Dynamics.swift

import UIKit

class ViewController: UIViewController {

  struct Constants {
    static let offset: CGFloat = 5
    static let height: CGFloat = 300
    static let maximum: CGFloat = Constants.height + Constants.offset
    static let maximumY = UIScreen.mainScreen().bounds.height - Constants.maximum
    static let velocity: CGFloat = 250
    static let arrel: CGFloat = 0.6
    static let sticky: CGFloat = 60
  }

  lazy var bottomView: BottomView = { [unowned self] in
    let view = BottomView()
    view.backgroundColor = UIColor(red:0.98, green:0.98, blue:0.97, alpha:1.00)
    view.layer.cornerRadius = 10
    view.translatesAutoresizingMaskIntoConstraints = false
    view.addGestureRecognizer(self.panGesture)

    return view
  }()

  lazy var panGesture: UIPanGestureRecognizer = { [unowned self] in
    let gesture = UIPanGestureRecognizer()
    gesture.addTarget(self, action: #selector(handlePanGesture(_:)))

    return gesture
  }()

  var animator: UIDynamicAnimator?
  var snapBehavior: UISnapBehavior?
  var itemBehavior: UIDynamicItemBehavior?
  var attachmentBehavior: UIAttachmentBehavior?
  var origin: CGPoint = CGPoint.zero
  var bottomDown: Bool = true

  override func viewDidLoad() {
    super.viewDidLoad()

    view.addSubview(bottomView)
    view.backgroundColor = UIColor(red: 0.08, green: 0.09, blue: 0.09, alpha: 1.00)

    animator = UIDynamicAnimator(referenceView: view)

    setupConstraints()
  }

  // MARK: - Gesture

  func handlePanGesture(gesture: UIPanGestureRecognizer) {
    let velocity = gesture.velocityInView(view)
    var translation = gesture.translationInView(view)
    translation.x = CGRectGetMidX(bottomView.bounds)

    if gesture.state == .Began {
      if let behavior = snapBehavior, itemBehavior = itemBehavior {
        animator?.removeBehavior(behavior)
        animator?.removeBehavior(itemBehavior)
      }

      origin = bottomView.frame.origin
      attachmentBehavior = UIAttachmentBehavior(
        item: bottomView, attachedToAnchor:
        CGPoint(x: origin.x + translation.x, y: origin.y + translation.y))

      bottomDown = origin.y > Constants.maximumY

      if let behavior = attachmentBehavior {
        animator?.addBehavior(behavior)
      }
    } else if gesture.state == .Changed {
      if bottomView.frame.origin.y < Constants.maximumY {
        let number = (origin.y + translation.y) / Constants.maximumY
        attachmentBehavior?.anchorPoint.y = Constants.maximumY * smaller(sqrt(number), Constants.arrel)
      } else {
        attachmentBehavior?.anchorPoint.y = origin.y + translation.y
      }
    } else {
      if let behavior = attachmentBehavior {
        animator?.removeBehavior(behavior)
      }

      let condition = abs(translation.y) > Constants.sticky
        || abs(velocity.y) > Constants.velocity ? velocity.y < 0 : !bottomDown
      condition ? snapView(Constants.maximum) : snapView(Constants.sticky)
    }
  }

  // MARK: - Dynamic methods

  func snapView(value: CGFloat) {
    let size = UIScreen.mainScreen().bounds.size

    snapBehavior = UISnapBehavior(item: bottomView, snapToPoint: CGPoint(
      x: size.width / 2,
      y: size.height + Constants.height / 2 - value))
    snapBehavior?.damping = 0.15
    itemBehavior = UIDynamicItemBehavior(items: [bottomView])
    itemBehavior?.allowsRotation = false

    if let behavior = snapBehavior, itemBehavior = itemBehavior {
      animator?.addBehavior(behavior)
      animator?.addBehavior(itemBehavior)
    }
  }

  // MARK: - Helper methods

  func smaller(ln: CGFloat, _ rn: CGFloat) -> CGFloat {
    return ln < rn ? rn : ln
  }

  override func preferredStatusBarStyle() -> UIStatusBarStyle {
    return .LightContent
  }

  // MARK: - Constraints

  func setupConstraints() {
    NSLayoutConstraint.activateConstraints([
      bottomView.widthAnchor.constraintEqualToAnchor(view.widthAnchor, constant: -Constants.offset * 2),
      bottomView.heightAnchor.constraintEqualToConstant(Constants.height),
      bottomView.rightAnchor.constraintEqualToAnchor(view.rightAnchor, constant: -Constants.offset),
      bottomView.topAnchor.constraintEqualToAnchor(view.bottomAnchor, constant: -Constants.sticky)
      ])
  }
}