Adding a display mask rectangle metadata track to a movie file

At a glance

Project structure

Source bundle/
├── AVAddDisplayMaskTrack.swift
├── MovieProcessor.swift
├── AVAddDisplayMaskTrack.xcodeproj/
│   ├── .xcodesamplecode.plist
│   └── project.pbxproj
└── Configuration/
    └── SampleCode.xcconfig

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 3 project/configuration file(s) and 5 source declaration(s).

Overall architecture

flowchart LR
    N1["AVAddDisplayMaskTrack"]
    N2["MovieProcessor"]
    N3["AVFoundation APIs"]
    N1 --> N2
    N2 --> N3

Reference code

AVAddDisplayMaskTrack.swift:14 — architecture anchor

@main
struct AVAddDisplayMaskTrack: AsyncParsableCommand {
    static let configuration = CommandConfiguration(
        abstract: "Appends a display mask metadata track to a movie file.",
        usage: """
            AVAddDisplayMaskTrack <input-path> <output-path> [display-mask-type]
            Examples:
               AVAddDisplayMaskTrack input.mov output.mov
               AVAddDisplayMaskTrack input.mov output.mov 2
            """,
        discussion: """
            Creates a new QuickTime movie file that contains the media tracks from the input movie file
            and an additional timed metadata track. The metadata track contains a display mask rectangle
            in boxed metadata format (`mebx`). The metadata track is associated to the enabled video track
            with a render ('rndr') track reference.
            """
    )
    // ...
}

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
    AVAddDisplayMaskTrack *-- CommandConfiguration : configuration
    AVAddDisplayMaskTrack *-- String : inputPath
    AVAddDisplayMaskTrack *-- String : outputPath
    AVAddDisplayMaskTrack *-- Int : displayMaskType

Ownership evidence

AVAddDisplayMaskTrack.swift:16 — stored dependency or nearest verified ownership anchor

@main
struct AVAddDisplayMaskTrack: 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

AVAddDisplayMaskTrack.swift:41 — representative execution boundary

    mutating func run() async throws {
        // ...
            let processor = MovieProcessor()
        // ...
    }

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

MovieProcessor.swift:11 — representative type boundary

class MovieProcessor {
    private var readerOutputs: [AVAssetReaderOutput] = []
    // ...
}

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

Access control

Reference code

MovieProcessor.swift:12 — representative boundary

class MovieProcessor {
    private var readerOutputs: [AVAssetReaderOutput] = []
    // ...
}

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 MovieProcessor
    participant asset
    participant sourceTrack
    MovieProcessor->>asset: load()
    MovieProcessor->>sourceTrack: load()
    MovieProcessor->>sourceTrack: load()
    MovieProcessor->>sourceTrack: load()
    MovieProcessor->>sourceTrack: load()
    MovieProcessor->>sourceTrack: load()

Reference code

MovieProcessor.swift:108 — setupExistingTracksForPassthroughAndReturnEnabledVideoInput()

    private func setupExistingTracksForPassthroughAndReturnEnabledVideoInput(from asset: AVAsset, reader: AVAssetReader, writer: AVAssetWriter) async throws -> (videoInput: AVAssetWriterInput, videoInfo: VideoTrackInfo) {
        // ...
        var enabledVideoInput: AVAssetWriterInput? = nil
        var enabledVideoInfo = VideoTrackInfo()
        let allTracks = try await asset.load(.tracks)
        print("Found \(allTracks.count) track(s).")
        for sourceTrack in allTracks {
            let readerOutput = AVAssetReaderTrackOutput(track: sourceTrack, outputSettings: nil)
            readerOutput.alwaysCopiesSampleData = false
            let writerInput = AVAssetWriterInput(mediaType: sourceTrack.mediaType, outputSettings: nil)
            writerInput.expectsMediaDataInRealTime = false
            if reader.canAdd(readerOutput) && writer.canAdd(writerInput) {
                reader.add(readerOutput)
                writer.add(writerInput)
                readerOutputs.append(readerOutput)
                writerInputs.append(writerInput)
            }
            if sourceTrack.mediaType == .video {
                if try await sourceTrack.load(.isEnabled) {
                    enabledVideoInput = writerInput
                    enabledVideoInfo.dimensions = try await sourceTrack.load(.naturalSize)
                    let trackTimeRange = try await sourceTrack.load(.timeRange)
                    enabledVideoInfo.trackDuration = trackTimeRange.end
                    enabledVideoInfo.timescale = try await sourceTrack.load(.naturalTimeScale)
                    enabledVideoInfo.frameRate = try await sourceTrack.load(.nominalFrameRate)
                }
            }
        }
        guard let enabledVideoInput = enabledVideoInput else {
            throw ProcessingError.noEnabledVideoTrack
        }
        return (enabledVideoInput, enabledVideoInfo)
    }

Naming conventions

• Types: Processor: MovieProcessor.
• Protocols: no local protocol declaration in the scanned source.
• Methods: run, processMovie, setupExistingTracksForPassthroughAndReturnEnabledVideoInput, addDisplayMaskMetadataTrack, performMediaProcessing, setupMediaDataTransfer, setupDisplayMaskMetadataTransfer, createMetadataGroupForDisplayMask.
• Files: AVAddDisplayMaskTrack.swift, MovieProcessor.swift.

Architecture takeaways

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