Timer.swift

import SwiftUI
import Combine

struct HMS {
  var h: Int
  var m: Int
  var s: Int
}

struct ContentView: View {
  @State var enteredValue = ""
  @State var seconds = 60
  @State var isRunning = false
  @State var elapsed = 0
  var timer = Timer.publish(every: 1.0, on: .main, in: .common).autoconnect()
  
  var body: some View {
    let hms = sToHMS(self.seconds - self.elapsed)
    return VStack {
      Spacer()
      if !self.isRunning {
        VStack(alignment: .leading, spacing: 4) {
          Text("Seconds:")
            .bold()
          TextField("Seconds", text: $enteredValue)
            .onReceive(Just(enteredValue)) { typedValue in
              if let newValue = Int(typedValue) {
                self.seconds = newValue
              }
          }.onAppear(perform:{self.enteredValue = "\(self.seconds)"})
            .font(.system(size: 56, weight: .bold, design: .default))
          Text(String(format: "(%01d hours, %01d minutes, %01d seconds)", hms.h, hms.m, hms.s))
          
        }
      } else {
        Text(String(format: "%02d:%02d:%02d", hms.h, hms.m, hms.s))
          .font(Font.system(size: 72, weight: .bold, design: .default).monospacedDigit())
      }
      
      Spacer()
      Spacer()
      
      Button(action: self.toggleRunning) {
        HStack {
          Text(self.isRunning ? "Stop" : "Start")
            .font(Font.system(size: 16, weight: .bold, design: .default))
            .foregroundColor(self.isRunning ? Color.white : Color.black)
          .frame(minWidth: 100, maxWidth: .infinity, minHeight: 44, maxHeight: 44, alignment: .center)
            .padding(.horizontal, 12)
            .padding(.vertical, 4)
            .background(Capsule().fill(self.isRunning ? Color.red : Color.orange))
            .cornerRadius(8)
        }
      }

        .buttonStyle(PlainButtonStyle())
    }
      .padding()
    .frame(maxWidth: .infinity, maxHeight: .infinity)
      .onReceive(self.timer) { time in
        if(self.isRunning && self.elapsed < self.seconds) {
          self.elapsed += 1
        }
    }.background(Rectangle().fill(Color.black)).colorScheme(.dark)
  }
  
  func toggleRunning() {
    if(self.isRunning) {
      self.elapsed = 0
    }
    
    self.isRunning.toggle()
  }
  
  func sToHMS(_ s: Int) -> HMS {
    return HMS(h: s / 3600, m: (s % 3600) / 60, s: (s % 3600) % 60)
    
  }
}