详细，在此就是记录下，方便查找

20170106123134176.gif

要实现这样的效果总共分四步：

1. 实现横向滚动 (UICollectionView的最基础实现)
2. 缩进效果

-(UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout 
insetForSectionAtIndex:(NSInteger)section  
{  
return UIEdgeInsetsMake(0, [self collectionInset], 0, [self collectionInset]);  
}  

3. 居中放大

   重写UICollectionViewFlowLayout方法
   
   -(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect  
   {  
   NSArray *arr = [self getCopyOfAttributes:[super layoutAttributesForElementsInRect:rect]];  
   //屏幕中线  
   CGFloat centerX = self.collectionView.contentOffset.x + self.collectionView.bounds.size.width/2.0f;  
   //刷新cell缩放  
   for (UICollectionViewLayoutAttributes *attributes in arr) {  
    CGFloat distance = fabs(attributes.center.x - centerX);  
    //移动的距离和屏幕宽度的的比例  
    CGFloat apartScale = distance/self.collectionView.bounds.size.width;  
    //把卡片移动范围固定到 -π/4到 +π/4这一个范围内  
    CGFloat scale = fabs(cos(apartScale * M_PI/4));  
    //设置cell的缩放 按照余弦函数曲线 越居中越趋近于1  
    attributes.transform = CGAffineTransformMakeScale(1.0, scale);  
   }  
   return arr
   }
   

   //防止报错 先复制attributes

   • (NSArray *)getCopyOfAttributes:(NSArray *)attributes
     {
     NSMutableArray *copyArr = [NSMutableArray new];
     for (UICollectionViewLayoutAttributes *attribute in attributes) {
     [copyArr addObject:[attribute copy]];
     }
     return copyArr;
     }

   //是否需要重新计算布局
   -(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
   {
   return true;
   }

4. 自动居中

   NSInteger _currentIndex;  
   CGFloat _dragStartX;  
   CGFloat _dragEndX; 
   

   //手指拖动开始
   -(void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
   {
   _dragStartX = scrollView.contentOffset.x;
   }

   //手指拖动停止
   -(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
   {
   _dragEndX = scrollView.contentOffset.x;
   dispatch_async(dispatch_get_main_queue(), ^{
   [self fixCellToCenter];
   });
   }

   //配置cell居中
   -(void)fixCellToCenter
   {
   //最小滚动距离
   float dragMiniDistance = self.bounds.size.width/20.0f;
   if (_dragStartX - _dragEndX >= dragMiniDistance) {
   _currentIndex -= 1;//向右
   }else if(_dragEndX - _dragStartX >= dragMiniDistance){
   _currentIndex += 1;//向左
   }
   NSInteger maxIndex = [_collectionView numberOfItemsInSection:0] - 1;
   _currentIndex = _currentIndex <= 0 ? 0 : _currentIndex;
   _currentIndex = _currentIndex >= maxIndex ? maxIndex : _currentIndex;

   [_collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:_currentIndex inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES];
   }