import UIKit

/// The container that lets you to flip two views.
class FlipView: UIView {

  /// Describes the side from which the view will flip.
  enum Side {

    /// Flips from the left side.
    case left

    /// Flips from the right side.
    case right

    /// Flips from the top side.
    case top

    /// Flips from the bottom side.
    case bottom

    /// The inversed flip side.
    var inversed: Side {
      switch self {
      case .left:
        return .right
      case .right:
        return .left
      case .top:
        return .bottom
      case .bottom:
        return .top
      }
    }

    /// The animation option.
    var animationOption: AnimationOptions {
      switch self {
      case .left:
        return .transitionFlipFromLeft
      case .right:
        return .transitionFlipFromRight
      case .top:
        return .transitionFlipFromTop
      case .bottom:
        return .transitionFlipFromBottom
      }
    }

  }

  /// The front view in the container.
  var frontView = UIView() {
    didSet {
      oldValue.removeFromSuperview()
      configure(frontView)
    }
  }

  /// The back view in the container.
  var backView = UIView() {
    didSet {
      oldValue.removeFromSuperview()
      configure(backView)
    }
  }

  /// The transition duration.
  var duration = 0.5

  /// The flag that indicates that the flip side will be inverted after showing the back side.
  var hasInversibleSide = false

  /// The flag that indicates which view to show.
  private var isFlipped = false

  /// Initializes the container with a frame.
  ///
  /// - Parameter frame: The container frame.
  override init(frame: CGRect) {
    super.init(frame: frame)

    configure(frontView)
    configure(backView)
  }

  /// From `NSCoding` protocol.
  required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  /// Configures a view in the container.
  ///
  /// - Parameter view: A view in the container.
  private func configure(_ view: UIView) {

    // Add a view to the container.
    addSubview(view)

    // Bring the front view to front of the container.
    bringSubviewToFront(frontView)

    view.translatesAutoresizingMaskIntoConstraints = false

    // Make a view size equal to the container size.
    NSLayoutConstraint.activate([
      view.widthAnchor.constraint(equalTo: widthAnchor),
      view.heightAnchor.constraint(equalTo: heightAnchor),
      view.centerXAnchor.constraint(equalTo: centerXAnchor),
      view.centerYAnchor.constraint(equalTo: centerYAnchor),
    ])
  }

  /// Flips the front view.
  ///
  /// - Parameter side: The side from which the view will flip.
  func flip(from side: Side) {

    // Choose views for transition.
    let from = isFlipped ? backView : frontView
    let to = isFlipped ? frontView : backView

    // Inverse the flip side if the view is flipped and the flip side is inversible.
    let side = isFlipped && hasInversibleSide ? side.inversed : side

    // Flip the front view.
    UIView.transition(
      from: from,
      to: to,
      duration: duration,
      options: [side.animationOption, .showHideTransitionViews]) { [unowned self] _ in
        self.isFlipped = !self.isFlipped
    }
  }

}