在WPF中，如何确定控件对用户是否可见?

I'm displaying a very big tree with a lot of items in it. Each of these items shows information to the user through its associated UserControl control, and this information has to be updated every 250 milliseconds, which can be a very expensive task since I'm also using reflection to access to some of their values. My first approach was to use the IsVisible property, but it doesn't work as I expected.I'm displaying a very big tree with a lot of it