DWFlowLayout *layout = [[DWFlowLayout alloc] init];
  //设置是否需要分页
  [layout setPagingEnabled:YES];
  self.collectionView.collectionViewLayout = layout;

  
  //为垂直缩放除以系数
  static CGFloat const ActiveDistance = 350;
  //越大，缩放越大。
  static CGFloat const ScaleFactor = 0.05;