避免 SwiftUI 视图的重复计算

public protocol DynamicProperty {
  static func _makeProperty<V>(in buffer: inout _DynamicPropertyBuffer, container: _GraphValue<V>, fieldOffset: Int, inputs: inout _GraphInputs)
  static var _propertyBehaviors: UInt32 { get }
  mutating func update()
}

@propertyWrapper public struct State<Value> : DynamicProperty {
  internal var _value: Value
  internal var _location: SwiftUI.AnyLocation<Value>? // SwiftUI 托管数据池中的数据引用
  public init(wrappedValue value: Value)
  public init(initialValue value: Value) {
        _value = value // 创建实例时，只会暂存初始值
    }
  public var wrappedValue: Value {
    get  //  guard let _location else { return _value} ...
    nonmutating set // 只能改动 _location 指向的数据
  }
  public var projectedValue: SwiftUI.Binding<Value> {
    get
  }
  // 在将视图加载到视图树中时，调用此方法，完成关联工作
  public static func _makeProperty<V>(in buffer: inout _DynamicPropertyBuffer, container: _GraphValue<V>, fieldOffset: Int, inputs: inout _GraphInputs)
}

struct TestView: View {
    @State private var number: Int = 10
    init(number: Int) {
        self.number = 11 // 更改无效
    }
    var body: some View {
        Text("\(number)") // 首次运行，显示 10
    }
}

@propertyWrapper public struct StateObject<ObjectType> : DynamicProperty where ObjectType : ObservableObject {
  internal enum Storage { // 通过内部定义的枚举来标注视图是否已经被加载、数据是否已被数据池托管
    case initially(() -> ObjectType)
    case object(ObservedObject<ObjectType>)
  }

  internal var storage: StateObject<ObjectType>.Storage
  public init(wrappedValue thunk: @autoclosure @escaping () -> ObjectType) {
        storage = .initially(thunk) // 初始化，视图尚未加载
    }
  @_Concurrency.MainActor(unsafe) public var wrappedValue: ObjectType {
    get
  }
  @_Concurrency.MainActor(unsafe) public var projectedValue: SwiftUI.ObservedObject<ObjectType>.Wrapper {
    get
  }
    // 在 DynamicProperty 要求的方法中，实现将实例保存在托管数据池，并将视图与托管实例的 objectWillChange 进行关联
  public static func _makeProperty<V>(in buffer: inout _DynamicPropertyBuffer, container: _GraphValue<V>, fieldOffset: Int, inputs: inout _GraphInputs)
}

@ObservedObject var store = Store() // 每次创建视图类型实例，都会重新创建 Store 实例

struct EnvObjectDemoView:View{
    @EnvironmentObject var store:Store
    var body: some View{
        Text("abc")
    }
}

struct MyEnvKey: EnvironmentKey {
    static var defaultValue = 10
}

extension EnvironmentValues {
    var myValue: Int {
        get { self[MyEnvKey.self] }
        set { self[MyEnvKey.self] = newValue }
    }
}

struct EnvDemo: View {
    @State var i = 100
    var body: some View {
        VStack {
            VStack {
                EnvSubView()
            }
            .environment(\.myValue, i)
            Button("change") {
                i = Int.random(in: 0...100)
            }
        }
    }
}

struct EnvSubView: View {
    @Environment(\.myValue) var myValue // 声明了，但并没有在 body 中使用
    var body: some View {
        let _ = print("sub view update")
        Text("Sub View")
    }
}

struct SubView{
    let name:String
    let age:Int
    
    var body: some View{
        VStack{
            Text(name)
            Text("\(age)")
        }
    }
}

struct ContentView {
    var body: some View{
        SubView(name: "fat" , age: 99)
    }
}

struct Student {
    var name: String
    var age: Int
}

struct RootView:View{
    @State var student = Student(name: "fat", age: 88)
    var body: some View{
        VStack{
            StudentNameView(student: student)
            StudentAgeView(student: student)
            Button("random age"){
                student.age = Int.random(in: 0...99)
            }
        }
    }
}

// 分成小视图
struct StudentNameView:View{
    let student:Student
    var body: some View{
        let _ = Self._printChanges()
        Text(student.name)
    }
}

struct StudentAgeView:View{
    let student:Student
    var body: some View{
        let _ = Self._printChanges()
        Text(student.age,format: .number)
    }
}

struct RootView:View{
    @State var student = Student(name: "fat", age: 88)
    var body: some View{
        VStack{
            StudentNameView(name: student.name) // 仅传递需要的数据
            StudentAgeView(age:student.age)
            Button("random age"){
                student.age = Int.random(in: 0...99)
            }
        }
    }
}

struct StudentNameView:View{
    let name:String // 需要的数据
    var body: some View{
        let _ = Self._printChanges()
        Text(name)
    }
}

struct StudentAgeView:View{
    let age:Int
    var body: some View{
        let _ = Self._printChanges()
        Text(age,format: .number)
    }
}

struct RootView: View {
    @State var student = Student(name: "fat", age: 88)
    var body: some View {
        VStack {
            StudentNameView(student: student)
            StudentAgeView(student: student)
            Button("random age") {
                student.age = Int.random(in: 0...99)
            }
        }
    }
}

struct StudentNameView: View, Equatable {
    let student: Student
    var body: some View {
        let _ = Self._printChanges()
        Text(student.name)
    }

    static func == (lhs: Self, rhs: Self) -> Bool {
        lhs.student.name == rhs.student.name
    }
}

struct StudentAgeView: View, Equatable {
    let student: Student
    var body: some View {
        let _ = Self._printChanges()
        Text(student.age, format: .number)
    }

    static func== (lhs: Self, rhs: Self) -> Bool {
        lhs.student.age == rhs.student.age
    }
}

struct ClosureDemo: View {
    @StateObject var store = MyStore()
    var body: some View {
        VStack {
            if let currentID = store.selection {
                Text("Current ID: \(currentID)")
            }
            List {
                ForEach(0..<100) { i in
                    CellView(id: i){ store.sendID(i) } // 使用尾随闭包的方式为子视图设定按钮动作
                }
            }
            .listStyle(.plain)
        }
    }
}

struct CellView: View {
    let id: Int
    var action: () -> Void
    init(id: Int, action: @escaping () -> Void) {
        self.id = id
        self.action = action
    }

    var body: some View {
        VStack {
            let _ = print("update \(id)")
            Button("ID: \(id)") {
                action()
            }
        }
    }
}

class MyStore: ObservableObject {
    @Published var selection:Int?

    func sendID(_ id: Int) {
        self.selection = id
    }
}

CellView(id: i){ store.sendID(i) } 

struct CellView: View, Equatable {
    let id: Int
    var action: () -> Void
    init(id: Int, action: @escaping () -> Void) {
        self.id = id
        self.action = action
    }

    var body: some View {
        VStack {
            let _ = print("update \(id)")
            Button("ID: \(id)") {
                action()
            }
        }
    }

    static func == (lhs: Self, rhs: Self) -> Bool { // 将 action 排除在比较之外
        lhs.id == rhs.id
    }
}

ForEach(0..<100) { i in
    CellView(id: i){ store.sendID(i) }
}

struct CellView: View {
    let id: Int
    var action: (Int) -> Void // 修改函数定义
    init(id: Int, action: @escaping (Int) -> Void) {
        self.id = id
        self.action = action
    }

    var body: some View {
        VStack {
            let _ = print("update \(id)")
            Button("ID: \(id)") {
                action(id)
            }
        }
    }
}

ForEach(0..<100) { i in
    CellView(id: i, action: store.sendID) // 直接传递 store 中的 sendID 方法，将 store 排除在外
}

struct EventSourceTest: View {
    @State private var enable = false

    var body: some View {
        VStack {
            let _ = Self._printChanges()
            Button(enable ? "Stop" : "Start") {
                enable.toggle()
            }
            TimeView(enable: enable) // 单独的视图，onReceive 只能导致 TimeView 被更新
        }
    }
}

struct TimeView:View{
    let enable:Bool
    @State private var timestamp = Date.now
    var body: some View{
        let _ = Self._printChanges()
        Text(timestamp, format: .dateTime.hour(.twoDigits(amPM: .abbreviated)).minute(.twoDigits).second(.twoDigits))
            .background(
                Group {
                    if enable { // 只在需要使用时，才加载触发器
                        Color.clear
                            .task {
                                while !Task.isCancelled {
                                    try? await Task.sleep(nanoseconds: 1000000000)
                                    NotificationCenter.default.post(name: .test, object: Date())
                                }
                            }
                            .onReceive(NotificationCenter.default.publisher(for: .test)) { notification in
                                if let date = notification.object as? Date {
                                    timestamp = date
                                }
                            }
                    }
                }
            )
    }
}

extension Notification.Name {
    static let test = Notification.Name("test")
}