WPF ComboBox - 在没有绑定项目时显示不同的内容

I have a ComboBox, and i want to change its look when the ItemsSource property is null. When it is in that state, i want to show a TextPanel with the text "Retrieving data" in it, and give it a look kind of similar to the watermarked textbox.I have a ComboBox, and i want to change its loo