Debugging AVFoundation audio mixes, compositions, and video compositions

At a glance

Project structure

Source bundle/
├── AVCompositionDebugViewer/
│   ├── AppDelegate.swift
│   ├── DebugView/
│   │   ├── CompDebugView.swift
│   │   ├── CompDebugViewCAUtils.swift
│   │   └── CompDebugViewUtils.swift
│   ├── Editor/
│   │   ├── SimpleEditorUtils.swift
│   │   └── SimpleEditor.swift
│   ├── PlayerViewController.swift
│   ├── SceneDelegate.swift
│   └── Base.lproj/
│       ├── LaunchScreen.storyboard
│       └── Main.storyboard
└── AVCompositionDebugViewer.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 6 project/configuration file(s) and 11 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["PlayerViewController"]
    N3["AVFoundation APIs"]
    N1 --> N2
    N2 --> N3

Reference code

AVCompositionDebugViewer/AppDelegate.swift:10 — architecture anchor

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        return true
    }
}

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 AVFoundation.

Ownership and state

classDiagram
    CompositionTrackSegmentInfo *-- String : description
    CompositionDebugView *-- CALayer : drawingLayer
    CompositionDebugView *-- CMTimeMake : duration
    CompositionDebugView *-- CGFloat : scaledDurationToWidth

Ownership evidence

AVCompositionDebugViewer/DebugView/CompDebugView.swift:30 — stored dependency or nearest verified ownership anchor

struct CompositionTrackSegmentInfo {
    var timeRange = CMTimeRange.zero
    var empty = true
    var mediaType = AVMediaType.video
    var description = String("")
}

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

AVCompositionDebugViewer/Editor/SimpleEditor.swift:95 — representative execution boundary

class SimpleEditor: NSObject, AVVideoCompositionValidationHandling {
    // ...
    private let assetQueue = DispatchQueue(label: "assetQueue")
    // ...
}

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

AVCompositionDebugViewer/AppDelegate.swift:11 — representative type boundary

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        return true
    }
}

No local protocol conformance is claimed as protocol-oriented design; external framework conformances are listed only as dependencies.

Access control

Reference code

AVCompositionDebugViewer/DebugView/CompDebugView.swift:64 — representative boundary

class CompositionDebugView: UIView {
    // ...
    private var drawingLayer: CALayer = CALayer()
    // ...
}

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: Controller: PlayerViewController; Delegate: AppDelegate, SceneDelegate; View: CompositionDebugView.
• Protocols: no local protocol declaration in the scanned source.
• Methods: application, drawVerticallyCentered, willMove, synchronize, draw, drawCompositionTracks, drawVideoComposition, drawAudioMix.
• Files: AVCompositionDebugViewer/AppDelegate.swift, AVCompositionDebugViewer/Editor/SimpleEditorUtils.swift, AVCompositionDebugViewer/PlayerViewController.swift, AVCompositionDebugViewer/SceneDelegate.swift, AVCompositionDebugViewer/Editor/SimpleEditor.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches UIKit, AVFoundation, AVKit 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.
• The source does not justify labeling the design protocol-oriented.

Source map

• Official Apple documentation