当高度不固定IOS时CollectionView冻结

Question

我正在使用自动布局来计算UICollectionView的高度。因此，每当调用sizeforitematindexpath时，我的UICollectionView就会冻结。

这是一个代码

  - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
 {

    CGFloat height;

    if ([alertheightDict objectForKey:indexPath]) {
        height = [[alertheightDict objectForKey:indexPath] floatValue];
    }else{
        //[[LogClass sharedInstance] log:[NSString stringWithFormat:@"offscreencells %@", self.offscreenCells]];
        //NotificationCustomCell *cell = [self.offscreenCells objectForKey:indexPath];
        NotificationCustomCell *cell = [[NotificationCustomCell alloc] init];//my custom cell
        NSMutableDictionary *dataSourceItem = [alertsArray objectAtIndex:indexPath.row];// my source array
        [cell updateCell:dataSourceItem :indexPath.row : 0];// updating cell

        [cell setNeedsUpdateConstraints];
        [cell updateConstraintsIfNeeded];

        cell.bounds = CGRectMake(0.0f, 0.0f, cellWidth, CGRectGetHeight(cell.bounds));

        [cell setNeedsLayout];
        [cell layoutIfNeeded];

        height = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
        [alertheightDict setObject:[NSString stringWithFormat:@"%f", height] forKey:indexPath];
    }

    return CGSizeMake(cellWidth , height);
   }  

这里有什么不对，冻结我的应用程序