CameraPreviewLayer.swift

//
//  CameraView.swift
//  ALCameraViewController
//
//  Created by Alex Littlejohn on 2015/06/17.
//  Copyright (c) 2015 zero. All rights reserved.
//

import UIKit
import AVFoundation
import Dispatch

// Adapted from ALCameraViewController
// https://github.com/AlexLittlejohn/ALCameraViewController/blob/fce8499e4ccd52a2389c4b215d718a544b7aba51/ALCameraViewController/Views/CameraView.swift

/// CameraView is an object that manages the preview layer of the camera.
///
/// Construct one and retrieve the `previewLayer`. Also call the `startSession`
/// and `stopSession` when appropriate.
class CameraPreviewLayer: NSObject {


    // MARK: Init

    init(targetSize: CGSize) {

        super.init()

        // AVFoundation boilerplate...
        session = AVCaptureSession()
        session.sessionPreset = AVCaptureSessionPresetPhoto

        device = cameraWithPosition(.back)
        if let device = device, device.hasFlash {
            do {
                try device.lockForConfiguration()
                device.flashMode = .auto
                device.unlockForConfiguration()
            } catch _ {}
        }

        let outputSettings = [AVVideoCodecKey: AVVideoCodecJPEG]

        do {
            input = try AVCaptureDeviceInput(device: device)
        } catch let error as NSError {
            input = nil
            fatalError("Error: \(error.localizedDescription)")
        }

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

        imageOutput = AVCaptureStillImageOutput()
        imageOutput.outputSettings = outputSettings

        session.addOutput(imageOutput)


        // Create the layer
        previewLayer = AVCaptureVideoPreviewLayer(session: session)
        previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
        previewLayer.frame = CGRect(origin: CGPoint.zero, size: targetSize)


        // Handle rotation
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(CameraPreviewLayer.deviceOrientationDidChange),
            name: NSNotification.Name.UIDeviceOrientationDidChange,
            object: nil
        )
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    deinit {
        stopSession()

        // Zeroing-weak observers? Who knows if that works...
        NotificationCenter.default.removeObserver(self)
    }


    // MARK: Public properties

    /// The AVCaptureVideoPreviewLayer that shows the camera live feed.
    var previewLayer: AVCaptureVideoPreviewLayer!


    // MARK: Public methods

    /// Starts the AVCaptureSession. If the session is stopped (or not yet
    /// started) the view will be black.
    func startSession() {

        cameraQueue.sync {
            self.session.startRunning()
        }

        // Detect orientation changes
        UIDevice.current.beginGeneratingDeviceOrientationNotifications()
    }

    /// Stops the AVCaptureSession. This will also be called on deinit
    /// automatically.
    func stopSession() {

        // We're no longer insterested in orientation changes
        UIDevice.current.endGeneratingDeviceOrientationNotifications()

        cameraQueue.sync {
            self.session.stopRunning()
        }
    }


    // MARK: Private methods

    private func cameraWithPosition(_ position: AVCaptureDevicePosition) -> AVCaptureDevice? {
        guard let devices = AVCaptureDevice.devices(withMediaType: AVMediaTypeVideo) as? [AVCaptureDevice] else {
            return nil
        }

        return devices.filter { $0.position == position }.first
    }

    private func rotatePreview() {

        switch UIDevice.current.orientation {
        case .portrait:
            previewLayer.connection.videoOrientation = AVCaptureVideoOrientation.portrait
        case .portraitUpsideDown:
            previewLayer.connection.videoOrientation = AVCaptureVideoOrientation.portraitUpsideDown
        case .landscapeLeft:
            previewLayer.connection.videoOrientation = AVCaptureVideoOrientation.landscapeRight
        case .landscapeRight:
            previewLayer.connection.videoOrientation = AVCaptureVideoOrientation.landscapeLeft
        default: break
        }
    }

    @objc private func deviceOrientationDidChange() {
        rotatePreview()
    }


    // MARK: Private properties

    var session: AVCaptureSession!
    var input: AVCaptureDeviceInput!
    var device: AVCaptureDevice!
    var imageOutput: AVCaptureStillImageOutput!
    
    let cameraQueue = DispatchQueue(label: "com.zero.ALCameraViewController.Queue", attributes: [])
    
}