SwiftUIで引数からViewを渡す。

struct TestView<Content: View>: View {
    
    let left: Content
    let right: Content

    init(left: Content, right: Content) {
        self.left = left
        self.right = right
    }
    
    var body: some View  {
        
        GeometryReader { geometry in
            HStack() {
                
                left
                    .frame(height: 50, alignment: .bottom)
                    .background(Color.red)
                right
                    .frame(height:50, alignment: .bottom)
                    .background(Color.blue)

            }
            .frame(height:50, alignment: .bottom)

        }
    }
}


        TestView(
            left: Text("100").customFont(size: 30),
            right: Text("kg").customFont(size: 20)
        ).frame(height: 40)

viewを囲って使

struct A: View {
    @State var isActive = false
    @State var number = Int.random(in: 1..<10).description

    var body: some View {
        NavigationView {
            Container {
                VStack {
                    Button(action: {
                        self.isActive.toggle()
                    }) {
                        Text("移動A\(number)")
                    }
                    NavigationLink(destination: B(),isActive: $isActive) {
                        EmptyView()
                    }
                }
            }
        }.navigationViewStyle(StackNavigationViewStyle())

    }
}


struct Container <Content : View> : View {
    
    var content : Content
    @State var isLogin = false

    init(@ViewBuilder content: () -> Content) {
        self.content = content()
    }
    
    var body: some View {
        ZStack {
            Color.yellow.edgesIgnoringSafeArea(.all)
            VStack {
                VStack() {
                    if isLogin {
                        content
                    } else {
                        Text("ログインへ")
                    }
                }
            }
        }.onAppear() {
            isLogin = false
        }
    }
}

@Stateでdidset,WillSetは使える。

 
 @State var form: Form = .none {
    
        didSet(oldValue) {}
        
        willSet(newValue) {}
    }