Creating custom audio effects

At a glance

Project structure

AUv3Filter/
├── iOS/
│   ├── AUv3Filter/
│   ├── AUv3FilterExtension/
│   └── AUv3FilterFramework/
├── macOS/
│   ├── AUv3Filter/
│   ├── AUv3FilterExtension/
│   └── AUv3FilterFramework/
└── Shared/
    ├── AudioUnitManager.swift
    ├── AUv3FilterDemoViewController.swift
    ├── AudioUnit/
    │   ├── AUv3FilterDemo.swift
    │   ├── AUv3FilterDemoParameters.swift
    │   └── Support/{FilterDSPKernelAdapter.*,FilterDSPKernel.hpp}
    └── Support/{Audio,User Interface}/

Structure observations

• The archive is deliberately multi-target: host apps, AU extensions, and frameworks exist for both iOS and macOS.
• Conditional type aliases and shared UI/service files minimize platform duplication; target-specific files provide lifecycle and factory glue.

Overall architecture

flowchart LR
    Host["iOS/macOS host UI"] --> Manager["AudioUnitManager"]
    Manager --> Unit["AUv3FilterDemo"]
    Manager --> Play["SimplePlayEngine"]
    Manager --> PluginUI["AUv3FilterDemoViewController"]
    PluginUI --> Params["AUParameterTree"]
    Unit --> Params
    Unit --> Adapter["FilterDSPKernelAdapter ObjC++"]
    Adapter --> Kernel["FilterDSPKernel C++"]
    Play --> Graph["AVAudioEngine graph"]
    Graph --> Unit

Reference code

Shared/AudioUnitManager.swift:104 — the manager registers, instantiates, binds, and connects the custom unit.

public init() {
    viewController = loadViewController()
    AUAudioUnit.registerSubclass(AUv3FilterDemo.self,
                                 as: componentDescription,
                                 name: componentName,
                                 version: UInt32.max)
    AVAudioUnit.instantiate(with: componentDescription) { audioUnit, error in
        self.audioUnit = audioUnit?.auAudioUnit as? AUv3FilterDemo
        self.connectParametersToControls()
        self.playEngine.connect(avAudioUnit: audioUnit)
    }
}

Interpretation

AudioUnitManager is the host-facing façade. The AU parameter tree is the control contract shared by host and plug-in UI. The adapter is the language and real-time boundary around the C++ kernel.

Ownership and state

classDiagram
    MainViewController *-- AudioUnitManager : creates
    AudioUnitManager *-- SimplePlayEngine : owns
    AudioUnitManager *-- AUv3FilterDemo : instantiated unit
    AudioUnitManager *-- AUv3FilterDemoViewController : loads
    AUv3FilterDemo *-- AUv3FilterDemoParameters : owns
    AUv3FilterDemo *-- FilterDSPKernelAdapter : owns via base
    FilterDSPKernelAdapter *-- FilterDSPKernel : C++ ivar
    AudioUnitManager o-- AUManagerDelegate : weak

Ownership evidence

Shared/AudioUnitManager.swift:29 — the manager exposes selected state but privately owns the audio-unit and playback infrastructure.

public class AudioUnitManager {
    private var audioUnit: AUv3FilterDemo?
    public weak var delegate: AUManagerDelegate?
    public private(set) var viewController: AUv3FilterDemoViewController!
    private let playEngine = SimplePlayEngine()
}

Class and protocol design

Access control

Reference code

Shared/AudioUnitManager.swift:13 — a public UI model wraps a file-confined Core Audio preset.

public struct Preset {
    fileprivate init(preset: AUAudioUnitPreset) { audioUnitPreset = preset }
    fileprivate let audioUnitPreset: AUAudioUnitPreset
    public var number: Int { audioUnitPreset.number }
    public var name: String { audioUnitPreset.name }
}

Logic ownership and placement

Design patterns

Naming conventions

• Manager, ViewController, Parameters, Adapter, and Kernel suffixes reveal layer roles.
• AUv3FilterDemo is reused as the component prefix across app, extension, framework, and implementation types.
• Target folders use platform first, then product role; reusable code lives under Shared.
• Parameter names use domain vocabulary: cutoff, resonance, factoryPresets.

Architecture takeaways

• Organize a cross-platform Audio Unit sample around a shared core and thin target adapters.
• Make the parameter tree the common contract between host UI, plug-in UI, AU state, and DSP.
• Keep Swift-to-C++ interop inside Objective-C++ and expose only an Objective-C adapter header.
• Public access is justified at target/framework boundaries; private/fileprivate state protects host and render invariants.
• Treat the host and extension as separate architectures connected by Audio Unit contracts.

Source map

• Official Apple documentation