Read original post by Fons Sonnemans at Reflection it Blog
In XAML applications you can use a Popup control to displays content on top of existing content. To show a Popup you must set the IsOpen property to true. You can subscribe yourself to the Closed eventhandler which is fired when the IsOpen property is (re)set to false. You can use this event to execute extra logic when the Popup is closed.
To make this easier I have created an ShowAsync() extension method which does this for you. I got my inspiration for this solution from this Awaiting Storyboard completion blog post by Nigel Sampson. He uses this technique to begin a storyboard using a BeginAsync() extension method.