public event EventHandler<ItemsRepeaterElementIndexChangedEventArgs> ElementIndexChanged

When you use ItemsRepeater to build a more complex control that supports specific interactions on the child elements (such as selection or click), it is useful to be able to keep an up-to-date identifier for the backing data item. This event is raised for each realized IControl where the index for the item it represents has changed. For example, when another item is added or removed in the data source, the index for items that come after in the ordering will be impacted.