title: UICollection view update callback
body: <p>I have been googling about this but is there an easy way to catch the event when data in collection has been added or removed? I have collection view containing list of images in one line horizontally and user can scroll right or left to see the images. I want to bring the scroll position to the first image from the right when another image is added. I can do that by using <code>scrollRectToVisible</code> but I am not sure where I can call this function to scroll. I was trying to find an event fired when the collection view is modified.</p>