How can I determine when a user has updated the text in a textbox before the Binding has updated the source?

I was expecting to find an "IsDirty" property on either the TextBox or the Binding ...

My problem is that the "Cancel" button Enabled property is bound to the ViewModel's IsDirty property and is disabled until the focus moves out of the textbox.

"IsDirty" needs to be defined as ViewModel.IsDirty || TextBox.IsDirty