问SwuiftUI折叠多行文字

EN

let bigText = """
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
"""

struct ContentView: View {

    // we need `true` on construction (not visible) to calculate
    // max content height
    @State var expanded = true    // << required initial true !!

    @State private var maxHeight: CGFloat?

    var body: some View {
        VStack(spacing: 20) {
            HStack {
                Button(expanded ? "Colapse" : "Expand") {
                    withAnimation {
                        self.expanded.toggle()
                    }
                }
            }
            VStack(spacing: 10) {
                Text(bigText)
                Text(bigText)
            }
            .background(GeometryReader { // read content height
                Color.clear.preference(key: ViewHeightKey.self,
                                              value: $0.frame(in: .local).size.height)
            })
            .onPreferenceChange(ViewHeightKey.self) {
                if nil == self.maxHeight {
                    self.maxHeight = $0     // << needed once !!
                }
            }
            .modifier(AnimatingFrameHeight(height: expanded ? maxHeight ?? .infinity : 0))
            .clipped()
            .background(Color.red)
            Text("Thist is another text underneath the huge one. ")
                .font(.system(.headline))
                .foregroundColor(.red)
            Spacer()
        }
        .onAppear {
            // this cases instance redraw on first render
            // so initial state will be invisible for user
            expanded = false    // << set if needed here !!
        }
    }
}

struct AnimatingFrameHeight: AnimatableModifier {
    var height: CGFloat = 0

    var animatableData: CGFloat {
        get { height }
        set { height = newValue }
    }

    func body(content: Content) -> some View {
        content.frame(height: height)
    }
}

struct ViewHeightKey: PreferenceKey {
    static var defaultValue: CGFloat { 0 }
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value = value + nextValue()
    }
}