Design-time support¶

A designer surface, such as the Visual Studio or Rider XAML preview, loads your views to draw them without running your app. WhenActivated needs an activation fetcher to know when a view is shown and hidden, and a designer never provides one. Without a check for that case, activation would throw every time a designer opened a view.

GetIsDesignMode, an extension method on IActivatableView, reports whether a designer surface is loading the view. It returns false unless a platform package overrides it for its own view types. The WPF package, for example, backs it with DesignerProperties.GetIsInDesignMode. WhenActivated calls it internally. With no activation fetcher registered and the view in design mode, it does nothing instead of throwing. A designer preview then needs none of the services the running app has. See Design mode for the check in place and what it returns for a view outside a designer.

Call GetIsDesignMode yourself to skip work a designer cannot run, such as creating a router or navigating it. The WPF grade-book example's MainWindow constructor does this before it builds a real AppShell.

For design-time data in XAML markup, bind through d:DataContext and a design-time implementation of your view model's interface, the way you would for any XAML-based framework. ReactiveUI adds nothing of its own on top of that. Data Binding covers the bindings a view sets up once it has a real view model.