How to initialize @StateObject with parameters in SwiftUI

Today I want to talk about something that I do a lot when creating a view in SwiftUI, but it is not obvious how to do it. That is, initialize @StateObject with parameters.

What is the proper way to initialize @StateObject with parameters

To understand the problem, I set up a DashboardView, which shows a greeting message to a logged-in user (User) using a view model DashboardViewModel. Here are the implementation details.

struct User {
    let name: String // 1
}

class DashboardViewModel: ObservableObject {
    @Published var greeting: String
    
    init(name: String) {
        greeting = "Hello, \(name)!" // 2
    }
}

struct DashboardView: View {
    var user: User // 3
    @StateObject var viewModel = DashboardViewModel(name: user.name) // 4
    
    var body: some View {
        Text(viewModel.greeting) // 5
            .padding()
    }
}

1 A user model contains the name of the user.
2 DashboardViewModel accepts the name parameter and converts it to a greeting message.
3 DashboardView accepts user, <4> passes user's name to DashboardViewModel, and <5> showing greeting message in the text view.

Problem: property initializers run before 'self' is available

Xcode will show the following error, which is not a surprise. You can't use an instance property as an argument for another property.

Cannot use instance member 'user' within property initializer; property initializers run before 'self' is available

Problem: Lazy

It is tempting to use lazy, but that doesn't work either. You can't use lazy with a property wrapper.

@StateObject lazy var viewModel = DashboardViewModel(name: user.name)

Property 'viewModel' with a wrapper cannot also be lazy

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Solution

The solution for this problem needs three modifications in our view and view models.

Add a default value

First, we need to add a default value to our DashboardViewModel. So we can initialize it without passing parameters.

class DashboardViewModel: ObservableObject {
    @Published var greeting: String
    
    init(name: String? = nil) { // 1
        if let name = name {
            greeting = "Hello, \(name)!"
        } else {
            greeting = "Hello there"
        }
    }
}

1 We update our initializer to accept an optional string with a default value of nil.

With this change, we can initialize our view model without a problem.

struct DashboardView: View {
    var user: User
    
    @StateObject var viewModel = DashboardViewModel()
    
    var body: some View {
        Text(viewModel.greeting)
            .padding()
        
    }
}

But we need a way to update our view model to a passing user, which comes to the second change.

Expose a way to update the value

Since we no longer pass a name via the initializer, we need to expose another way to update it. I introduce a new update method in this case.

class DashboardViewModel: ObservableObject {
    @Published var greeting: String
    
    init(name: String? = nil) {
        if let name = name {
            greeting = "Hello, \(name)!"
        } else {
            greeting = "Hello there"
        }
    }
    
    func update(name: String) { // 1
        greeting = "Hello, \(name)!"
    }
}

1 An update method that accepts a new name and updates the greeting message.

Update the value in onAppear

The only thing left is to update our view model with a user name. We do this in onAppear().

struct DashboardView: View {
    var user: User
    
    @StateObject var viewModel = DashboardViewModel()
    
    var body: some View {
        Text(viewModel.greeting)
            .padding()
            .onAppear {
                viewModel.update(name: user.name) // 1
            }
        
    }
}

1 We update our view model as soon as the view appears.

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Conclusion

Even I named this post a "Proper way to initialize @StateObject with parameters in SwiftUI", I'm not sure this is the best way or not. It is the only way I found so far^[1]. I'm still new to SwiftUI and love to learn the better way to do this if you got one. You can direct message or tweet to me on Twitter.