import Foundation
import SwiftUI

struct BlinkDemo : View {

    struct Blink : View {
        let period: TimeInterval
        @State var timer: Timer? = nil
        @State var isVisible: Bool = true

        var body: some View {
            let text = Text("DISCO")
            return Group {
                if isVisible {
                    text
                } else {
                    text.opacity(0)
                }
            }.onAppear {
                self.timer = Timer.scheduledTimer(withTimeInterval: self.period, repeats: true) { _ in
                    self.isVisible.toggle()
                }
            }.onDisappear {
                self.timer?.invalidate()
                self.timer = nil
            }
        }
    }

    @State var period: TimeInterval = 1.0;

    var body: some View {
        HStack {
            Button(action: self.slowDown) {
                Text("Slow down")
            }
            Blink(period: self.period)
            Button(action: self.speedUp) {
                Text("Speed up")
            }
        }
    }

    func speedUp() {
        self.period *= 0.8;
    }

    func slowDown() {
        self.period *= 1.25;
    }
}

#if DEBUG
struct Blink_Previews : PreviewProvider {
    static var previews: some View {
        BlinkDemo()
    }
}
#endif