public protocol DrawerCoordinating: class {
    /// An object vended by the conforming object, whose responsibility is to control
    /// the presentation, animation, and interactivity of/with the drawer.
    var drawerDisplayController: DrawerDisplayController? { get }
}

public protocol DrawerPresentable: class {
    /// The height at which the drawer must be presented when it's in its
    /// partially expanded state. If negative, its value is clamped to zero.
    var heightOfPartiallyExpandedDrawer: CGFloat { get }
}

extension PresenterViewController {
    func doModalPresentation() {
        guard let vc = storyboard?.instantiateViewController(withIdentifier: "presented")
            as? PresentedViewController else { return }

        // you can provide the configuration values in the initialiser...
        var configuration = DrawerConfiguration(/* ..., ..., ..., */)

        // ... or after initialisation. All of these have default values so change only
        // what you need to configure differently. They're all listed here just so you
        // can see what can be configured. The values listed are the default ones,
        // except where indicated otherwise.
        configuration.totalDurationInSeconds = 3 // default is 0.4
        configuration.durationIsProportionalToDistanceTraveled = false
        // default is UISpringTimingParameters()
        configuration.timingCurveProvider = UISpringTimingParameters(dampingRatio: 0.8)
        configuration.fullExpansionBehaviour = .leavesCustomGap(gap: 100) // default is .coversFullScreen
        configuration.supportsPartialExpansion = true
        configuration.dismissesInStages = true
        configuration.isDrawerDraggable = true
        configuration.isFullyPresentableByDrawerTaps = true
        configuration.numberOfTapsForFullDrawerPresentation = 1
        configuration.isDismissableByOutsideDrawerTaps = true
        configuration.numberOfTapsForOutsideDrawerDismissal = 1
        configuration.flickSpeedThreshold = 3
        configuration.upperMarkGap = 100 // default is 40
        configuration.lowerMarkGap =  80 // default is 40
        configuration.maximumCornerRadius = 15

        var handleViewConfiguration = HandleViewConfiguration()
        handleViewConfiguration.autoAnimatesDimming = true
        handleViewConfiguration.backgroundColor = .gray
        handleViewConfiguration.size = CGSize(width: 40, height: 6)
        handleViewConfiguration.top = 8
        handleViewConfiguration.cornerRadius = .automatic
        configuration.handleViewConfiguration = handleViewConfiguration

        let borderColor = UIColor(red: 205.0/255.0, green: 206.0/255.0, blue: 210.0/255.0, alpha: 1)
        let drawerBorderConfiguration = DrawerBorderConfiguration(borderThickness: 0.5,
                                                                  borderColor: borderColor)
        configuration.drawerBorderConfiguration = drawerBorderConfiguration

        let drawerShadowConfiguration = DrawerShadowConfiguration(shadowOpacity: 0.25,
                                                                  shadowRadius: 4,
                                                                  shadowOffset: .zero,
                                                                  shadowColor: .black)
        configuration.drawerShadowConfiguration = drawerShadowConfiguration

        drawerDisplayController = DrawerDisplayController(presentingViewController: self,
                                                          presentedViewController: vc,
                                                          configuration: configuration,
                                                          inDebugMode: true)

        present(vc, animated: true)
    }
}

extension PresentedViewController: DrawerPresentable {
    var heightOfPartiallyExpandedDrawer: CGFloat {
        guard let view = self.view as? PresentedView else { return 0 }
        return view.dividerView.frame.origin.y
    }
}

extension PresentedViewController {
    @IBAction func dismissButtonTapped() {
        dismiss(animated: true)
    }
}

    /// The total duration, in seconds, for the drawer to transition from its
    /// collapsed state to its fully-expanded state, or vice-versa. The default
    /// value is 0.4 seconds.
    public var totalDurationInSeconds: TimeInterval

    /// When the drawer transitions between its collapsed and partially-expanded
    /// states, or between its partially-expanded and its fully-expanded states, in
    /// either direction, the distance traveled by the drawer is some fraction of
    /// the total distance traveled between the collapsed and fully-expanded states.
    /// You have a choice between having those fractional transitions take the same
    /// amount of time as the full transition, and having them take a time that is
    /// a fraction of the total time, where the fraction used is the fraction of
    /// space those partial transitions travel. In the first case, all transitions
    /// have the same duration (`totalDurationInSeconds`) but different speeds, while
    /// in the second case different transitions have different durations but the same
    /// speed. The default is `false`, that is, all transitions last the same amount
    /// of time.
    public var durationIsProportionalToDistanceTraveled: Bool

    /// The type of timing curve to use for the animations. The full set of cubic
    /// Bezier curves and spring-based curves is supported. Note that selecting a
    /// spring-based timing curve may cause the `totalDurationInSeconds` parameter
    /// to be ignored because the duration, for a fully general spring-based timing
    /// curve provider, is computed based on the specifics of the spring-based curve.
    /// The default is `UISpringTimingParameters()`, which is the system's global
    /// spring-based timing curve.
    public var timingCurveProvider: UITimingCurveProvider

    /// Whether the drawer expands to cover the entire screen, the entire screen minus
    /// the status bar, or the entire screen minus a custom gap. The default is to cover
    /// the full screen.
    public var fullExpansionBehaviour: FullExpansionBehaviour

    /// When `true`, the drawer is presented first in its partially expanded state.
    /// When `false`, the presentation is always to full screen and there is no
    /// partially expanded state. The default value is `true`.
    public var supportsPartialExpansion: Bool

    /// When `true`, dismissing the drawer from its fully expanded state can result
    /// in the drawer stopping at its partially expanded state. When `false`, the
    /// dismissal is always straight to the collapsed state. Note that
    /// `supportsPartialExpansion` being `false` implies `dismissesInStages` being
    /// `false` as well but you can have `supportsPartialExpansion == true` and
    /// `dismissesInStages == false`, which would result in presentations to the
    /// partially expanded state but all dismissals would be straight to the collapsed
    /// state. The default value is `true`.
    public var dismissesInStages: Bool

    /// Whether or not the drawer can be dragged up and down. The default value is `true`.
    public var isDrawerDraggable: Bool

    /// Whether or not the drawer can be fully presentable by tapping on it.
    /// The default value is `true`.
    public var isFullyPresentableByDrawerTaps: Bool

    /// How many taps are required for fully presenting the drawer by tapping on it.
    /// The default value is 1.
    public var numberOfTapsForFullDrawerPresentation: Int

    /// Whether or not the drawer can be dismissed by tapping anywhere outside of it.
    /// The default value is `true`.
    public var isDismissableByOutsideDrawerTaps: Bool

    /// How many taps are required for dismissing the drawer by tapping outside of it.
    /// The default value is 1.
    public var numberOfTapsForOutsideDrawerDismissal: Int

    /// How fast one needs to "flick" the drawer up or down to make it ignore the
    /// partially expanded state. Flicking fast enough up always presents to full screen
    /// and flicking fast enough down always collapses the drawer. A typically good value
    /// is around 3 points per screen height per second, and that is also the default
    /// value of this property.
    public var flickSpeedThreshold: CGFloat

    /// There is a band around the partially expanded position of the drawer where
    /// ending a drag inside will cause the drawer to move back to the partially
    /// expanded position (subjected to the conditions set by `supportsPartialExpansion`
    /// and `dismissesInStages`, of course). Set `inDebugMode` to `true` to see lines
    /// drawn at those positions. This value represents the gap *above* the partially
    /// expanded position. The default value is 40 points.
    public var upperMarkGap: CGFloat

    /// There is a band around the partially expanded position of the drawer where
    /// ending a drag inside will cause the drawer to move back to the partially
    /// expanded position (subjected to the conditions set by `supportsPartialExpansion`
    /// and `dismissesInStages`, of course). Set `inDebugMode` to `true` to see lines
    /// drawn at those positions. This value represents the gap *below* the partially
    /// expanded position. The default value is 40 points.
    public var lowerMarkGap: CGFloat

    /// The animating drawer also animates the radius of its top left and top right
    /// corners, from 0 to the value of this property. Setting this to 0 prevents any
    /// corner animations from taking place. The default value is 15 points.
    public var maximumCornerRadius: CGFloat

    /// The configuration options for the handle view, should it be shown. Set this
    /// property to `nil` to hide the handle view. The default value is
    /// `HandleViewConfiguration()`.
    public var handleViewConfiguration: HandleViewConfiguration?

    /// The configuration options for the drawer's border, should it be shown. Set this
    /// property to `nil` so as not to have a drawer border. The default value is `nil`.
    public var drawerBorderConfiguration: DrawerBorderConfiguration?

    /// The configuration options for the drawer's shadow, should it be shown. Set this
    /// property to `nil` so as not to have a drawer shadow. The default value is `nil`.
    public var drawerShadowConfiguration: DrawerShadowConfiguration?

    public enum FullExpansionBehaviour: Equatable {
        case coversFullScreen
        case dosNotCoverStatusBar
        case leavesCustomGap(gap: CGFloat)
    }

public struct HandleViewConfiguration {
    /// Whether or not to automatically dim the handle view as the drawer approaches
    /// its collapsed or fully expanded states. The default is `true`. Set it to `false`
    /// when configuring the drawer not to cover the full screen so that the handle view
    /// is always visible in that case.
    public var autoAnimatesDimming: Bool

    /// The handle view's background color. The default value is `UIColor.gray`.
    public var backgroundColor: UIColor

    /// The handle view's bounding rectangle's size. The default value is
    /// `CGSize(width: 40, height: 6)`.
    public var size: CGSize

    /// The handle view's vertical distance from the top of the drawer. In other words,
    /// the constant to be used when setting up the layout constraint
    /// `handleView.topAnchor.constraint(equalTo: presentedView.topAnchor, constant: top)`
    /// The default value is 8 points.
    public var top: CGFloat

    /// The handle view's corner radius. The default is `CornerRadius.automatic`, which
    /// results in a corner radius equal to half the handle view's height.
    public var cornerRadius: CornerRadius
}

public struct DrawerBorderConfiguration {
    /// The drawer's layer’s border thickness. The default value is 0,
    /// so effectively the default is not to have any border at all.
    public let borderThickness: CGFloat

    /// The drawer's layer’s border's color. The default value is `nil`, so
    /// effectively the default is not to have any border at all.
    public let borderColor: UIColor?

    public init(borderThickness: CGFloat = 0, borderColor: UIColor? = nil)
}

public struct DrawerShadowConfiguration {
    /// The drawer's layer’s shadow's opacity. The default value is 0, so
    /// effectively the default is not to have any shadow at all.
    public let shadowOpacity: CGFloat

    /// The blur radius (in points) used to render the drawer's layer’s shadow.
    /// The default value is 0, so effectively the default is not to have any
    /// shadow at all.
    public let shadowRadius: CGFloat

    /// The offset (in points) of the drawer's layer’s shadow. The default value is
    /// `CGSize.zero`, so effectively the default is not to have any shadow at all.
    public let shadowOffset: CGSize

    /// The drawer's layer’s shadow's color. The default value is `nil`, so
    /// effectively the default is not to have any shadow at all.
    public let shadowColor: UIColor?

    public init(shadowOpacity: CGFloat = 0,
                shadowRadius: CGFloat = 0,
                shadowOffset: CGSize = .zero,
                shadowColor: UIColor? = nil)
}

    if isMovingUpQuickly { show fully expanded }
    if isMovingDownQuickly { collapse all the way (ie, dismiss) }

    if isAboveUpperMark {
        if isMovingUp || isNotMoving {
            show fully expanded
        } else { // is moving down
            collapse to the partially expanded state or all the way (ie, dismiss),
            depending on the values of `supportsPartialExpansion` and `dismissesInStages`
        }
    }

    if isAboveLowerMark { // ie, in the band surrounding the partially expanded state
        if isMovingDown {
            collapse all the way (ie, dismiss)
        } else { // not moving or moving up
            expand to the partially expanded state or all the way (ie, full-screen),
            depending on the value of `supportsPartialExpansion`
        }
    }

    // below the band surrounding the partially expanded state
    collapse all the way (ie, dismiss)