Why does my SwiftUI view not get onChange updates from a @Binding member of a @StateObject?

Question

Given the setup I've outlined below, I'm trying to determine why ChildView's .onChange(of: _) is not receiving updates.

import SwiftUI

struct SomeItem: Equatable {
    var doubleValue: Double
}

struct ParentView: View {
    @State
    private var someItem = SomeItem(doubleValue: 45)

    var body: some View {
        Color.black
            .overlay(alignment: .top) {
                Text(someItem.doubleValue.description)
                    .font(.system(size: 50))
                    .foregroundColor(.white)
            }
            .onTapGesture { someItem.doubleValue += 10.0 }
            .overlay { ChildView(someItem: $someItem) }
    }
}

struct ChildView: View {
    @StateObject
    var viewModel: ViewModel

    init(someItem: Binding<SomeItem>) {
        _viewModel = StateObject(wrappedValue: ViewModel(someItem: someItem))
    }

    var body: some View {
        Rectangle()
            .fill(Color.red)
            .frame(width: 50, height: 70, alignment: .center)
            .rotationEffect(
                Angle(degrees: viewModel.someItem.doubleValue)
            )
            .onTapGesture { viewModel.changeItem() }
            .onChange(of: viewModel.someItem) { _ in
                print("Change Detected", viewModel.someItem.doubleValue)
            }
    }
}


@MainActor
final class ViewModel: ObservableObject {
    @Binding
    var someItem: SomeItem

    public init(someItem: Binding<SomeItem>) {
        self._someItem = someItem
    }

    public func changeItem() {
        self.someItem = SomeItem(doubleValue: .zero)
    }
}

Interestingly, if I make the following changes in ChildView, I get the behavior I want.

• Change @StateObject to @ObservedObject

• Change _viewModel = StateObject(wrappedValue: ViewModel(someItem: someItem)) to viewModel = ViewModel(someItem: someItem)

From what I understand, it is improper for ChildView's viewModel to be @ObservedObject because ChildView owns viewModel but @ObservedObject gives me the behavior I need whereas @StateObject does not.

Here are the differences I'm paying attention to:

• When using @ObservedObject, I can tap the black area and see the changes applied to both the white text and red rectangle. I can also tap the red rectangle and see the changes observed in ParentView through the white text.
• When using @StateObject, I can tap the black area and see the changes applied to both the white text and red rectangle. The problem lies in that I can tap the red rectangle here and see the changes reflected in ParentView but ChildView doesn't recognize the change (rotation does not change and "Change Detected" is not printed).

Is @ObservedObject actually correct since ViewModel contains a @Binding to a @State created in ParentView?

Answer 1

Actually we don't use view model objects at all in SwiftUI, see [Data Essentials in SwiftUI WWDC 2020]. As shown in the video at 4:33 create a custom struct to hold the item, e.g. ChildViewConfig and init it in an @State in the parent. Set the childViewConfig.item in a handler or add any mutating custom funcs. Pass the binding $childViewConfig or $childViewConfig.item to the to the child View if you need write access. It's all very simple if you stick to structs and value semantics.