Code-along: Elevating an app with Swift concurrency

At a glance

Project structure

Source bundle/
├── End/
│   └── SwiftSticker/
│       ├── SwiftStickerApp.swift
│       ├── ViewModel/
│       │   └── StickerViewModel.swift
│       ├── Model/
│       │   ├── PhotoProcessor.swift
│       │   ├── ProcessedPhoto.swift
│       │   └── ColorExtractor.swift
│       └── Views/
│           └── PhotoPickerView.swift
└── Start/
    └── SwiftSticker/
        ├── SwiftStickerApp.swift
        ├── ViewModel/
        │   └── StickerViewModel.swift
        ├── Model/
        │   ├── PhotoProcessor.swift
        │   ├── ProcessedPhoto.swift
        │   └── ColorExtractor.swift
        └── Views/
            └── PhotoPickerView.swift

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 36 source declaration(s).

Overall architecture

flowchart LR
    Bundle["Sample bundle"]
    V1["End"]
    V2["Start"]
    Boundary["SwiftUI APIs"]
    Bundle --> V1
    V1 --> Boundary
    Bundle --> V2
    V2 --> Boundary

Reference code

End/SwiftSticker/SwiftStickerApp.swift:12 — architecture anchor

@main
struct SwiftStickerApp: App {
    // ...
                PhotoPickerView(
                    viewModel: StickerViewModel()
                )
    // ...
}

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
    StickerViewModel *-- Array : invalidPhotos
    StickerViewModel o-- Selection : selection
    PhotoPickerView *-- StickerViewModel : viewModel
    PhotoProcessor *-- ColorExtractor : colorExtractor

Ownership evidence

End/SwiftSticker/ViewModel/StickerViewModel.swift:21 — stored dependency or nearest verified ownership anchor

@Observable
class StickerViewModel {
    // ...
    var invalidPhotos: [SelectedPhoto.ID] = []
    // ...
}

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

End/SwiftSticker/Model/PhotoProcessor.swift:15 — representative execution boundary

    @concurrent
    func process(data: Data) async -> ProcessedPhoto? {
        async let sticker = extractSticker(from: data)
        // ...
    }

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

End/SwiftSticker/SwiftStickerApp.swift:13 — representative type boundary

@main
struct SwiftStickerApp: App {
    // ...
                    viewModel: StickerViewModel()
    // ...
}

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

Access control

Reference code

End/SwiftSticker/Model/ColorExtractor.swift:15 — representative boundary

nonisolated final class ColorExtractor {
    private let k = 4
    // ...
}

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: SwiftStickerApp; Processor: PhotoProcessor; View: PhotoPickerView; ViewModel: StickerViewModel.
• Protocols: no local protocol declaration in the scanned source.
• Methods: loadPhoto, processAllPhotos, process, extractColors, extractSticker.
• Files: End/SwiftSticker/SwiftStickerApp.swift, Start/SwiftSticker/SwiftStickerApp.swift, End/SwiftSticker/ViewModel/StickerViewModel.swift, Start/SwiftSticker/ViewModel/StickerViewModel.swift, End/SwiftSticker/Model/PhotoProcessor.swift, End/SwiftSticker/Views/PhotoPickerView.swift.

Architecture takeaways

• SwiftStickerApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches SwiftUI, PhotosUI, Accelerate, CoreImage 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