How to Build a Metronome App in SwiftUI

import SwiftData
import Foundation

@Model
final class TempoPreset {
    var name: String
    var bpm: Double
    var beatsPerMeasure: Int   // numerator:  2, 3, 4, 5, 6, 7…
    var noteValue: Int         // denominator: 4 = quarter, 8 = eighth

    init(
        name: String,
        bpm: Double,
        beatsPerMeasure: Int = 4,
        noteValue: Int = 4
    ) {
        self.name = name
        self.bpm = bpm
        self.beatsPerMeasure = beatsPerMeasure
        self.noteValue = noteValue
    }
}

// MetronomeApp.swift:
// WindowGroup { ContentView() }
//   .modelContainer(for: TempoPreset.self)

struct ContentView: View {
    @State private var engine = MetronomeEngine()
    @State private var bpm: Double = 120

    var body: some View {
        VStack(spacing: 28) {
            Text("\(Int(bpm))")
                .font(.system(size: 96, weight: .bold, design: .rounded))
                .contentTransition(.numericText())
                .animation(.spring, value: bpm)

            Slider(value: $bpm, in: 40...240, step: 1)
                .onChange(of: bpm) { engine.bpm = bpm }
                .padding(.horizontal)

            BeatIndicatorRow(engine: engine)  // ForEach over currentBeat

            Button(engine.isRunning ? "Stop" : "Start") {
                if engine.isRunning { engine.stop() } else { engine.start() }
            }
            .buttonStyle(.borderedProminent)
            .tint(engine.isRunning ? .red : .accentColor)
            .controlSize(.large)
        }
        .padding()
    }
}

@Observable
final class MetronomeEngine {
    var bpm: Double = 120
    var beatsPerMeasure: Int = 4
    var currentBeat: Int = 0
    var isRunning = false

    private let audioEngine = AVAudioEngine()
    private let playerNode  = AVAudioPlayerNode()
    private var accentBuf: AVAudioPCMBuffer?
    private var tickBuf:   AVAudioPCMBuffer?
    private var timer: DispatchSourceTimer?

    func start() {
        isRunning = true; currentBeat = 0
        let src = DispatchSource.makeTimerSource(
            queue: .global(qos: .userInteractive))
        src.schedule(deadline: .now(),
                     repeating: 60.0 / bpm,
                     leeway: .milliseconds(1))
        src.setEventHandler { [weak self] in self?.tick() }
        src.resume(); timer = src
    }

    func stop() { isRunning = false; timer?.cancel(); timer = nil }

    private func tick() {
        let beat = (currentBeat % beatsPerMeasure) + 1
        DispatchQueue.main.async { self.currentBeat = beat }
        playerNode.scheduleBuffer(beat == 1 ? accentBuf! : tickBuf!,
                                  at: nil, options: .interrupts)
        playerNode.play()
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
  "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>NSPrivacyTracking</key>
  <false/>
  <key>NSPrivacyTrackingDomains</key>
  <array/>
  <key>NSPrivacyCollectedDataTypes</key>
  <array/>
  <key>NSPrivacyAccessedAPITypes</key>
  <array>
    <dict>
      <key>NSPrivacyAccessedAPIType</key>
      <string>NSPrivacyAccessedAPICategoryFileTimestamp</string>
      <key>NSPrivacyAccessedAPITypeReasons</key>
      <array><string>C617.1</string></array>
    </dict>
  </array>
</dict>
</plist>