Note First of all, ensure you install the Avalonia.ReactiveUI package and add a call to UseReactiveUI() to your AppBuilder — that's super important. Note, that Avalonia.ReactiveUI package is supported by AvaloniaUI team, so if anything goes wrong, head over to AvaloniaUI Gitter. Also see Avalonia.ReactiveUI Docs.

For WhenActivated to work, you need to use custom base classes from Avalonia.ReactiveUI package, such as ReactiveWindow<TViewModel> or ReactiveUserControl<TViewModel>. Of course, you can also implement the IViewFor<TViewModel> interface by hand in your class, but ensure to store the ViewModel inside an AvaloniaProperty. If you wish to add activation support to your view models, then implement the IActivatableViewModel interface. A view model implementing IActivatableViewModel might look like the one below:

public class ViewModel : ReactiveObject, IActivatableViewModel
{
    public ViewModelActivator Activator { get; } = new ViewModelActivator();

    public ViewModel()
    {
        this.WhenActivated(disposables =>
        {
            /* Handle activation */
            Disposable
                .Create(() => { /* Handle deactivation */ })
                .DisposeWith(disposables);
        });
    }
}

Remember to place a call to WhenActivated to the code-behind of your view, otherwise your view model's WhenActivated block won't get called.

// This represents the View.xaml.cs code-behind file.
public partial class View : ReactiveWindow<ViewModel>
{
    public View()
    {
        // If you put a WhenActivated block here, your activatable view model 
        // will also support activation, otherwise it won't.
        this.WhenActivated(disposables => { /* Handle interactions etc. */ });
        AvaloniaXamlLoader.Load(this);
    }
}

<!-- This represents the View.xaml markup file. -->
<Window xmlns="https://github.com/avaloniaui">
  <TextBlock Text="Hello, world!" />
</Window>

The WhenActivated block is called when the AttachedToVisualTree event is published, and the disposable is disposed of when the DetachedFromVisualTree event is published. The behavior is the same for both the views and the view models. Unfortunately, Avalonia won't generate strongly typed x:Name references to controls, so the only way for now is to use the FindControl method that will find a control by the name specified in XAML, or to use {Binding Path} syntax.

Note The FindControl method shouldn't be used inside an expression. Instead, create a custom property which calls the FindControl method, or store the control in a variable. See an example of how to use ReactiveUI type-safe bindings with AvaloniaUI.

public partial class View : ReactiveWindow<ViewModel>
{
    // Assume the Button control has the Name="ExampleButton" attribute defined in XAML.
    public Button ExampleButton => this.FindControl<Button>("ExampleButton");

    public View()
    {
        this.WhenActivated(disposables => 
        {
            // Bind the 'ExampleCommand' to 'ExampleButton' defined above.
            this.BindCommand(ViewModel, x => x.ExampleCommand, x => x.ExampleButton)
                .DisposeWith(disposables);
        });
        AvaloniaXamlLoader.Load(this);
    }
}