Resolve Nested ObservableObject issues in Observation

ObservableObjectをプロパティに持つObsevableObjectはCombineの監視の仕組み上問題があります。Observationでは、Combineの監視とは異なる手段で監視することでこの問題を解決しています。
CombineとObservationの監視の仕組みの違いを見て、元の問題がどう解決されるかを紹介します。

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5. final class Parent: ObservableObject {
   @Published var child: Child
   init(child: Child) {
   self.child = child
   }
   }
   final class Child: ObservableObject {
   @Published var age: Int
   init(age: Int) {
   self.age = age
   }
   func haveBirthday() {
   age += 1
   }
   }
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6. /FTUFE0CTFSWBCMF0CKFDU
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   final class Parent: ObservableObject {
   @Published var child: Child
   init(child: Child) {
   self.child = child
   }
   }
   final class Child: ObservableObject {
   @Published var age: Int
   init(age: Int) {
   self.age = age
   }
   func haveBirthday() {
   age += 1
   }
   }
   struct BirthdayView: View {
   @StateObject var parent = Parent(child: Child(age: 1))
   var body: some View {
   Text("child is \(parent.child.age) years old")
   Button("Have Birthday", action: { parent.child.haveBirthday() })
   }
   }
   

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7. final class Parent: ObservableObject {
   @Published var child: Child
   private var cancellables = Set()
   init(child: Child) {
   self.child = child
   child.objectWillChange.sink {
   [weak self] in self?.objectWillChange.send()
   }
   .store(in: &cancellables)
   }
   }
   final class Child: ObservableObject {
   @Published var age: Int
   init(age: Int) {
   self.age = age
   }
   func haveBirthday() {
   age += 1
   }
   }
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8. final class Parent: ObservableObject {
   @Published var child: Child
   private var cancellables = Set()
   init(child: Child) {
   self.child = child
   child.objectWillChange.sink {
   [weak self] in self?.objectWillChange.send()
   }
   .store(in: &cancellables)
   }
   }
   final class Child: ObservableObject {
   @Published var age: Int
   init(age: Int) {
   self.age = age
   }
   func haveBirthday() {
   age += 1
   }
   }
   struct BirthdayView: View {
   @StateObject var parent = Parent(child: Child(age: 1))
   var body: some View {
   Text("child is \(parent.child.age) years old")
   Button("Have Birthday", action: { parent.child.haveBirthday() })
   }
   }
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9. /FTUFE!0CTFSWBCMF
   

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10. @Observable final class Parent {
    var child: Child
    init(child: Child) {
    self.child = child
    }
    }
    @Observable final class Child {
    var age: Int
    init(age: Int) {
    self.age = age
    }
    func haveBirthday() {
    age += 1
    }
    }
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    @Observable final class Parent {
    var child: Child
    init(child: Child) {
    self.child = child
    }
    }
    @Observable final class Child {
    var age: Int
    init(age: Int) {
    self.age = age
    }
    func haveBirthday() {
    age += 1
    }
    }
    struct BirthdayView: View {
    @State var parent = Parent(child: Child(age: 1))
    var body: some View {
    Text("child is \(parent.child.age) years old")
    Button("Have Birthday", action: { parent.child.haveBirthday() })
    }
    }
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    final class Child {
    @ObservationTracked var age: Int
    init(age: Int) {
    self.age = age
    }
    func haveBirthday() {
    age += 1
    }
    @ObservationIgnored private let _$observationRegistrar = Observation.ObservationRegistrar()
    internal nonisolated func access(
    keyPath: KeyPath
    ) {
    _$observationRegistrar.access(self, keyPath: keyPath)
    }
    internal nonisolated func withMutation(
    keyPath: KeyPath,
    _ mutation: () throws -> MutationResult
    ) rethrows -> MutationResult {
    try _$observationRegistrar.withMutation(of: self, keyPath: keyPath, mutation)
    }
    }
    extension Child: Observation.Observable {
    }
    

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    final class Child {
    var age: Int{
    @storageRestrictions(initializes: _age)
    init(initialValue) {
    _age = initialValue
    }
    get {
    access(keyPath: \.age)
    return _age
    }
    set {
    withMutation(keyPath: \.age) {
    _age = newValue
    }
    }
    }
    …
    }
    

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