Building an App to Notify Users of COVID-19 Exposure

At a glance

Project structure

Source bundle/
├── ExposureNotificationApp-iOS12/
│   └── AppDelegate.swift
├── ExposureNotificationApp/
│   ├── Custom View Controllers/
│   │   ├── ExposureDetailsViewController.swift
│   │   ├── OnboardingViewController.swift
│   │   ├── StepViewController.swift
│   │   ├── TestVerificationViewController.swift
│   │   ├── ExposureNotificationsInfoViewController.swift
│   │   ├── ExposuresViewController.swift
│   │   ├── NotifyOthersViewController.swift
│   │   └── TestResultDetailsViewController.swift
│   └── Developer/
│       ├── DeveloperDiagnosisKeysViewController.swift
│       └── DeveloperViewController.swift
└── Common/
    └── Model/
        └── LocalStore.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 17 project/configuration file(s) and 101 source declaration(s).

Overall architecture

flowchart LR
    Bundle["Sample bundle"]
    V1["ExposureNotificationApp-iOS12"]
    V2["ExposureNotificationApp"]
    Boundary["ExposureNotification APIs"]
    Bundle --> V1
    V1 --> Boundary
    Bundle --> V2
    V2 --> Boundary

Reference code

ExposureNotificationApp-iOS12/AppDelegate.swift:12 — architecture anchor

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    ///  iOS 13 and later use the `window` property from the scene delegate, but this is needed for the
    ///  storyboard to function correctly on iOS 12.5.
    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        return true
    }
}

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
    AppDelegate o-- UIWindow : window
    ExposureDetailsViewController o-- RelativeDateTimeFormatter : relativeDateFormatter
    ExposureDetailsCell o-- UILabel : headerLabel
    ExposureDetailsCell o-- UILabel : diagnosisVerificationLabel

Ownership evidence

ExposureNotificationApp-iOS12/AppDelegate.swift:17 — stored dependency or nearest verified ownership anchor

@main
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

Common/Model/ExposureManager.swift:242 — representative execution boundary

                DispatchQueue.main.async {
                    if let error = error {
                        print("Error showing error user notification: \(error)")
                    }
                }

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

ExposureNotificationApp-iOS12/AppDelegate.swift:13 — representative type boundary

@main
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

ExposureNotificationApp-iOS12/View Controllers/MainViewController.swift:16 — representative boundary

class MainViewController: UITableViewController {
    // ...
    private var segueRowData: TableRowData?
    // ...
}

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: AboutTestIdentifiersViewController, BeforeYouGetStartedViewController, ContentViewController, CustomStepViewController, DeveloperDiagnosisKeysViewController; DataSource: DataSource; Delegate: AppDelegate, SceneDelegate; Manager: ExposureManager; Store: LocalStore; View: DeveloperQRCodeScannerView, EntryStackView, EntryView, RequirementView, TableHeaderView.
• Protocols: no local protocol declaration in the scanned source.
• Methods: application, viewDidLoad, tableView, modifySnapshot, learnMore, enableExposureNotifications, enablePushNotifications, viewDidAppear.
• Files: ExposureNotificationApp-iOS12/AppDelegate.swift, ExposureNotificationApp/Custom View Controllers/ExposureDetailsViewController.swift, ExposureNotificationApp/Custom View Controllers/OnboardingViewController.swift, ExposureNotificationApp/Custom View Controllers/StepViewController.swift, ExposureNotificationApp/Custom View Controllers/TestVerificationViewController.swift, ExposureNotificationApp/Developer/DeveloperDiagnosisKeysViewController.swift.

Architecture takeaways

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