title: WPF DataTemplate: How to create a tooltip just-in-time?
body: <p>I am using DataTemplates to render items in an ItemsControl. I want to show a tooltip for each item. If I use a binding such as ToolTip="{Binding MyToolTip,Mode=OneWay}", WPF gets the tooltip once at the beginning and does not update the tooltip when it is opened.</p>
  
  <p>My items implement INotifyPropertyChanged, so in principle I could generate change notifications for MyToolTip. The problem is, the tooltip is produced based on many pieces of information in the underlying model. These pieces of information change frequently and it would be both cumbersome and inefficient to generate change notifications for the MyToolTip property. Besides, I do not want tooltips for ALL items to be produced initially. Instead I would like to force a fresh tooltip to be generated when the tooltip is opened. How can I do it?</p>