Scene phases

[kaascevich]

Adds a scenePhase environment value and associated types and backend functions.

• If the property is declared on the App struct, it returns active if the app as a whole is active, or inactive otherwise. (For GTK and WinUI, this simply checks if any of the app's windows are active; for AppKit and UIKit it checks the appropriate property of NS/UIApplication.shared.)
• If the property is declared on a View, it returns active if the enclosing window is active, or inactive otherwise.

This resolves #488.

Currently, there are only two scene states: active and inactive. We can consider adding a background state to match SwiftUI, though that'd require some extra bikeshedding on the exact behavior of background on desktop platforms -- personally I'd prefer to leave that for a future PR, but if it's important enough to add it now we can add it now.

Implemented backends

The property itself

• AppKitBackend
• GtkBackend
• Gtk3Backend
• UIKitBackend
• WinUIBackend

Rerendering when scene phase changes

• AppKitBackend
• GtkBackend (only for individual windows, not the whole app)
• Gtk3Backend (only for individual windows, not the whole app)
• UIKitBackend
• WinUIBackend

[MiaKoring]

what is the problem with rerendering on UIKit and WinUI? /cur

[kaascevich]

AFAIK there isn’t any, I just haven’t gotten around to it yet.

[MiaKoring]

ah, okay :)