slow-camera.swift

import UIKit
import AVFoundation

class ViewController: UIViewController, AVCapturePhotoCaptureDelegate {
    var captureSession: AVCaptureSession!
    var photoOutput: AVCapturePhotoOutput!
    var previewLayer: AVCaptureVideoPreviewLayer!
    var captureButton: UIButton!
    var captureStartTime: Date?

    override func viewDidLoad() {
        super.viewDidLoad()
        
        setupCameraSession()
        setupPreviewLayer()
        setupCaptureButton()
    }
    
    func setupCameraSession() {
        captureSession = AVCaptureSession()
        captureSession.sessionPreset = .photo
        
        let wideCamera = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back)!
        let input = try! AVCaptureDeviceInput(device: wideCamera)
        
        captureSession.addInput(input)
        
        photoOutput = AVCapturePhotoOutput()
        
        let prewarmSettings = AVCapturePhotoSettings(format: [AVVideoCodecKey: AVVideoCodecType.jpeg])
        prewarmSettings.photoQualityPrioritization = .speed
        
        photoOutput.setPreparedPhotoSettingsArray([prewarmSettings]) { didCompletePrewarming, error in
            print("Prewarming complete: \(didCompletePrewarming) with error: \(String(describing: error))")
        }

        if captureSession.canAddOutput(photoOutput) {
            captureSession.addOutput(photoOutput)
        }
        
        captureSession.startRunning()
    }
    
    func setupPreviewLayer() {
        previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        previewLayer.frame = view.bounds
        previewLayer.videoGravity = .resizeAspectFill
        view.layer.addSublayer(previewLayer)
    }
    
    func setupCaptureButton() {
        captureButton = UIButton(type: .system)
        captureButton.setTitle("Capture", for: .normal)
        captureButton.titleLabel?.font = UIFont.preferredFont(forTextStyle: .largeTitle)
        captureButton.backgroundColor = UIColor.systemBlue
        captureButton.setTitleColor(.white, for: .normal)
        captureButton.translatesAutoresizingMaskIntoConstraints = false
        captureButton.addTarget(self, action: #selector(capturePhoto), for: .touchUpInside)
        
        view.addSubview(captureButton)
        
        NSLayoutConstraint.activate([
            captureButton.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            captureButton.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
        ])
    }
    
    @objc func capturePhoto() {
        captureStartTime = Date.now
        
        let settings = AVCapturePhotoSettings(format: [AVVideoCodecKey: AVVideoCodecType.jpeg])
        settings.photoQualityPrioritization = .speed

        photoOutput.capturePhoto(with: settings, delegate: self)
    }
    
    func photoOutput(_ output: AVCapturePhotoOutput, didFinishProcessingPhoto photo: AVCapturePhoto, error: Error?) {
        let callbackTime = Date.now
        
        let elapsedTime = callbackTime.timeIntervalSince(captureStartTime!)
        let image = UIImage(data: photo.fileDataRepresentation()!)!
        print("Time from button tap to delegate callback: \(elapsedTime * 1000) ms. Image size: \(image.size)")
        
        captureStartTime = nil
    }
}