CollectioView滚动到指定section的方法

项目中的需求：collectionView顶部有一个scrollView组成的标签，点击标签，让collectionView滚动到指定的行，滚动collectionView自动切换到顶部指定的标签

实现方法如下：

1. 保证collectionView全部加载完毕，我这里通过一个bool的标志位来标示

  -(void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
  {
        if([indexPath row] == ((NSIndexPath*)[[collectionView indexPathsForVisibleItems] lastObject]).row){
            self.isLoaded = YES;
        }
   }

2. 标签点击了CollectionView滚动到指定行

        if (self.isLoaded) {

            // scroll to selected index
            NSIndexPath* cellIndexPath = [NSIndexPath indexPathForItem:0 inSection:index+1];
            UICollectionViewLayoutAttributes* attr = [self.collectionView.collectionViewLayout layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:cellIndexPath];
            UIEdgeInsets insets = self.collectionView.scrollIndicatorInsets;

            CGRect rect = attr.frame;
            rect.size = self.collectionView.frame.size;
            rect.size.height -= insets.top + insets.bottom;
            CGFloat offset = (rect.origin.y + rect.size.height) - self.collectionView.contentSize.height;
            if ( offset > 0.0 ) rect = CGRectOffset(rect, 0, -offset);

            [weakSelf.collectionView scrollRectToVisible:rect animated:YES];
        }

3. CollectionView滚动到指定行的时候，同时切换标签滚动

    -(void)scrollViewDidScroll:(UIScrollView *)scrollView{
        static NSInteger currentIndex = 0;

        NSInteger index=scrollView.contentOffset.y;
        CGRect visibleRect = (CGRect){.origin = self.collectionView.contentOffset, .size = self.collectionView.bounds.size};
        CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
        NSIndexPath *visibleIndexPath = [self.collectionView indexPathForItemAtPoint:visiblePoint];
//        NSLog(@"%@",visibleIndexPath);
        if (currentIndex == visibleIndexPath.section || visibleIndexPath == nil) {
            return;
        }
        currentIndex = visibleIndexPath.section;

       [self.itemTool itemScrollToPositionWithIndex:currentIndex isJump:YES];
    }