当状态发生变化时如何运行操作？

enum SectionType: String, CaseIterable {
    case top = "Top"
    case best = "Best"
}

struct ContentView : View {
    @State private var selection: Int = 0

    var body: some View {
        SegmentedControl(selection: $selection) {
            ForEach(SectionType.allCases.identified(by: \.self)) { type in
                Text(type.rawValue).tag(type)
            }
        }
    }
}

如何运行代码（例如print("Selection changed to \(selection)")当。。。的时候$selection状态变化？我浏览了文档，但找不到任何东西。

iOS 14.0+

您可以使用onChange(of:perform:) https://developer.apple.com/documentation/swiftui/view/onchange(of:perform:)修饰符，像这样：

struct ContentView: View {
    
    @State private var isLightOn = false

    var body: some View {
        Toggle("Light", isOn: $isLightOn)
            .onChange(of: isLightOn) { value in
                if value {
                    print("Light is now on!")
                } else {
                    print("Light is now off.")
                }
            }
    }
}

iOS 13.0+

以下作为延伸Binding，因此只要值发生变化就可以执行闭包。

extension Binding {
    
    /// When the `Binding`'s `wrappedValue` changes, the given closure is executed.
    /// - Parameter closure: Chunk of code to execute whenever the value changes.
    /// - Returns: New `Binding`.
    func onUpdate(_ closure: @escaping () -> Void) -> Binding<Value> {
        Binding(get: {
            wrappedValue
        }, set: { newValue in
            wrappedValue = newValue
            closure()
        })
    }
}

像这样使用例如：

struct ContentView: View {
    
    @State private var isLightOn = false
    
    var body: some View {
        Toggle("Light", isOn: $isLightOn.onUpdate(printInfo))
    }
    
    private func printInfo() {
        if isLightOn {
            print("Light is now on!")
        } else {
            print("Light is now off.")
        }
    }
}

^{This example doesn't need to use a separate function. You only need a closure.}