Deferred Start for AV Capture Outputs

import AVFoundation
import UIKit

class FastCameraViewController: UIViewController {

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

    override func viewDidLoad() {
        super.viewDidLoad()
        setupPreviewLayer()
        sessionQueue.async { self.configureSession() }
    }

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

    private func configureSession() {
        session.beginConfiguration()

        // Automatic deferred start is enabled by default when compiled
        // against iOS 26+ SDK, but set it explicitly for clarity.
        session.automaticallyRunsDeferredStart = true

        // Add camera input
        guard
            let device = AVCaptureDevice.default(.builtInWideAngleCamera,
                                                 for: .video,
                                                 position: .back),
            let input = try? AVCaptureDeviceInput(device: device),
            session.canAddInput(input)
        else { return }
        session.addInputWithNoConnections(input)

        // Configure preview layer — NOT deferred, needed for first frame
        previewLayer.setSessionWithNoConnection(session)
        if let videoPort = input.ports(for: .video,
                                        sourceDeviceType: device.deviceType,
                                        sourceDevicePosition: device.position).first {
            let previewConnection = AVCaptureConnection(inputPort: videoPort,
                                                        videoPreviewLayer: previewLayer)
            session.addConnection(previewConnection)
        }

        // Configure photo output — DEFER it, not needed for preview
        if session.canAddOutput(photoOutput) {
            session.addOutput(photoOutput)
            // Defer this output so it doesn't block first preview frame
            photoOutput.isDeferredStartEnabled = true
            // Allow captures even before deferred init completes
            photoOutput.isResponsiveCaptureEnabled = true
        }

        session.commitConfiguration()

        // Attach delegate to receive deferred start callbacks
        session.deferredStartDelegate = self

        session.startRunning()
    }
}

extension FastCameraViewController: AVCaptureSessionDeferredStartDelegate {
    func captureSessionWillRunDeferredStart(_ session: AVCaptureSession) {
        // Called just before deferred outputs begin initializing
        print("Deferred start beginning — preview already visible")
    }

    func captureSessionDidRunDeferredStart(_ session: AVCaptureSession) {
        // All deferred outputs are now fully initialized
        DispatchQueue.main.async {
            print("Photo output ready — show remaining UI elements now")
        }
    }
}