You could use the RadBusyIndicator. It is a ContentControl so you can put it on your MainPage and wrap all the content inside (for example put the LayoutRoot Grid inside the RadBusyIndicator so it could be set as content) and when you set the IsBusy = false all the controls within will show disabled and will not be clickable. Nice notification and overlay will appear indicating that you application is doing some work on the background and when you are ready set the IsBusy to false.
Remember the most important part is to have:
The RadBusy indicator documentation is here
You could also search for the RadBusyIndicator on your examples site
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