Fix NavigationView broken state after re-render

[MaxDesiatov]

After #241 was merged, NavigationView started using @ObservedObject instead of @State for its destination storage. This uncovered a bug where @ObservedObject properties that have their values provided in-place lost their subscriptions after a re-renderer. This manifested in navigation links not working in the demo after any scene phase changes that trigger a re-render. This was caused by a new NavigationContext being created on every render, but a corresponding MountedCompositeElement instance kept dangling subscription to the old NavigationContext instance, while new subscriptions weren't created.

This PR splits subscriptions on MountedCompositeElement into separate "transient" and "persistent" arrays. Persistent subscriptions are set up once when an element is mounted, and are automatically cleaned up when the element is unmounted. Scene phase and color scheme subscriptions on MountedApp are now primary examples of these persistent subscriptions, but @StateObject is another future use case to be implemented in a separate PR. Transient subscriptions are recreated on every re-render to allow new @ObservedObject instances to establish their subscriptions.

NavigationView in the demo still resets to the empty destination view when the scene phase observer is triggered, but NavigationLink selections are fixed now. Resetting to the empty destination view on scene phase changes will be fixed when @StateObject implementation is available in a separate PR.

[j-f1]

LGTM! Why not make all the subscriptions persistent?

[MaxDesiatov]

They were persistent initially, but as I mentioned in the description, when re-rendered new ObservableObject instances are created, which invalidates old subscriptions.

[j-f1]

when re-rendered new ObservableObject instances are created, which invalidates old subscriptions.

Is that intended behavior or a deviation from SwiftUI?

[MaxDesiatov]

That's intended and is the reason for adding @StateObject this year to the upstream API, which persists across re-renders.

[MaxDesiatov]

BTW, if the instance is created outside of the element hierarchy or in an ancestor element that's not re-rendered, it persists. That was the recommended way to work around it, but NavigationView needs to instantiate the context on its own.

Also, more details are available here: https://stackoverflow.com/questions/62544115/what-is-the-difference-between-observedobject-and-stateobject-in-swiftui