SwiftUI 中的 Expandable List 只展开一次，不会收缩

Question

我正在尝试（但失败了！）在 SwiftUI 中实现可扩展的问题和答案列表。

struct FAQ: Equatable, Identifiable {
    static func ==(lhs: FAQ, rhs: FAQ) -> Bool {
        return lhs.id == rhs.id
    }
    let id = UUID()
    let question: String
    let answers: [String]
    var isExpanded: Bool = false
}

struct ContentView: View {
    @State private (set) var faqs: [FAQ] = [
        FAQ(question: "What is the fastest animal on land?", answers: ["The cheetah"]),
        FAQ(question: "What colours are in a rainbox?", answers: ["Red", "Orange", "Yellow", "Blue", "Indigo", "Violet"])
    ]

    var body: some View {
        List {
            ForEach(faqs) { faq in
                Section(header: Text(faq.question)
                    .onTapGesture {
                        if let index = self.faqs.firstIndex(of: faq) {
                            self.faqs[index].isExpanded.toggle()
                        }
                    }
                ) {
                    if faq.isExpanded {
                        ForEach(faq.answers, id: \.self) {
                            Text("• " + $0).font(.body)
                        }
                    }
                }
            }
        }
    }
}

点击任何问题成功将答案展开到视图中，但再次点击同一标题不会缩小答案，点击第二个问题也不会扩大这些答案。

通过明智地放置prints，我可以看到isExpanded 在点击的第一个问题上切换为true，但随后不会切换回false。

谁能解释我做错了什么？

Answer 1

问题在于您的@State var faq: [FAQ] 行。 @State 属性包装器允许您的视图监视数组中的更改，但是更改数组元素之一的属性不会被视为数组中的更改。

您可能想要创建一个小型视图模型，而不是使用状态变量，如下所示：

class ViewModel: ObservableObject {
    @Published var faqs: [FAQ] = [
           FAQ(question: "What is the fastest animal on land?", answers: ["The cheetah"]),
           FAQ(question: "What colours are in a rainbox?", answers: ["Red", "Orange", "Yellow", "Blue", "Indigo", "Violet"])
       ]
}

并更新您的 ContentView：

struct ContentView: View {
    @ObservedObject var model = ViewModel()

    var body: some View {
        List {
            ForEach(model.faqs.indices) { index in
                Section(header: Text(self.model.faqs[index].question)
                    .onTapGesture {
                        self.model.faqs[index].isExpanded.toggle()
                    }
                ) {
                    if self.model.faqs[index].isExpanded {
                        ForEach(self.model.faqs[index].answers, id: \.self) {
                            Text("• " + $0).font(.body)
                        }
                    }
                }
            }
        }
    }
}

ContentView 中的 @ObservedObject 属性包装器现在监视其 ViewModel 对象宣布的任何更改，@Published 包装器告诉 ViewModel 类宣布数组发生的任何事情，包括更改它的元素。

Answer 2

通过将视图拆分为较小的自包含视图，可以轻松实现这一点。

模型不需要知道答案是否显示在屏幕上。因此，无需查看数组即可知道视图是否已展开。

此外，当某个部分展开时，不需要重新绘制整个列表。有关视图何时重绘的更多信息，请参阅this article。

struct FAQ {
    var question: String
    var answers: [String]
}

struct InfoView: View {
    let information: [FAQ] = [
        FAQ(question: "What is the fastest animal on land?", answers: ["The cheetah"]),
        FAQ(question: "What colours are in a rainbox?", answers: ["Red", "Orange", "Yellow", "Blue", "Indigo", "Violet"])
    ]

    var body: some View {
        List {
            ForEach(information, id: \.question) { info in
                InfoSection(info: info)
            }
        }
    }
}

struct InfoSection: View {

    let info: FAQ
    @State var showsAnswer = false

    var body: some View {
        Section(header: questionHeader) {
            if showsAnswer {
                ForEach(info.answers, id: \.self) { answer in
                    Text("• " + answer)
                }
            }
        }
    }

    var questionHeader: some View {
        Text(info.question)
            .foregroundColor(.primary)
            .onTapGesture {
            self.showsAnswer.toggle()
        }
    }
}