Writing fragmented MPEG-4 files for HTTP Live Streaming

At a glance

Project structure

Source bundle/
├── fmp4writer/
│   ├── main.swift
│   ├── SegmentGeneration.swift
│   ├── Configuration.swift
│   ├── Error.swift
│   ├── IndexFileGeneration.swift
│   └── TrackLoading.swift
├── Configuration/
│   └── SampleCode.xcconfig
└── fmp4writer.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 3 project/configuration file(s) and 6 source declaration(s).

Overall architecture

flowchart LR
    N1["Segment"]
    N2["AVFoundation APIs"]
    N1 --> N2

Reference code

fmp4writer/SegmentGeneration.swift:12 — architecture anchor

struct Segment {
    let index: Int
    let data: Data
    let isInitializationSegment: Bool
    let report: AVAssetSegmentReport?
}

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
    main *-- URL : commandName
    main *-- FMP4WriterConfiguration : config
    main o-- Completion : result
    main o-- Any : segmentGenerationToken

Ownership evidence

fmp4writer/main.swift:14 — stored dependency or nearest verified ownership anchor

    let commandName = URL(fileURLWithPath: commandPath).lastPathComponent

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

fmp4writer/SegmentGeneration.swift:94 — representative execution boundary

class ReaderWriter<S>: NSObject, AVAssetWriterDelegate where S: Subject, S.Output == Segment, S.Failure == Error {
    // ...
        let audioQueue = DispatchQueue(label: "audio read/write")
    // ...
}

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

fmp4writer/SegmentGeneration.swift:12 — representative type boundary

struct Segment {
    let index: Int
    let data: Data
    let isInitializationSegment: Bool
    let report: AVAssetSegmentReport?
}

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

Access control

Reference code

fmp4writer/SegmentGeneration.swift:36 — representative boundary

class ReaderWriter<S>: NSObject, AVAssetWriterDelegate where S: Subject, S.Output == Segment, S.Failure == Error {
    private let assetReader: AVAssetReader
    // ...
}

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: feature-specific names rather than reusable layer suffixes.
• Protocols: no local protocol declaration in the scanned source.
• Methods: generateSegments, start, assetWriter, finish, transferSamplesUntilWriterInputPushesBack, offsettingTiming, reduceToIndexFile, fileName.
• Files: feature/project roles rather than a strict one-type-per-file rule.

Architecture takeaways

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