Encoding video for offline transcoding

At a glance

Project structure

Source bundle/
├── Swift/
│   ├── VTEncoderForTranscoding/
│   │   ├── CommandOptions.swift
│   │   └── VTEncoderForTranscoding.swift
│   └── VTEncoderUtils/
│       ├── VideoSource.swift
│       ├── RealTimeVideoSource.swift
│       ├── VTEncoderUtil.swift
│       └── VideoSink.swift
└── Objective-C/
    ├── VTEncoderForTranscoding/
    │   ├── main.m
    │   └── VTEncoderForTranscoding.h
    └── VTEncoderUtils/
        ├── VideoSource.h
        ├── VideoSource.m
        ├── VideoSink.h
        └── VideoSink.m

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, Swift.
• The verified tree contains 6 project/configuration file(s) and 24 source declaration(s).

Overall architecture

flowchart LR
    Bundle["Sample bundle"]
    V1["Objective-C"]
    V2["Swift"]
    Boundary["VideoToolbox APIs"]
    Bundle --> V1
    V1 --> Boundary
    Bundle --> V2
    V2 --> Boundary

Reference code

Swift/VTEncoderForTranscoding/CommandOptions.swift:13 — architecture anchor

@main
struct CommandOptions: AsyncParsableCommand {
    // ...
    static var configuration = CommandConfiguration(
        usage: """
            VTEncoderForTranscoding <source-movie> [<options>]
            """)
    // ...
}

Interpretation

The branches represent separate entry-bearing project variants in the downloaded bundle, not runtime calls between those variants. Each branch is intentionally collapsed at the documented framework boundary; the detailed target-local flow remains in the cited files. Ownership is claimed only where the next section cites a stored property or assignment.

Ownership and state

classDiagram
    CommandOptions *-- CommandConfiguration : configuration
    CommandOptions *-- String : sourceMovie
    CommandOptions *-- Int : bitrate
    CommandOptions o-- CodecType : codecType

Ownership evidence

Swift/VTEncoderForTranscoding/CommandOptions.swift:16 — stored dependency or nearest verified ownership anchor

@main
struct CommandOptions: AsyncParsableCommand {
    // ...
}

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

Swift/VTEncoderForTranscoding/CommandOptions.swift:169 — representative execution boundary

    mutating func run() async throws {
        // ...
        let codec: CMVideoCodecType
        // ...
    }

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

Swift/VTEncoderForTranscoding/CommandOptions.swift:14 — representative type boundary

@main
struct CommandOptions: AsyncParsableCommand {
    // ...
    var sourceMovie: String
    // ...
}

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

Access control

Reference code

Objective-C/VTEncoderUtils/VideoSource.h:23 — representative boundary

#ifndef VideoSource_h
@property(nonatomic, readonly) float frameRate;
#endif /* VideoSource_h */

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

Main application flow

sequenceDiagram
    participant AsyncIterator
    participant sampleBufferQueue
    participant SampleBufferQueue
    participant source
    AsyncIterator->>sampleBufferQueue: dequeue()
    AsyncIterator->>SampleBufferQueue: SampleBufferQueue()
    AsyncIterator->>source: sourceTrack()
    AsyncIterator->>AsyncIterator: next()

Reference code

Swift/VTEncoderUtils/VideoSource.swift:294 — next()

        public mutating func next() async throws -> (CVImageBuffer, CMTime)? {
            if let sampleBufferQueue = self.sampleBufferQueue {
                guard framesEmitted < self.frameCount else { return nil }
                guard let sbuf = await sampleBufferQueue.dequeue() else {
                    return nil
                }
                guard let imageBuffer = sbuf.imageBuffer else {
                    return nil
                }

                framesEmitted += 1

                return (imageBuffer, sbuf.presentationTimeStamp)
            } else {
                let sampleBufferQueue = try await SampleBufferQueue(sourceAsset: source.sourceAsset,
                                                                    videoTrack: try await source.sourceTrack,
                                                                    outputPixelFormat: source.outputPixelFormat,
                                                                    alwaysCopiesSampleData: source.alwaysCopiesSampleData,
                                                                    frameCount: self.frameCount)

                self.sampleBufferQueue = sampleBufferQueue

                return try await self.next()
            }
        }

Naming conventions

• Types: feature-specific names rather than reusable layer suffixes.
• Protocols: no local protocol declaration in the scanned source.
• Methods: validate, run, frames, enqueue, dequeue, wait, signal, next.
• Files: Swift/VTEncoderForTranscoding/CommandOptions.swift, Swift/VTEncoderUtils/VideoSource.swift, Swift/VTEncoderUtils/RealTimeVideoSource.swift, Objective-C/VTEncoderUtils/VideoSource.h, Objective-C/VTEncoderUtils/VideoSource.m, Objective-C/VTEncoderUtils/VideoSink.h.

Architecture takeaways

• CommandOptions is the main source-visible entry or composition anchor for this sample.
• Framework work reaches AVFoundation, VideoToolbox, ArgumentParser 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