title: how to release the memory of a image in the imageView of a collectionViewCell when push to another controller
body: <p>the collectionView is just a image waterfall, each image cost about 200k ~ 2M memory,its ok to show them though not smoothly enough. 
  what's the question?  it push to another controller by click some image, the memory this collectionView take does not release, but the images's memory drive me mad. </p>
  
  <p>i tried this </p>
  
  <p>in controller </p>
  
  <pre><code>- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    [self.gifView disAppearReload];
    }
  </code></pre>
  
  <p>in collectionView</p>
  
  <pre><code>- (void)disAppearReload {
    self.withoutImg = YES;
    [self.collectionView reloadData];
    }
    - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
  
      WaterfallCell *cell = [WaterfallCell cellWithCollentionView:collectionView IndexPath:indexPath];
      cell.delegate = self;
      cell.withoutImg = self.isWithoutImg;
      cell.cellModel = self.cellModels[indexPath.row];
      return cell;
   }
  </code></pre>
  
  <p>in collectionCell</p>
  
  <pre><code>- (void)setCellModel:(ImageModel *)cellModel {
    _cellModel = cellModel;
    if (self.isWithoutImg) {        
      self.imageBtn.imageView.image = nil;
      }
    else {
      [self.imageBtn sd_setImageWithURL:[NSURL URLWithString:cellModel.imgUrl] forState:UIControlStateNormal placeholderImage:nil options:SDWebImageLowPriority];
      }
   }
  </code></pre>
  
  <p>it does not work, the cell code can't be called, but the same way in UITableView does work.   help ~~~ </p>