ViewController.swift
import UIKit
import AVFoundation
import MobileCoreServices
import PryntTrimmerView
import Photos
import AVKit

class ViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate {



    @IBOutlet weak var trimmerView: TrimmerView!
    @IBOutlet weak var playerView: UIView!



    var player: AVPlayer?
    var playbackTimeCheckerTimer: Timer?
    var trimmerPositionChangedTimer: Timer?
    var imagePickerController = UIImagePickerController()
    var videoURL: URL?


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }



    @IBAction func selectAsset(_ sender: Any) {

        print("カメラロールから動画を選択する")
        imagePickerController.sourceType = .photoLibrary
        imagePickerController.delegate = self
        imagePickerController.mediaTypes = ["public.movie"]
        present(imagePickerController, animated: true, completion: nil)

    }

    func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
        videoURL = info["UIImagePickerControllerReferenceURL"] as? URL
        print(videoURL!)
        //playerView.image = previewImageFromVideo(videoURL!)!
        //playerView.contentMode = .scaleAspectFit
        //動画を洗濯したら元の画面に戻る
        imagePickerController.dismiss(animated: true, completion: nil)


        //関数実行
        previewImageFromVideo(videoURL!)!
    }

    func previewImageFromVideo(_ url:URL) -> UIImage? {
        print("動画からサムネイルを生成する")
        let asset = AVAsset(url:url)
        print(asset)
        print(type(of: asset))


        trimmerView.asset = asset
        trimmerView.delegate = self
        addVideoPlayer(with: asset, playerView: playerView)


        //下いらなくね
        let imageGenerator = AVAssetImageGenerator(asset:asset)
        imageGenerator.appliesPreferredTrackTransform = true
        var time = asset.duration
        time.value = min(time.value,2)
        do {
            let imageRef = try imageGenerator.copyCGImage(at: time, actualTime: nil)
            return UIImage(cgImage: imageRef)
        } catch {
            return nil
        }
    }


//    override func loadAsset(_ asset: AVAsset) {
//        
//        trimmerView.asset = asset
//        trimmerView.delegate = self
//        addVideoPlayer(with: asset, playerView: playerView)
//        
//        
//        print(asset)
//        print(  type(of: asset)  )
//        // 出力　-> AVURLAsset
//        
//        print("i can do it")
//    }


    private func addVideoPlayer(with asset: AVAsset, playerView: UIView) {
        let playerItem = AVPlayerItem(asset: asset)
        player = AVPlayer(playerItem: playerItem)

        NotificationCenter.default.addObserver(self, selector: #selector(ViewController.itemDidFinishPlaying(_:)),
                                               name: NSNotification.Name.AVPlayerItemDidPlayToEndTime, object: playerItem)

        let layer: AVPlayerLayer = AVPlayerLayer(player: player)
        layer.backgroundColor = UIColor.white.cgColor
        layer.frame = CGRect(x: 0, y: 0, width: playerView.frame.width, height: playerView.frame.height)
        layer.videoGravity = AVLayerVideoGravityResizeAspectFill
        playerView.layer.sublayers?.forEach({$0.removeFromSuperlayer()})
        playerView.layer.addSublayer(layer)
    }

    func itemDidFinishPlaying(_ notification: Notification) {
        if let startTime = trimmerView.startTime {
            player?.seek(to: startTime)
        }
    }

    func startPlaybackTimeChecker() {

        stopPlaybackTimeChecker()
        playbackTimeCheckerTimer = Timer.scheduledTimer(timeInterval: 0.1, target: self,
                                                        selector:
            #selector(ViewController.onPlaybackTimeChecker), userInfo: nil, repeats: true)
    }

    func stopPlaybackTimeChecker() {

        playbackTimeCheckerTimer?.invalidate()
        playbackTimeCheckerTimer = nil
    }

    func onPlaybackTimeChecker() {

        guard let startTime = trimmerView.startTime, let endTime = trimmerView.endTime, let player = player else {
            return
        }

        let playBackTime = player.currentTime()
        trimmerView.seek(to: playBackTime)

        if playBackTime >= endTime {
            player.seek(to: startTime, toleranceBefore: kCMTimeZero, toleranceAfter: kCMTimeZero)
            trimmerView.seek(to: startTime)
        }
    }


    @IBAction func play(_ sender: Any) {

        guard let player = player else { return }

        if !player.isPlaying {
            player.play()
            startPlaybackTimeChecker()
        } else {
            player.pause()
            stopPlaybackTimeChecker()
        }


    }

    @IBAction func trim(_ sender: Any) {
    }




}

extension ViewController: TrimmerViewDelegate {
    func positionBarStoppedMoving(_ playerTime: CMTime) {
        player?.seek(to: playerTime, toleranceBefore: kCMTimeZero, toleranceAfter: kCMTimeZero)
        player?.play()
        startPlaybackTimeChecker()
    }

    func didChangePositionBar(_ playerTime: CMTime) {
        stopPlaybackTimeChecker()
        player?.pause()
        player?.seek(to: playerTime, toleranceBefore: kCMTimeZero, toleranceAfter: kCMTimeZero)
        let duration = (trimmerView.endTime! - trimmerView.startTime!).seconds
        print(duration)
    }
}

extension AVPlayer {

    var isPlaying: Bool {
        return self.rate != 0 && self.error == nil
    }
}

AssetSelectionViewController.swift
import UIKit
import Photos

class AssetSelectionViewController: UIViewController {

    var fetchResult: PHFetchResult<PHAsset>?

    override func viewDidLoad() {
        super.viewDidLoad()
        loadLibrary()
    }

    func loadLibrary() {
        PHPhotoLibrary.requestAuthorization { (status) in
            if status == .authorized {
                self.fetchResult = PHAsset.fetchAssets(with: .video, options: nil)
            }
        }
    }

    func loadAssetRandomly() {
        guard let fetchResult = fetchResult, fetchResult.count > 0 else {
            print("Error loading assets.")
            return
        }

        let randomAssetIndex = Int(arc4random_uniform(UInt32(fetchResult.count - 1)))
        let asset = fetchResult.object(at: randomAssetIndex)
        PHCachingImageManager().requestAVAsset(forVideo: asset, options: nil) { (avAsset, audioMix, info) in
            DispatchQueue.main.async {
                if let avAsset = avAsset {
                    self.loadAsset(avAsset)
                }
            }
        }
    }

    func loadAsset(_ asset: AVAsset) {
        // override in subclass
    }
}