Logging a User into Your App with Face ID or Touch ID

At a glance

Project structure

Source bundle/
├── Authenticator/
│   ├── AppDelegate.swift
│   ├── ViewController.swift
│   ├── Base.lproj/
│   │   ├── LaunchScreen.storyboard
│   │   └── Main.storyboard
│   └── Info.plist
├── Authenticator.xcodeproj/
│   ├── .xcodesamplecode.plist
│   └── project.pbxproj
└── Configuration/
    └── SampleCode.xcconfig

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 3 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["ViewController"]
    N3["LocalAuthentication APIs"]
    N1 --> N2
    N2 --> N3

Reference code

Authenticator/AppDelegate.swift:10 — architecture anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

}

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 Local Authentication.

Ownership and state

classDiagram
    AppDelegate o-- UIWindow : window
    ViewController o-- UIButton : loginButton
    ViewController o-- UIView : stateView
    ViewController o-- UILabel : faceIDLabel

Ownership evidence

Authenticator/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

Authenticator/ViewController.swift:26 — representative execution boundary

class ViewController: UIViewController {
    // ...
    @MainActor
    var state = AuthenticationState.loggedout {

        // Update the UI on a change.
        didSet {
            loginButton.isHighlighted = state == .loggedin  // The button text changes on highlight.
            stateView.backgroundColor = state == .loggedin ? .green : .red

            // FaceID runs right away on evaluation, so you might want to warn the user.
            //  In this app, show a special Face ID prompt if the user is logged out, but
            //  only if the device supports that kind of authentication.
            faceIDLabel.isHidden = (state == .loggedin) || (context.biometryType != .faceID)
        }
    }
    // ...
}

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

Authenticator/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

Authenticator/AppDelegate.swift:11 — representative boundary

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

}

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: ViewController; Delegate: AppDelegate.
• Protocols: no local protocol declaration in the scanned source.
• Methods: viewDidLoad, tapButton.
• Files: Authenticator/AppDelegate.swift, Authenticator/ViewController.swift.

Architecture takeaways

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