UIViewPropertyAnimatorSample.swift

class ViewController: UIViewController {

    enum ImageViewState: Int {
        case open
        case close

        var opposite: ImageViewState {
            switch self {
            case .open:
                return .close
            case .close:
                return .open
            }
        }
    }

    private lazy var imageView: UIImageView = {
        let imageView = UIImageView()
        imageView.image = #imageLiteral(resourceName: "background")
        imageView.isUserInteractionEnabled = true
        return imageView
    }()

    private let blurView: UIVisualEffectView = {
        let blurView = UIVisualEffectView()
        blurView.isUserInteractionEnabled = false
        return blurView
    }()
    private var imageViewTopConstraint = NSLayoutConstraint()
    private var offset: CGFloat {
        return view.frame.maxY - 80
    }

    // UIViewPropertyAnimatorの生成
    private var animator = UIViewPropertyAnimator(duration: 3.0, curve: .easeInOut)
    private var animationProgress: CGFloat = 0.0

    private var imageViewState: ImageViewState = .close

    override func viewDidLoad() {
        super.viewDidLoad()
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageViewTopConstraint = imageView.topAnchor.constraint(equalTo: view.topAnchor)
        view.addSubview(imageView)

        NSLayoutConstraint.activate(
            [
                imageViewTopConstraint,
                imageView.leftAnchor.constraint(equalTo: view.leftAnchor),
                imageView.rightAnchor.constraint(equalTo: view.rightAnchor),
                imageView.heightAnchor.constraint(equalTo: view.heightAnchor)
            ]
        )

        blurView.translatesAutoresizingMaskIntoConstraints = false
        imageView.addSubview(blurView)
        NSLayoutConstraint.activate(
            [
                blurView.topAnchor.constraint(equalTo: imageView.topAnchor),
                blurView.leftAnchor.constraint(equalTo: imageView.leftAnchor),
                blurView.rightAnchor.constraint(equalTo: imageView.rightAnchor),
                blurView.bottomAnchor.constraint(equalTo: imageView.bottomAnchor)
            ]
        )

        let recognizer = UIPanGestureRecognizer()
        recognizer.addTarget(self, action: #selector(self.imageViewPanned(recognizer:)))
        imageView.addGestureRecognizer(recognizer)
    }

    private func setupAnimator(state: ImageViewState) {
        // 逆再生をoff
        animator.isReversed = false

        // アニメーション実行中の場合はアニメーションを設定しない
        if animator.isRunning {
            return
        }
        // アニメーションの追加
        animator.addAnimations {
            switch state {
            case .close:
                self.imageViewTopConstraint.constant = -self.offset
            case .open:
                self.imageViewTopConstraint.constant = 0
            }
            self.view.layoutIfNeeded()
        }
        // アニメーションの追加
        animator.addAnimations {
            switch state {
            case .close:
                self.blurView.effect = UIBlurEffect(style: .light)
            case .open:
                self.blurView.effect = nil
            }
        }
　      // アニメーション完了時の処理
        animator.addCompletion { position in
            switch position {
            case .start:
                self.imageViewState = state
            case .end:
                self.imageViewState = state.opposite
            case .current: ()
            }

            switch self.imageViewState {
            case .close:
                self.imageViewTopConstraint.constant = 0.0
            case .open:
                self.imageViewTopConstraint.constant = -self.offset
            }
        }
    }

    @objc private func imageViewPanned(recognizer: UIPanGestureRecognizer) {
        switch recognizer.state {
        case .began:
            // Animatorの設定
            setupAnimator(state: imageViewState)
            // アニメーションの一時停止
            animator.pauseAnimation()
            // アニメーションの進捗率を保持
            animationProgress = animator.fractionComplete
        case .changed:
            let translation = recognizer.translation(in: imageView)
            var fraction = -translation.y / offset
            if imageViewState == .open { fraction *= -1 }
            // ユーザー操作とアニメーションの進捗率を連動させインタラクティブアニメーションを実現するキーポイント
            animator.fractionComplete = fraction + animationProgress

        case .ended:
            let yVelocity = recognizer.velocity(in: imageView).y
            if yVelocity == 0 {
                // アニメーション再開
                animator.continueAnimation(withTimingParameters: nil, durationFactor: 0)
            }

            switch imageViewState {
            case .close:
                if yVelocity > 0 {
                    // 逆再生モードON
                    animator.isReversed = true
                }
            case .open:
                if yVelocity < 0 {
                    // 逆再生モードON
                    animator.isReversed = true
                }
            }
            // アニメーション再開
            animator.continueAnimation(withTimingParameters: nil, durationFactor: 0)

        default: ()
        }
    }

}