Injecting dependencies in SwiftUI preview

[JanC]

hey there,
I'm having troubles to inject a dependency for a SwiftUI preview. I created a dependency with liveValue and previewValue and that works as expected

private enum APIClientKey: DependencyKey {
    static let liveValue: APIClient = RealClient()
    static let previewValue: APIClient = MyMockClient(result: "preview default value")
}

Then I'd like to preview a specific use case where I configure the MyMockClient dependency so I create my preview like this:

#Preview {
    NavigationStack {
        MySampleView(
            store: Store(
                initialState: MySampleFeature.State(),
                reducer: { MySampleFeature() },
                withDependencies: {
                    $0.apiClient = MyMockClient(result: "mocked result")
                }
            )
        )
    }
}

However, the preview still uses the instance of my mock created in static let previewValue: APIClient = MyMockClient(result: "preview default value").

I put some logs in the initialiser of MyMockClient and when I run the preview, I see it's created 2x when I run the SwiftUI preview:

MyMockClient.init mocked result
MyMockClient.init preview default value

The complete code to reproduce it:

import ComposableArchitecture
import Foundation
import SwiftUI

protocol APIClient {
    func fetch() -> String
}

class MyMockClient: APIClient {
    let result: String

    init(result: String = "live") {
        print("MyMockClient.init \(result)")
        self.result = result
    }

    func fetch() -> String {
        result
    }
}

private enum APIClientKey: DependencyKey {
    static let liveValue: APIClient = MyMockClient(result: "live value")
    static let previewValue: APIClient = MyMockClient(result: "preview default value")
}

extension DependencyValues {
    var apiClient: APIClient {
        get { self[APIClientKey.self] }
        set { self[APIClientKey.self] = newValue }
    }
}

@Reducer
struct MySampleFeature {
    @Dependency(\.apiClient) var apiClient

    @ObservableState
    struct State: Equatable {
        var title: String = ""
    }

    enum Action {
        case viewDidAppear
    }

    var body: some ReducerOf<Self> {
        Reduce { state, action in
            switch action {
            case .viewDidAppear:
                state.title = apiClient.fetch()
                return .none
            }
        }
    }
}

struct MySampleView: View {
    @Bindable
    var store: StoreOf<MySampleFeature>

    var body: some View {
        List {
            Text("Hello \(store.title)")
        }

        .onAppear(perform: {
            store.send(.viewDidAppear)
        })
        .navigationTitle("Sample")
    }
}

#Preview {
    NavigationStack {
        MySampleView(
            store: Store(
                initialState: MySampleFeature.State(),
                reducer: { MySampleFeature() },
                withDependencies: {
                    $0.apiClient = MyMockClient(result: "mocked result")
                }
            )
        )
    }
}

I also tried with the alternative syntax but the result is the same

#Preview {
    NavigationStack {
        MySampleView(
            store: Store(
                initialState: MySampleFeature.State(),
                reducer: {
                    MySampleFeature()
                        .dependency(\.apiClient, MyMockClient(result: "mocked result"))
                }
            )
        )
    }
}

What did I miss?

thanks in advance

[mbrandonw]

Hi @JanC, when I run your code in a preview it works as I expect:

The preview says "Hello mocked result". If you are seeing different can you provide a project that reproduces the problem?

[JanC]

So I also noticed that my described behavior sometimes works as expected. I'm trying find out what could be the reason.

[mbrandonw]

I'm sorry but I'm never able to ever produce the problem. I have run the preview many times and I have restarted Xcode, as well as tried Xcode 15.4 and Xcode 16 beta 6. Is there anything else special about your set up?

[JanC]

I think it could be related to Xcode 16 beta:

Xcode 15.3.x seems to work fine:

[mbrandonw]

Which beta are you using? It works fine for me in Xcode 16 beta 6.

[JanC]

Also Xcode 16 beta 6 🤷‍♂️

Anyway, it seems like an issue related to my local setup and probably not TCA. I'll try to debug further and will close this