title: Change attributes of a UICollectionViewCell in didSelectItemAtIndexPath
body: <p>I am configuring a <code>UICollectionViewCell</code> in a subclass, it adds 2 subviews to the <code>contentView</code> property, both are <code>UIImageView</code> and both have the <code>hidden</code> property set to <code>YES</code>. These subviews are "checked" and "unchecked" images that overlay the primary UIImageView in the cell to indicate whether or not the current cell is selected using UICollectionView's "multiple select" feature.</p>
  
  <p>When the cell is tapped, <code>collectionView:didSelectItemAtIndexPath:</code> is called on the delegate, and I'd like to <code>setHidden:NO</code> on the "checked" UIImageView. Calling this on the cell does nothing at all -- the cell is seemingly locked in its originally drawn state.</p>
  
  <p>Is it possible to make changes to a cell outside <code>collectionView:cellForItemAtIndexPath:</code>? I have tried manually adding subviews within <code>collectionView:didSelectItemAtIndexPath:</code>, but it just makes absolutely no change to the UI. I have verified that the delegate method is getting called, it's just not making my cell changes.</p>
  
  <pre><code>- (void) collectionView(UICollectionView *)cv didSelectItemAtIndexPath(NSIndexPath *)indexPath {
      ShotCell *cell = [self collectionView:cv cellForItemAtIndexPath:indexPath];
      UILabel *testLabel = UILabel.alloc.init;
      testLabel.text = @"FooBar";
      testLabel.sizeToFit;
      [cell.contentView.addSubview testLabel];
  }
  </code></pre>