title: change UICollectionView cell's alpha value not working
body: <p>I want to tap the cell of the UICollectionView and the cell becomes half transparent.</p>
  
  <pre><code>    - (void)collectionView:(UICollectionView *)collectionView
  didSelectItemAtIndexPath:(NSIndexPath *)indexPath
  {
      GLEpisodeCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:[GLEpisodeCell identifier] forIndexPath:indexPath];//GLEpisodeCell is subclass of UICollectionViewCell
      cell.alpha=0.5;
      cell.contentView.alpha=0.5;
  }
  </code></pre>
  
  <p>But this doesn't work. The cell is still opacity.</p>