Unable to initialize @StateObject ViewModel in SwiftUI

Question

I am trying to inject the managedObjectContext in ViewModel and for some weird reason it is throwing a weird error.

struct ContentView: View {
   
    @Environment(\.managedObjectContext) var viewContext
    @StateObject var addBudgetVM: AddBudgetViewModel
    
    init() {
        // THIS LINE CAUSES ISSUES 
        addBudgetVM = AddBudgetViewModel(context: viewContext)
    }
    
    var body: some View {
       // some code here
    }
    
}

Cannot assign to property: 'addBudgetVM' is a get-only property

Here is the implementation of AddBudgetViewModel

import Foundation
import CoreData

class AddBudgetViewModel: ObservableObject {
    
    @Published var name: String = ""
    var context: NSManagedObjectContext
    
    init(context: NSManagedObjectContext) {
        self.context = context
    }
    
    func save() {
        
    }
    
}

see this answer: https://stackoverflow.com/questions/62635914/initialize-stateobject-with-a-parameter-in-swiftui — workingdog support Ukraine, Nov 07 '21 at 02:10
You could make the context optional on your view model and set it on `onAppear` — jnpdx, Nov 07 '21 at 03:22
I agree with @JoakimDanielson. You should inject the view model from the superview — Paulw11, Nov 07 '21 at 09:48
If I inject the ViewModel into the View it will create an instance of the VM for each time the View is rendered. — Mary Doe, Nov 07 '21 at 12:58
If you inject the view model then a new view model instance will be created if the superview is re-rendered (The one that injected the view model), but not if things change with the view that is using the view model. — Paulw11, Nov 08 '21 at 04:37
This is SwiftUI you shouldn’t be using view model objects you should instead be using structs and value semantics. Also ObservableObject and @Published are part of the Combine framework but you don’t have a Combine pipeline. — malhal, Nov 18 '21 at 01:00

score 1 · Answer 1 · answered Nov 08 '21 at 04:44

I find the best approach is to inject the view model from the superview. This nicely separates the view, which consumes the view model, from the creation of the view model. It improves testability as it makes it easy to inject a mock view model.

Something like:


struct ParentView: View {
      @Environment(\.managedObjectContext) var viewContext

      var body: some View {
          //...
          AddBudgetView(viewModel: AddBudgetViewModel(context: viewContext))
          //...
      }
}


struct AddBudgetView: View {

      viewModel: AddBudgetViewModel 

      var body: some View {
          // ...
      }
}

class AddBudgetViewModel: ObservableObject {

    private (set) var context: NSManagedObjectContext

    init(context: NSManagedObjectContext) {
    
        self.context = context
    }

   func save() {
    
   }
}

I probably wouldn't even have Core Data code in the view model. I would put all of that in a main model and pass the main model instance to the view model initialiser instead of the managed object context. The view model can then call methods on the main model to create and save items. This way your view model is isolated from the storage implementation detail and you could make changes to your store, such as replacing Core Data, without having to touch view models all over the place.

Unable to initialize @StateObject ViewModel in SwiftUI

1 Answers1