AVCaptureSession Deferred Start API

import AVFoundation
import UIKit

class CameraViewController: UIViewController {

    private let session = AVCaptureSession()
    private let photoOutput = AVCapturePhotoOutput()
    private let previewLayer = AVCaptureVideoPreviewLayer()
    private let sessionQueue = DispatchQueue(label: "com.example.camera.session")

    override func viewDidLoad() {
        super.viewDidLoad()
        setupPreviewLayer()
        // Configure session off the main thread to avoid blocking UI
        sessionQueue.async { self.configureSession() }
    }

    private func setupPreviewLayer() {
        previewLayer.videoGravity = .resizeAspectFill
        previewLayer.frame = view.bounds
        view.layer.insertSublayer(previewLayer, at: 0)
    }

    private func configureSession() {
        guard let camera = AVCaptureDevice.default(.builtInWideAngleCamera,
                                                   for: .video,
                                                   position: .back),
              let input = try? AVCaptureDeviceInput(device: camera) else { return }

        session.beginConfiguration()
        session.sessionPreset = .photo

        // Enable automatic deferred start — system will defer after first preview frame
        session.automaticallyRunsDeferredStart = true

        if session.canAddInput(input) {
            session.addInput(input)
        }

        // Preview layer is NOT deferred — it must be ready immediately
        previewLayer.session = session
        // isDeferredStartEnabled defaults to false, so preview layer initializes eagerly

        // Photo output IS deferred — not needed until user taps shutter
        if session.canAddOutput(photoOutput) {
            session.addOutput(photoOutput)
            photoOutput.isDeferredStartEnabled = true
            // Allow capture requests even before deferred init completes
            photoOutput.isResponsiveCaptureEnabled = true
        }

        session.commitConfiguration()

        // Set delegate to receive deferred-start lifecycle callbacks
        session.deferredStartDelegate = self

        session.startRunning()
    }

    @IBAction func shutterTapped(_ sender: UIButton) {
        let settings = AVCapturePhotoSettings()
        photoOutput.capturePhoto(with: settings, delegate: self)
    }
}

extension CameraViewController: AVCaptureSessionDeferredStartDelegate {
    func captureSessionWillRunDeferredStart(_ session: AVCaptureSession) {
        // Good moment to prepare any non-critical background resources
        print("Deferred start beginning — photo output about to initialize")
    }

    func captureSessionDidRunDeferredStart(_ session: AVCaptureSession) {
        // All deferred outputs are now fully initialized
        DispatchQueue.main.async {
            // Fade in non-critical UI elements now that full session is ready
            print("Deferred start complete — all outputs ready")
        }
    }
}

extension CameraViewController: AVCapturePhotoCaptureDelegate {
    func photoOutput(_ output: AVCapturePhotoOutput,
                     didFinishProcessingPhoto photo: AVCapturePhoto,
                     error: Error?) {
        guard error == nil, let data = photo.fileDataRepresentation() else { return }
        print("Captured photo: \(data.count) bytes")
    }
}