Asynchronously loading images into table and collection views

At a glance

Project structure

Source bundle/
├── Async Image Loading/
│   ├── AppDelegate.swift
│   ├── CollectionViewController.swift
│   ├── TableViewController.swift
│   ├── ImageURLProtocol.swift
│   ├── Item.swift
│   ├── ImageCache.swift
│   ├── Async Image Loading.entitlements
│   ├── Base.lproj/
│   │   ├── LaunchScreen.storyboard
│   │   └── Main.storyboard
│   └── Info.plist
└── Async Image Loading.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 7 project/configuration file(s) and 8 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["CollectionViewController"]
    N3["UIKit APIs"]
    N1 --> N2
    N2 --> N3

Reference code

Async Image Loading/AppDelegate.swift:10 — architecture anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    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 UIKit.

Ownership and state

classDiagram
    AppDelegate o-- UIWindow : window
    CollectionViewController o-- UICollectionViewDiffableDataSource : dataSource
    TableViewController o-- UITableViewDiffableDataSource : dataSource
    ImageURLProtocol *-- Bool : cancelledOrComplete

Ownership evidence

Async Image Loading/AppDelegate.swift:13 — stored dependency or nearest verified ownership anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    // ...
    var window: UIWindow?
    // ...
}

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

Async Image Loading/ImageCache.swift:24 — representative execution boundary

            DispatchQueue.main.async {
                completion(item, cachedImage)
            }

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

Async Image Loading/AppDelegate.swift:11 — representative type boundary

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    // ...
    var window: UIWindow?
    // ...
}

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

Access control

Reference code

Async Image Loading/CollectionViewController.swift:13 — representative boundary

class CollectionViewController: UICollectionViewController {
    // ...
    private var imageObjects = [Item]()
    // ...
}

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: CollectionViewController, TableViewController; Delegate: AppDelegate.
• Protocols: no local protocol declaration in the scanned source.
• Methods: application, createLayout, viewDidLoad, canInit, canonicalRequest, requestIsCacheEquivalent, startLoading, stopLoading.
• Files: Async Image Loading/AppDelegate.swift, Async Image Loading/CollectionViewController.swift, Async Image Loading/TableViewController.swift, Async Image Loading/ImageURLProtocol.swift, Async Image Loading/Item.swift, Async Image Loading/ImageCache.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches UIKit 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