ViewModels creation is not supported in Preview

2 min read 06-10-2024

ViewModels creation is not supported in Preview

ViewModel Creation in Preview: Not Supported, But Why?

You're trying to use a ViewModel within a Preview, and you're getting an error message like "ViewModel creation is not supported in Preview." This can be frustrating, especially when you want to visualize how your UI will look with specific data.

Understanding the Problem

The core issue is that Previews in SwiftUI are meant for static UI rendering, not dynamic updates driven by ViewModels. Previews are designed for visualising your UI structure and layout, not for showcasing the behavior of your app with real-time data.

Replicating the Scenario

Let's imagine a simple scenario where you're trying to preview a ContentView that uses a UserViewModel to display user data:

struct User: Identifiable {
    let id = UUID()
    let name: String
}

class UserViewModel: ObservableObject {
    @Published var user: User

    init(user: User) {
        self.user = user
    }
}

struct ContentView: View {
    @ObservedObject var viewModel = UserViewModel(user: User(name: "John Doe"))

    var body: some View {
        VStack {
            Text("Name: \(viewModel.user.name)")
        }
    }
}

#Preview {
    ContentView()
}

In this example, you're attempting to create a UserViewModel within the ContentView preview. This will result in the "ViewModel creation is not supported in Preview" error.

Why Previews Don't Support ViewModel Creation

Preview limitations: Previews are primarily focused on rendering UI elements statically. They don't simulate the full lifecycle of your app, including dependencies like view models.
Potential complexity: Introducing dynamic data handling through ViewModels within Previews could introduce unnecessary complexity and potentially break the previewing process.
Performance impact: Creating ViewModels for every preview could lead to performance issues, especially with complex applications.

Workarounds and Best Practices

Static Data: Instead of creating a ViewModel within the preview, use static data directly within the ContentView. This way, you can visually inspect how your UI will look with specific data:
```
#Preview {
    ContentView(user: User(name: "Jane Doe"))
}
```
Mock Data: Create a mock data structure that mimics the data your ViewModel would provide. This offers a more controlled environment for testing UI variations:
```
#Preview {
    ContentView(viewModel: UserViewModel(user: MockUser(name: "Jane Doe")))
}
```

Environment Variables: Use environment variables in your ContentView to manage the data being displayed. This allows you to change the data without modifying the ViewModel directly:

struct ContentView: View {
    @Environment(\.user) var user // Use a custom environment variable
    
    var body: some View {
        VStack {
            Text("Name: \(user.name)")
        }
    }
}

#Preview {
    ContentView()
        .environment(\.user, User(name: "Jane Doe")) 
}

PreviewProvider with Static Data: Define a dedicated PreviewProvider to control the data used in previews:

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(viewModel: UserViewModel(user: User(name: "Jane Doe")))
    }
}

Conclusion

Understanding the reasons behind the "ViewModel creation not supported in Preview" error helps you develop better strategies for previewing your SwiftUI views. By using static data, mock objects, environment variables, or dedicated PreviewProviders, you can ensure a smooth previewing experience without compromising your app's dynamic data handling.