Supporting real-time ML inference on the CPU

At a glance

Project structure

Source bundle/
├── BNNSBitcrusher/
│   ├── BNNSBitcrusherApp.swift
│   ├── Model/
│   │   ├── AudioUnitViewModel.swift
│   │   └── AudioUnitHostModel.swift
│   ├── Common/
│   │   ├── Audio/
│   │   │   └── SimplePlayEngine.swift
│   │   ├── MIDI/
│   │   │   └── MIDIManager.swift
│   │   └── UI/
│   │       └── ViewControllerRepresentable.swift
│   └── ContentView.swift
└── BNNSBitcrusherExtension/
    ├── Common/
    │   ├── Parameters/
    │   │   └── ParameterSpecBase.swift
    │   ├── UI/
    │   │   ├── ObservableAUParameter.swift
    │   │   └── AudioUnitViewController.swift
    │   └── Audio Unit/
    │       └── BNNSBitcrusherExtensionAudioUnit.mm
    └── UI/
        └── BNNSBitcrusherExtensionMainView.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: C/Objective-C header, Objective-C++, Python, Swift.
• The verified tree contains 6 project/configuration file(s) and 23 source declaration(s).

Overall architecture

flowchart LR
    N1["BNNSBitcrusherApp"]
    N2["ContentView"]
    N3["AudioUnitViewModel"]
    N4["SimplePlayEngine"]
    N5["SwiftUI / AudioToolbox APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4
    N4 --> N5

Reference code

BNNSBitcrusher/BNNSBitcrusherApp.swift:11 — architecture anchor

@main
struct BNNSBitcrusherApp: App {
    @ObservedObject private var hostModel = AudioUnitHostModel()

    var body: some Scene {
        WindowGroup {
            ContentView(hostModel: hostModel)
        }
    }
}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into Accelerate.

Ownership and state

classDiagram
    BNNSBitcrusherApp o-- AudioUnitHostModel : hostModel
    AudioUnitViewModel *-- Bool : showAudioControls
    AudioUnitViewModel *-- Bool : showMIDIContols
    AudioUnitViewModel *-- String : title

Ownership evidence

BNNSBitcrusher/BNNSBitcrusherApp.swift:13 — stored dependency or nearest verified ownership anchor

@main
struct BNNSBitcrusherApp: App {
    @ObservedObject private var hostModel = AudioUnitHostModel()
    // ...
}

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

Reference code

BNNSBitcrusher/Common/Audio/SimplePlayEngine.swift:38 — representative execution boundary

            DispatchQueue.main.async {
                // ...
                            let genericViewController = AUGenericViewController()
                // ...
            }

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

BNNSBitcrusherExtension/Common/Parameters/ParameterSpecBase.swift:11 — representative type boundary

public protocol NodeSpec {}

The source explicitly defines local protocol relationships: ParameterGroupSpec → NodeSpec, ParameterTreeSpec → NodeSpec, ParameterSpec → NodeSpec.

Access control

Reference code

BNNSBitcrusher/BNNSBitcrusherApp.swift:13 — representative boundary

@main
struct BNNSBitcrusherApp: App {
    @ObservedObject private var hostModel = AudioUnitHostModel()
    // ...
}

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Naming conventions

• Types: App: BNNSBitcrusherApp; Builder: ParameterGroupBuilder; Controller: AudioUnitViewController; Engine: SimplePlayEngine; Manager: MIDIManager; Model: AudioUnitHostModel; View: BNNSBitcrusherExtensionMainView, ContentView; ViewModel: AudioUnitViewModel.
• Protocols: NodeSpec.
• Methods: validateID, buildBlock, createNode, createParameterGroup, createParameter, createAUParameterNodes, createAUParameterTree, create.
• Files: BNNSBitcrusher/BNNSBitcrusherApp.swift, BNNSBitcrusher/Model/AudioUnitViewModel.swift, BNNSBitcrusherExtension/Common/UI/ObservableAUParameter.swift, BNNSBitcrusher/Common/Audio/SimplePlayEngine.swift, BNNSBitcrusher/Common/MIDI/MIDIManager.swift, BNNSBitcrusher/ContentView.swift.

Architecture takeaways

• BNNSBitcrusherApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches SwiftUI, AudioToolbox, CoreAudioKit, CoreMIDI through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation