AVCamBarcode: detecting barcodes and faces

At a glance

Project structure

AVCamBarcode/
├── AppDelegate.swift
├── CameraViewController.swift
├── PreviewView.swift
├── ItemSelectionViewController.swift
├── Base.lproj/
│   ├── Main.storyboard
│   └── LaunchScreen.storyboard
└── Assets.xcassets/

Structure observations

• Most capture, authorization, configuration, overlay, and UI coordination logic is colocated in CameraViewController.
• PreviewView owns preview-layer and resizable-region rendering behavior.
• ItemSelectionViewController<Item> is the only extracted reusable UI component and communicates completion through a protocol.

Overall architecture

flowchart LR
    Storyboard["Storyboard"] --> Controller["CameraViewController"]
    Controller --> Session["AVCaptureSession"]
    Session --> Metadata["AVCaptureMetadataOutput"]
    Metadata -->|delegate callback| Controller
    Controller --> Overlay["MetadataObjectLayer objects"]
    Overlay --> Preview["PreviewView / AVCaptureVideoPreviewLayer"]
    Selector["ItemSelectionViewController"] -->|delegate selection| Controller

Reference code

AVCamBarcode/CameraViewController.swift:257 — metadata-output wiring

if session.canAddOutput(metadataOutput) {
    session.addOutput(metadataOutput)
    metadataOutput.setMetadataObjectsDelegate(self, queue: metadataObjectsQueue)
    metadataOutput.metadataObjectTypes = metadataOutput.availableMetadataObjectTypes
}

Interpretation

The controller is both presentation coordinator and capture coordinator. Metadata arrives on a dedicated queue, is transformed into Core Animation overlays on the main queue, and is displayed over the preview layer. This is a compact UIKit sample rather than a separated service/view-model architecture.

Ownership and state

classDiagram
    CameraViewController *-- AVCaptureSession : creates
    CameraViewController *-- AVCaptureMetadataOutput : creates
    CameraViewController *-- MetadataObjectLayer : creates and retains
    CameraViewController o-- PreviewView : storyboard outlet
    ItemSelectionViewController o-- ItemSelectionViewControllerDelegate : weak
    PreviewView *-- CAShapeLayer : region controls

Ownership evidence

AVCamBarcode/CameraViewController.swift:173 — capture and UI references

private let session = AVCaptureSession()
private let sessionQueue = DispatchQueue(label: "session queue")
@IBOutlet private var previewView: PreviewView!
private let metadataOutput = AVCaptureMetadataOutput()

Class and protocol design

Access control

Reference code

AVCamBarcode/PreviewView.swift:137 — constrained region mutation

@objc private(set) dynamic var regionOfInterest = CGRect.null

func setRegionOfInterestWithProposedRegionOfInterest(
    _ proposedRegionOfInterest: CGRect
) {
    // Clamp, assign, and redraw.
}

No public, open, or fileprivate declaration appears in the Swift source.

Logic ownership and placement

Design patterns

Main application flow

sequenceDiagram
    participant Output as Metadata output
    participant Controller as CameraViewController
    participant Main as Main queue
    participant Preview as Preview layer
    Output->>Controller: didOutput metadataObjects
    Controller->>Controller: nonblocking semaphore wait
    Controller->>Main: transform objects and build layers
    Main->>Preview: replace overlay layers
    Main->>Controller: signal semaphore

Reference code

AVCamBarcode/CameraViewController.swift:792 — overlay callback

func metadataOutput(_ output: AVCaptureMetadataOutput,
                    didOutput metadataObjects: [AVMetadataObject],
                    from connection: AVCaptureConnection) {
    if metadataObjectsOverlayLayersDrawingSemaphore.wait(timeout: .now()) == .success {
        DispatchQueue.main.async {
            // Replace overlays, then signal.
        }
    }
}

Naming conventions

• Types: UIKit role suffixes (ViewController, View) and framework-aligned MetadataObjectLayer.
• Protocol: collaborator role with Delegate suffix.
• Methods: lifecycle/configuration commands (configureSession, changeCamera) and delegate signatures dictated by AVFoundation/UIKit.
• State: is... for Boolean status; ...Queue, ...Output, and ...Layer suffixes expose object roles.
• Files: one primary UIKit type per Swift file; storyboard resources are grouped in Base.lproj.

Architecture takeaways

• The source favors a conventional, controller-heavy UIKit sample over MVVM or service extraction.
• Separate serial queues protect capture configuration and metadata delivery; Core Animation changes return to the main queue.
• The preview view owns interactive geometry, while the controller owns detection semantics and overlay creation.
• private(set) makes a view invariant explicit: outside code can observe the region but cannot bypass validation.
• The generic selection controller is protocol-oriented at one narrow reuse point rather than across the whole app.

Source map

• Official Apple documentation