duckSern1108's questions

我有一个简单的注册表格，当用户按下时Continue Button，如果他们没有输入任何字段，我会显示一个弹出窗口。

struct ContentView: View {
    @State var firstName = ""
    @State var lastName = ""
    
    @State private var validationMessage = ""
    @State private var showValidationMessage = false
    
    private var isFormValid: Bool {
        !firstName.isEmpty &&
        !lastName.isEmpty
    }
    
    private func checkGeneralFormCompletion() {
        if isFormValid {
            showValidationMessage = false
        } else {
            let errorText = {
                if firstName.isEmpty {
                    return "First name empty"
                } else if lastName.isEmpty {
                    return "Last name empty"
                } else {
                    return ""
                }
            }()
            validationMessage = errorText
            showValidationMessage = !errorText.isEmpty
        }
    }
    
    var body: some View {
        
        VStack(spacing: 12) {
            let _ = Self._printChanges()
            
            TextField("First Name", text: $firstName)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding(.bottom, 10)
            
            TextField("Last Name", text: $lastName)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding(.bottom, 10)
            
            Button(action: {
                checkGeneralFormCompletion()
            }) {
                Text("Continue")
                    .foregroundColor(.white)
                    .padding(.horizontal, 45)
                    .padding([.top, .bottom], 10)
                    .background( Color.red)
                    .cornerRadius(5)
            }
            
            .popover(isPresented: self.$showValidationMessage,
                     attachmentAnchor: .point(.top),
                     arrowEdge: .top,
                     content: { [validationMessage] in
                let _ = print("validationMessage :: popover :: ", validationMessage)
                VStack {
                    Text(validationMessage)
                }
                .multilineTextAlignment(.center)
                .lineLimit(0)
                .foregroundStyle(.black)
                .font(.system(size: 18, weight: .semibold, design: .rounded))
                .padding()
                .presentationCompactAdaptation(.none)
                .fixedSize(horizontal: false, vertical: true)
                .frame(minWidth: 200)
            })
            .padding(.top, 70)
            
        }
        .padding()
//        .onChange(of: validationMessage) { oldValue, newValue in
//            print("CHANGE text VALIDATE MSG")
//        }
//        .onChange(of: showValidationMessage) { oldValue, newValue in
//            print("CHANGE VALIDATE MSG")
//        }
    }
}

如果我注释掉强捕获[validationMessage] in，当用户第一次按下按钮时，弹出窗口将显示空文本并且body不会重新渲染（基于调用）。但是如果我为of_printChanges提供强捕获或添加修饰符，则主体将重新渲染并且弹出窗口将显示正确的文本。closure.popover.onChangeVStack

我的问题是，为什么当我提供 的强捕获时@State，它会导致body重新渲染？任何见解或建议都将不胜感激！

protocol Animal {
    func makeNoise()
    static var species: String { get }
}
struct Dog: Animal {
    func makeNoise() { print("Woof") }
    static var species: String = "Canus familiaris"
}
struct Cat: Animal {
    func makeNoise() { print("Meow") }
    static var species: String = "Felis catus"
}

var animal: Animal // `Animal` is used here as a type.
animal = Dog()
animal.makeNoise() // Prints "Woof".
animal = Cat()
animal.makeNoise() // Prints "Meow".

func declareAnimalSpecies<T: Animal>(_ animal: T) {
    animal.makeNoise()
    print("My species is known as (T.species)")
}

let dog = Dog()
declareAnimalSpecies(dog)
// Prints:
// "Woof"
// "My species is known as Canus familiaris"
declareAnimalSpecies(animal) // <- not show error here
// error: protocol type 'Animal' cannot conform to 'Animal'...`

我从swift git中获取示例代码。据我所知，animal的类型是，any Animal因此当它传递给函数 时declareAnimalSpecies，编译器将推断 T 是any Animal，any Animal cannot conform to Animal因此我预计它会显示该错误。