【SwiftUI】プロパティの変更を検知する方法【onChange】

onChange(of value: Equatable, perform action: (Equatable) -> Void)) -> View

struct ContentView: View {
    
    @State var currentText = ""
    let maxTextLength = 15
    
    var body: some View {
        VStack {
            Spacer(minLength: 32)
            Text("入力文字数：\(currentText.count)")
            TextEditor(text: $currentText)
                .onChange(of: currentText) { value in
                    if value.count > maxTextLength {
                        currentText.removeLast(currentText.count - maxTextLength)
                    }
                }
            Spacer(minLength: 32)
        }
    }
}

struct ContentView: View {
    
    @State var showTimerView = false
    
    var body: some View {
        NavigationView {
            VStack {
                Text("Content View")
                NavigationLink(destination: TimerView(active: $showTimerView), isActive: $showTimerView) {
                    Text("Show Timer View")
                }
            }
        }
    }
}

struct TimerView: View {
    
    @State var counter = 5
    @Binding var active: Bool
    
    var body: some View {
        VStack {
            Text("Timer View")
            Text("Timer Count: \(counter)")
                .onChange(of: counter) { value in
                    if value == 0 {
                        active = false
                    }
                }
        }
        .onAppear() {
            Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
                self.counter -= 1
                if self.counter == 0 {
                    timer.invalidate()
                }
            }
        }
    }
}