WPF - how to prevent that UserControl is switched before saving?

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WPF

user-controls

Hi there,

I am trying to create a very very simple application.
Lets call this app: DAMN!

Okay - DAMN! contains three UserControls, which represent three different views in the app. Each UserControl contains a textbox and a save-button.

Once the text has changed, the save-button will be enabled.
From now, it is not allowed to switch to another "page" (UserControl-View), until the save-button is clicked.

But I really have no idea, how to do that. Maybe my initial concept of the app layout is wrong and can be done better.

So I uploaded a little demo-app.
Could you be so kind and help me with this?

Downloadable here :
Click!

Thx a lot.