Precise Timer Example in SwiftUI

PreciseTimerExample.swift

import SwiftUI

struct ContentView: View {

    @State var startDate: Date?

    var body: some View {
        TimelineView(MetricsTimelineSchedule(from: .now)) { _ in
            VStack(spacing: 16.0) {
                ElapsedTimeView(elapsedTime: elapsedTime())
                HStack(spacing: 16.0) {
                    Button {
                        startDate = .now
                    } label: {
                        Text("Start")
                    }
                    Button {
                        startDate = nil
                    } label: {
                        Text("Stop")
                            .foregroundStyle(Color.red)
                    }
                }
            }
        }
    }

    private func elapsedTime() -> TimeInterval {
        guard let startDate else {
            return 0
        }
        return -startDate.timeIntervalSinceNow
    }
}

#Preview {
    ContentView()
}


struct MetricsTimelineSchedule: TimelineSchedule {
    var startDate: Date

    init(from startDate: Date) {
        self.startDate = startDate
    }

    func entries(from startDate: Date, mode: TimelineScheduleMode) -> AnyIterator<Date> {
        var baseSchedule = PeriodicTimelineSchedule(
            from: self.startDate,
            by: (mode == .lowFrequency ? 1.0 : 1.0 / 30.0)
        )
            .entries(from: startDate, mode: mode)
        
        return AnyIterator<Date> {
            return baseSchedule.next()
        }
    }
}


struct ElapsedTimeView: View {
    var elapsedTime: TimeInterval = 0
    @State private var timeFormatter = ElapsedTimeFormatter()

    var body: some View {
        Text(NSNumber(value: elapsedTime), formatter: timeFormatter)
            .font(.system(.largeTitle, design: .monospaced))
            .foregroundStyle(Color.yellow)
    }
}

final class ElapsedTimeFormatter: Formatter {
    let componentsFormatter: DateComponentsFormatter = {
        let formatter = DateComponentsFormatter()
        formatter.allowedUnits = [.minute, .second]
        formatter.zeroFormattingBehavior = .pad
        return formatter
    }()

    override func string(for value: Any?) -> String? {
        guard let time = value as? TimeInterval else {
            return nil
        }

        guard let formattedString = componentsFormatter.string(from: time) else {
            return nil
        }

        let hundredths = Int((time.truncatingRemainder(dividingBy: 1)) * 100)
        let decimalSeparator = Locale.current.decimalSeparator ?? "."
        return String(format: "%@%@%0.2d", formattedString, decimalSeparator, hundredths)
    }
}