How to initialize StateObject within SwiftUI with parameter - swift

Question

I'm working on a SwiftUI app using the MVVM architecture. I have the problem that I need to pass data from a parent View into its child viewModel, but I'm not sure how to pass a parameter from the view into the viewModel.
The child View which should own the viewModel is here:
struct InstanceView: View {
#ObservedObject var instance: Instance
#StateObject var viewModel = InstanceViewViewModel(instance: instance)
var body: some View {
...
}
}
And this is the viewModel:
class InstanceViewViewModel: ObservableObject {
#ObservedObject var instance: Instance
...
}
Obviously the View doesn't work, I get the error Cannot use instance member 'instance' within property initializer; property initializers run before 'self' is available
If I try using init() in the View to assign a value to viewModel:
#ObservedObject var instance: Instance
#StateObject var viewModel: InstanceViewViewModel
init(instance: Instance) {
self.instance = instance
self.viewModel = InstanceViewViewModel(instance: instance)
}
But I get the error Cannot assign to property: 'viewModel' is a get-only property.
I have read that you should give the viewModel property (instance) a default value, and then set it in the View with .onAppear(). However, in my case, Instance is a Core Data object so I can't really create one to use as a default value.
I also read that I could maybe use _instance = StateObject(wrappedValue: InstanceViewViewModel(instance: instance)) in the View's init, and swiftUI would be smart enough to only initialize the StateObject once. However I'm pretty sure this is a bad practice.
I also can't use lazy on a StateObject
So is there any other way to achieve this? Could a solution be somehow getting the value InstanceView is being initialized with on this line:
#StateObject var viewModel = InstanceViewViewModel(instance: instance)
, outside of an init, so I don't need to reference self, like you would do inside of an init? (this is probably a stupid idea)
Or should I be implementing MVVM in a different way?
Thanks!

Answer 1

SwiftUI property wrappers are often automagic. #StateObject modifies the property in a way that the initialization has to be at the point of declaration. The pattern suggested by Apple is to create the #StateObject in the parent of the view and pass it down either as an #ObservedObject or #EnvironmentObject.

Answer 2

It's not good practice to use MVVM view model objects with SwiftUI because in SwiftUI, the View struct value type is designed to hold the view data which is regarded as less buggy than using objects. The use of property wrappers, e.g. #State give these value types similar semantics to reference types, i.e. objects, so we get the best of both worlds. If you move your view data into view model objects instead of using the View struct correctly, then you lose these features and re-introduce the consistency bugs that SwiftUI was designed to eliminate. These structs form an efficiently updated (via dependency tracking) and diffed hierarchy and SwiftUI internally takes the result and they update UIKit (or AppKit) controls for us in a platform specific way, we don’t interact with the actual UIViews, NSView i.e. the V in the traditional MVC/MVVM sense at all.
The ‘ObservableObject’ protocol for reference types is part of the Combine framework so we only use that when we require a Combine pipeline which doesn’t appear to be required in your case. ‘#StateObject’ defines a source of truth, thus it cannot possibly take any parameters because then it wouldn’t be a source of truth.