How to Build an Ear Training App in SwiftUI

import SwiftData
import Foundation

enum IntervalName: String, Codable, CaseIterable {
    case minorSecond   = "Minor 2nd"
    case majorSecond   = "Major 2nd"
    case minorThird    = "Minor 3rd"
    case majorThird    = "Major 3rd"
    case perfectFourth = "Perfect 4th"
    case perfectFifth  = "Perfect 5th"
    case majorSixth    = "Major 6th"
    case octave        = "Octave"

    var semitones: Int {
        switch self {
        case .minorSecond:   return 1
        case .majorSecond:   return 2
        case .minorThird:    return 3
        case .majorThird:    return 4
        case .perfectFourth: return 5
        case .perfectFifth:  return 7
        case .majorSixth:    return 9
        case .octave:        return 12
        }
    }
}

@Model final class ExerciseSession {
    var id: UUID      = UUID()
    var date: Date    = Date.now
    var interval: String = ""
    var total: Int    = 0
    var correct: Int  = 0
    var accuracy: Double { total > 0 ? Double(correct) / Double(total) : 0 }
}

struct QuizView: View {
    @Environment(\.modelContext) private var ctx
    @State private var player   = IntervalPlayer()
    @State private var current  : IntervalName = .majorThird
    @State private var choices  : [IntervalName] = []
    @State private var selected : IntervalName?
    @State private var revealed = false

    var body: some View {
        VStack(spacing: 24) {
            Text("What interval is this?").font(.title2.weight(.semibold))
            Button("▶  Play") { player.play(interval: current) }
                .buttonStyle(.borderedProminent).controlSize(.large)
            LazyVGrid(columns: [.init(.flexible()), .init(.flexible())], spacing: 12) {
                ForEach(choices, id: \.self) { c in
                    Button(c.rawValue) {
                        guard !revealed else { return }
                        selected = c; revealed = true
                    }
                    .buttonStyle(.bordered)
                    .tint(revealed ? (c == current ? .green : c == selected ? .red : nil) : nil)
                }
            }
            if revealed { Button("Next") { newQuestion() }.buttonStyle(.borderedProminent) }
        }
        .padding().onAppear { newQuestion() }
    }

    private func newQuestion() {
        current = IntervalName.allCases.randomElement()!
        var pool = Array(IntervalName.allCases.shuffled().prefix(4))
        if !pool.contains(current) { pool[0] = current; pool.shuffle() }
        choices = pool; selected = nil; revealed = false
    }
}

import AVFoundation

@Observable final class IntervalPlayer {
    private let engine  = AVAudioEngine()
    private let sampler = AVAudioUnitSampler()

    init() {
        try? AVAudioSession.sharedInstance()
            .setCategory(.playback, mode: .default)
        try? AVAudioSession.sharedInstance().setActive(true)
        engine.attach(sampler)
        engine.connect(sampler, to: engine.mainMixerNode, format: nil)
        try? engine.start()
        if let url = Bundle.main.url(forResource: "GeneralUser", withExtension: "sf2") {
            try? sampler.loadSoundBankInstrument(
                at: url, program: 0, bankMSB: 0x79, bankLSB: 0)
        }
    }

    func play(interval: IntervalName, root: UInt8 = 60) {
        let top = root + UInt8(clamping: interval.semitones)
        sampler.startNote(root, withVelocity: 90, onChannel: 0)
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.65) {
            self.sampler.startNote(top, withVelocity: 90, onChannel: 0)
        }
        DispatchQueue.main.asyncAfter(deadline: .now() + 1.8) {
            self.sampler.stopNote(root, onChannel: 0)
            self.sampler.stopNote(top,  onChannel: 0)
        }
    }
}

<!-- PrivacyInfo.xcprivacy — add to app target, not an extension -->
<?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>NSPrivacyAccessedAPICategoryUserDefaults</string>
            <key>NSPrivacyAccessedAPITypeReasons</key>
            <array><string>CA92.1</string></array>
        </dict>
    </array>
</dict>
</plist>