ios-viewmodel

iOS ViewModel Rules

ObservableObject pattern

ViewModels conform to ObservableObject and expose state via @Published properties.

class TunerViewModel: ObservableObject {
    @Published var detectedNote: String = ""
    @Published var centsOff: Double = 0.0
}

@StateObject vs @ObservedObject

• Use @StateObject for view-owned ViewModels (the view creates and owns the instance)
• Use @ObservedObject for ViewModels passed in from a parent view

struct TunerView: View {
    @StateObject private var viewModel = TunerViewModel()
}

struct ChildView: View {
    @ObservedObject var viewModel: TunerViewModel
}

KMP Swift naming conventions

• KMP classes drop the Shared prefix: PitchDetector.shared, UkuleleTuning.highG
• KMP numeric types keep their Kotlin names: KotlinFloatArray, KotlinDouble (no prefix)

KMP collection bridging

Kotlin/Native bridges List and Set to different Swift types. Force-casting the entire collection to a Swift Array crashes for Sets.

Kotlin type	Swift bridge type	as! [NSNumber] safe?
List<Int>	[KotlinInt] (Array)	Yes
Set<Int>	Set<KotlinInt>	No — crashes (Set is not Array)

// ❌ DON'T: cast the whole collection — crashes if source is a Kotlin Set
let values = Set((notes as! [NSNumber]).map { $0.int32Value })

// ✅ DO: iterate elements individually — works for both List and Set
let values = Set(notes.map { ($0 as! NSNumber).int32Value })

Existing ViewModels

FretboardViewModel, MetronomeViewModel, FavoritesViewModel, SettingsViewModel, ChordLibraryViewModel, SongbookViewModel, MelodyViewModel, ProgressionsViewModel, LearnViewModel, PitchMonitorViewModel, ScalePracticeViewModel, PracticeTimerViewModel, CustomPatternsViewModel, PlayAlongViewModel, ChordTransitionsViewModel, SetlistViewModel