【SwiftUI】Stop watch sample 👉 https://android.benigumo.com/20240225/stop-watch/

StopWatch.swift

import SwiftUI

@Observable final class ScheduledTimer {
  var elapsed = TimeInterval.zero
  private var timer: Timer?

  func start() {
    timer = Timer.scheduledTimer(withTimeInterval: 0.001, repeats: true) { _ in
      self.elapsed += 0.001
    }
  }
  
  func reset() {
    elapsed = .zero
  }

  func stop() {
    timer?.invalidate()
  }
}

@Observable final class TaskedTimer {
  var elapsed = TimeInterval.zero
  private var task: Task<(), Never>?

  func start() {
    if task?.isCancelled ?? true { // keep task single
      task = Task { 
        let start = Date().addingTimeInterval(-elapsed)
        do {
          while true  {
            try await Task.sleep(for: .milliseconds(1))
            elapsed = Date().timeIntervalSince(start)
          }
        } catch {
          if Task.isCancelled {
            print("cancelled.")
          }
        }
      }
    }
  }

  func reset() {
    elapsed = .zero
  }

  func stop() {
    task?.cancel()
  }
}

struct StopWatch: View {
  private var st = ScheduledTimer()
  private var tt = TaskedTimer()

  @State private var running = false

  var body: some View {
    VStack {
      Text(String(format: "st : %.3f", st.elapsed))
      Text(String(format: "tt : %.3f", tt.elapsed))

      HStack {
        Button(running ? "stop" : "start") {
          if running {
            st.stop()
            tt.stop()
          } else {
            st.start()
            tt.start()
          }
          running.toggle()
        }
        Button("reset") {
          st.reset()
          tt.reset()
        }
        .disabled(running || st.elapsed == .zero || tt.elapsed == .zero)
      }
      .buttonStyle(.borderedProminent)
    }
    .monospaced()
  }
}

#Preview {
  StopWatch()
    .frame(width: 300, height: 300)
}

https://www.youtube.com/watch?v=bjyg28tI6I8