Implementing User Authentication with Sign in with Apple

At a glance

Project structure

Source bundle/
├── Juice/
│   ├── AppDelegate.swift
│   ├── LoginViewController.swift
│   ├── ResultViewController.swift
│   ├── Supporting Files/
│   │   ├── KeychainItem.swift
│   │   ├── Base.lproj/
│   │   │   └── LaunchScreen.storyboard
│   │   └── Info.plist
│   ├── Base.lproj/
│   │   └── Main.storyboard
│   └── Juice.entitlements
├── Configuration/
│   └── SampleCode.xcconfig
└── Juice.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 5 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["LoginViewController"]
    N3["AuthenticationServices APIs"]
    N1 --> N2
    N2 --> N3

Reference code

Juice/AppDelegate.swift:11 — architecture anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    /// - Tag: did_finish_launching
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        let appleIDProvider = ASAuthorizationAppleIDProvider()
        appleIDProvider.getCredentialState(forUserID: KeychainItem.currentUserIdentifier) { (credentialState, error) in
            switch credentialState {
            case .authorized:
                break // The Apple ID credential is valid.
            case .revoked, .notFound:
                // The Apple ID credential is either revoked or was not found, so show the sign-in UI.
                DispatchQueue.main.async {
                    self.window?.rootViewController?.showLoginViewController()
                }
            default:
                break
            }
        }
        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 Authentication Services.

Ownership and state

classDiagram
    AppDelegate o-- UIWindow : window
    LoginViewController o-- UIStackView : loginProviderStackView
    ResultViewController o-- UILabel : userIdentifierLabel
    ResultViewController o-- UILabel : givenNameLabel

Ownership evidence

Juice/AppDelegate.swift:14 — 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

Juice/AppDelegate.swift:25 — representative execution boundary

                DispatchQueue.main.async {
                    self.window?.rootViewController?.showLoginViewController()
                }

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

Juice/AppDelegate.swift:12 — 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

Juice/LoginViewController.swift:93 — representative boundary

    private func saveUserInKeychain(_ userIdentifier: String) {
        // ...
            try KeychainItem(service: "com.example.apple-samplecode.juice", account: "userIdentifier").saveItem(userIdentifier)
        // ...
    }

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: LoginViewController, ResultViewController; Delegate: AppDelegate.
• Protocols: no local protocol declaration in the scanned source.
• Methods: application, viewDidLoad, viewDidAppear, setupProviderLoginView, performExistingAccountSetupFlows, handleAuthorizationAppleIDButtonPress, authorizationController, saveUserInKeychain.
• Files: Juice/AppDelegate.swift, Juice/LoginViewController.swift, Juice/ResultViewController.swift, Juice/Supporting Files/KeychainItem.swift.

Architecture takeaways

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