CollectioView滾動到指定section的方法

項目中的需求：collectionView頂部有一個scrollView組成的標簽，點擊標簽，讓collectionView滾動到指定的行，滾動collectionView自動切換到頂部指定的標簽

實現方法如下：

1. 保證collectionView全部加載完畢，我這里通過一個bool的標志位來標示

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

2. 標簽點擊了CollectionView滾動到指定行

        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滾動到指定行的時候，同時切換標簽滾動

    -(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];
    }