SwiftUIでハーフモーダル的なものを出す

HarfModal.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        ZStack(alignment: .top) {
            MapView()
            SlideOverCard {
                VStack {
                    Text("Half Modal")
                        .font(.headline)
                        .foregroundColor(Color.white)
                    Spacer()
                }
                .frame(maxWidth: .infinity, maxHeight: .infinity)
            }
            .frame(height: 800)
            .padding(.horizontal, 10.0)

        }
        .edgesIgnoringSafeArea(.vertical)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct SlideOverCard<Content: View> : View {
    @GestureState private var dragState = DragState.inactive
    @State var position = CardPosition.top

    var content: () -> Content

    var body: some View {
        let drag = DragGesture()
            .updating($dragState) { drag, state, transaction in
                state = .dragging(translation: drag.translation)
            }
            .onEnded(onDragEnded)

        return Group {
            VStack {
                Handle()
                    .background(Color.clear)
                self.content()
            }
        }
        .background(Color(#colorLiteral(red: 0.2549019754, green: 0.2745098174, blue: 0.3019607961, alpha: 1)))
        .cornerRadius(10.0)
        .shadow(color: Color(.sRGBLinear, white: 0, opacity: 0.13), radius: 10.0)
        .offset(y: position.rawValue + dragState.translation.height)
        .animation(dragState.isDragging ? nil : .interpolatingSpring(stiffness: 300.0, damping: 30.0, initialVelocity: 10.0))
        .opacity(0.5)
        .gesture(drag)
    }

    private func onDragEnded(drag: DragGesture.Value) {
        let verticalDirection = drag.predictedEndLocation.y - drag.location.y
        let cardTopEdgeLocation = self.position.rawValue + drag.translation.height
        let positionAbove: CardPosition
        let positionBelow: CardPosition
        let closestPosition: CardPosition

        if cardTopEdgeLocation <= CardPosition.middle.rawValue {
            positionAbove = .top
            positionBelow = .middle
        } else {
            positionAbove = .middle
            positionBelow = .bottom
        }

        if (cardTopEdgeLocation - positionAbove.rawValue) < (positionBelow.rawValue - cardTopEdgeLocation) {
            closestPosition = positionAbove
        } else {
            closestPosition = positionBelow
        }

        if verticalDirection > 0 {
            self.position = positionBelow
        } else if verticalDirection < 0 {
            self.position = positionAbove
        } else {
            self.position = closestPosition
        }
    }
}

enum CardPosition: CGFloat {
    case top = 100
    case middle = 500
    case bottom = 850
}

enum DragState {
    case inactive
    case dragging(translation: CGSize)

    var translation: CGSize {
        switch self {
        case .inactive:
            return .zero
        case .dragging(let translation):
            return translation
        }
    }

    var isDragging: Bool {
        switch self {
        case .inactive:
            return false
        case .dragging:
            return true
        }
    }
}