Adding PreviewTrait support for SwiftUI previews

[alexanderwe]

Hi everyone,

I was working with Factory in SwiftUI previews and was always registering the dependencies that I need in my Previews in their body. While it works it didn't felt idiomatic to do so from a callers perspective. Therefore I was looking into PreviewTraits and came up with the following implementation:

@available(iOS 18.0, macOS 15.0, tvOS 18.0, watchOS 11.0, visionOS 2.0, macCatalyst 18.0, *)
 public static func register<C: SharedContainer, S>(
        on container: C.Type,
        _ path: KeyPath<C, Factory<S>> & Sendable,
        _ factory: @escaping @Sendable () -> S
    ) -> Self {
        .modifier(FactoryPreviewTrait(path: path, factory: factory))
    }


@available(iOS 18.0, macOS 15.0, tvOS 18.0, watchOS 11.0, visionOS 2.0, macCatalyst 18.0, *)
struct FactoryPreviewTrait<C: SharedContainer, S>: PreviewModifier {

    let path: KeyPath<C, Factory<S>> & Sendable
    let factory: @Sendable () -> S

    init(path: KeyPath<C, Factory<S>> & Sendable, factory: @escaping @Sendable () -> S) {
        self.path = path
        self.factory = factory
        C.shared[keyPath: path].register(factory: factory)
    }

    func body(content: Content, context _: Void) -> some View {
        C.shared[keyPath: path].register(factory: factory)
        return content
    }
}

Usage:

#Preview(traits: .register(on: PaymentsContainer.self, \.processor) { MockProcessor() }) {
     ContentView()
 }

I would like to gather some feedback if you think something like this would make sense to be part of the library or if its too narrow since its just a SwiftUI specific implementation and rather consumers should write similar extensions like this themselves.

Happy to hear your feedback