Building high-performance lists and collection views

At a glance

Project structure

Source bundle/
└── CollectionViewSample/
    ├── AppDelegate.swift
    ├── Stores/
    │   ├── AssetStore.swift
    │   └── ModelStore.swift
    ├── DestinationPost.swift
    ├── PostGridViewController.swift
    ├── SceneDelegate.swift
    ├── Utilities/
    │   ├── PlaceholderStore.swift
    │   ├── MemoryLimitedCache.swift
    │   ├── URLSession+DownloadTaskPublisher.swift
    │   └── UnfairLock.swift
    └── Views/
        ├── DestinationPostPropertiesView.swift
        └── SectionBackgroundView.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 5 project/configuration file(s) and 25 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["PostGridViewController"]
    N3["AnyModelStore"]
    N4["UIKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

CollectionViewSample/AppDelegate.swift:10 — architecture anchor

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    // MARK: UISceneSession Lifecycle

    func application(_ application: UIApplication,
                     configurationForConnecting connectingSceneSession: UISceneSession,
                     options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

}

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
    AssetStore *-- UIImage : placeholderFallbackImage
    AssetStore *-- UnfairLock : lock
    AssetStore *-- MemoryLimitedCache : preparedImages
    AssetStore o-- FileBasedCache : localAssets

Ownership evidence

CollectionViewSample/Stores/AssetStore.swift:39 — stored dependency or nearest verified ownership anchor

class AssetStore: ModelStore {
    static let placeholderFallbackImage = UIImage(systemName: "airplane.circle.fill")!
    // ...
}

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

CollectionViewSample/Stores/AssetStore.swift:46 — representative execution boundary

class AssetStore: ModelStore {
    // ...
    private let placeholderQueue = DispatchQueue(label: "com.apple.DestinationUnlocked.placeholderGenerationQueue",
                                                 qos: .utility,
                                                 attributes: [],
                                                 autoreleaseFrequency: .workItem,
                                                 target: nil)
    // ...
}

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

CollectionViewSample/Stores/ModelStore.swift:10 — representative type boundary

protocol ModelStore {
    associatedtype Model: Identifiable

    func fetchByID(_ id: Model.ID) -> Model
}

The source explicitly defines local protocol relationships: AssetStore → ModelStore, FileBasedCache → Cache, MemoryLimitedCache → Cache.

Access control

Reference code

CollectionViewSample/PostGridViewController.swift:20 — representative boundary

class PostGridViewController: UIViewController {
    // ...
    fileprivate var prefetchingIndexPathOperations = [IndexPath: AnyCancellable]()
    // ...
}

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: PostGridViewController; Delegate: AppDelegate, SceneDelegate; Store: AnyModelStore, AssetStore, ModelStore, PlaceholderStore; View: DestinationPostPropertiesView, SectionBackgroundDecorationView.
• Protocols: ModelStore, Cache.
• Methods: application, flatMapIfNil, withOutput, fetchByID, loadAssetByID, prepareAssetIfNeeded, prepareAsset, makeDownloadRequest.
• Files: CollectionViewSample/AppDelegate.swift, CollectionViewSample/Stores/AssetStore.swift, CollectionViewSample/Stores/ModelStore.swift, CollectionViewSample/DestinationPost.swift, CollectionViewSample/PostGridViewController.swift, CollectionViewSample/SceneDelegate.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches UIKit, UniformTypeIdentifiers, CoreGraphics, ImageIO 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.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation