Issue overriding dependencies for previews

[nashysolutions]

My preview isn't using the mock :(. I don't know what I'm doing wrong.

struct RoomsView_Previews: PreviewProvider {
    
    private static var service: RoomsService {
        withDependencies {
            let rooms = [Room(id: UUID(), name: "Decisions")]
            let data = try! Encoder.make().encode(rooms)
            $0.networkClient = .mock(using: data)
        } operation: {
            RoomsService()
        }
    }
    
    static var previews: some View {
        NavigationStack {
            RoomsView(service: service)
        }
    }
}

struct NetworkClient {
    
    typealias Payload = (Data, URLResponse)
    typealias Transaction = (_ request: URLRequest) async throws -> Payload
    
    var execute: Transaction
    
    init(execute: @escaping Transaction) {
        self.execute = execute
    }
}

extension NetworkClient {
    
    static func mock(using data: Data) -> NetworkClient {
        NetworkClient(execute: { _ in
            return (data, stub)
        })
    }
    
    private static var stub: URLResponse {
        HTTPURLResponse(
            url: URL(string: "https://www.google.co.uk")!,
            statusCode: 200,
            httpVersion: nil,
            headerFields: nil
        )!
    }
}

struct NetworkClientDependencyKey: DependencyKey {
    
    static var liveValue: NetworkClient {
        let session = URLSession.shared
        return NetworkClient(execute: { request in
            try await session.data(for: request)
        })
    }
}

If I add a preview value here, it works.

struct NetworkClientDependencyKey: DependencyKey {
    
    static var liveValue: NetworkClient {
        let session = URLSession.shared
        return NetworkClient(execute: { request in
            try await session.data(for: request)
        })
    }
    
    static var previewValue: NetworkClient {
        let rooms = [Room(id: UUID(), name: "Judgements")]
        let data = try! Encoder.make().encode(rooms)
        return .mock(using: data)
    }
}

[joprice]

I have the same issue. I can only use the previewValue, so I can't have different data in different previews. I've tried many different approaches with withDependencies and all of them crash the preview. Is there a pattern that works where a default preview is defined and some of the previews can override the mock selectively?

[mbrandonw]

Hi @nashysolutions, there is not enough information here to know what the problem is. You need to share at least RoomsService, but ideally you would distill the problem down into a minimal project for us to look at.

[kamcma]

Something like this.

import Dependencies
import SwiftUI

struct NumberClient {
  var getNumber: @Sendable () -> Int
}

extension NumberClient: DependencyKey {
  static let liveValue = NumberClient(
    getNumber: { 42 }
  )

  static let previewValue = NumberClient(
    getNumber: { 1 }
  )
}

extension DependencyValues {
  var numberClient: NumberClient {
    get { self[NumberClient.self] }
    set { self[NumberClient.self] = newValue }
  }
}

class FeatureModel: ObservableObject {
  var value: Int

  init() {
    @Dependency(\.numberClient) var numberClient
    self.value = numberClient.getNumber()
  }
}

struct FeatureView: View {
  @State var model = FeatureModel()

  var body: some View {
    Text("\(model.value)")
  }
}

#Preview("default preview dependency") {
  FeatureView(
    model: FeatureModel()
  )
}

#Preview("preview with dependency 2") {
  FeatureView(
    model: withDependencies {
      $0.numberClient = NumberClient(
        getNumber: { 2 }
      )
    } operation: {
      FeatureModel()
    }
  )
}

#Preview("preview with dependency 3") {
  FeatureView(
    model: withDependencies {
      $0.numberClient = NumberClient(
        getNumber: { 3 }
      )
    } operation: {
      FeatureModel()
    }
  )
}

If you're using TCA, don't use withDependencies directly; use the Store initializer that takes a dependencies argument.

[mbrandonw]

Hi @kamcma, just so that I understand, you are sharing this as an example of it working, correct? Because this runs how I would expect in Xcode.

[kamcma]

Yes, sorry for being unclear. Was just providing a working example. Sometimes people use withDependencies in incorrect ways. (Myself included, in the past.)

[nashysolutions]

After re-visiting this code, I realised that it was a threading issue on my end. I'm sorry for wasting your time, and greatly appreciate your responses.

[acosmicflamingo]

Don't be so hard on yourself; this post may save future developers some hassle because they will find this GitHub issue and find out that it was just a threading issue on their end :)