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Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web.

Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit:
The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child view models, one for the top, middle & bottom.
The ViewHost is nothing more than a ContentControl, its not only a place holder for the desired content but it also has the overlay and progress bar shown above.
Where is the XAML for the overlay defined?

The XAML is defined in a style - see below, I've defined it like this so I don't have to explicitly write any extra XAML when I want to use the ViewHost control - if I had an app with multiple Views I wouldn't have to keep defining the Grid for the overlay.

As you can see I have a specific view model - IBusyViewModel, this has a property called BusyMonitor which is used to trigger both the visibility of the grid containing the busy overlay as well as the actual overlay. I do this because the BusyMonitor property on the IBusyViewModel could be null and I don't want a binding exception thrown if I bound to BusyMonitor.IsBusy and BusyMonitor is null.
As you can see I have a specific view model - IBusyViewModel, this is bindable obejct because the IsBusy property is bound to the busy overlay. I've also included an Rx observable to allow any ViewModel to observe when the value changes.
Instances of this interface are then injected into the MainViewModel instance:
Also injected into the ContentViewModel & FooterViewModel, in this example these view models don't do very much:
The interesting view model in this example is the HeaderViewModel. The IObservableCommand handles the button click, which executes the ExecuteClick method. This sets the BusyMonitor.IsBusy property to true initially and when the observable timer pumps the BusyMonitor.IsBusy is set back to false.
Now at this point you might be thinking...

'but you've got 4 view models and 4 instances of the IBusyMonitor interface, how can setting IsBusy to true in the HeaderViewModel affect the whole application?'

The answer in fact is the exact opposite, they are all sharing the same instance of the IBusyMonitor interface and this is achieved by using an IoC container with a singleton registration for the IBusyMonitor interface, I'm using AutoFac for this example:
That pretty much rounds it up, the code is available for download:

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