SwiftUI 绑定(bind) bool 外部 View - 'Accessing State' s 值不在 View 上安装'

Question

我试图在一个简单的测验中将逻辑与我的 UI 分开，例如示例。但是当我完成这些问题时，我正在为如何离开而苦苦挣扎。
在 Xcode 12 中，我收到错误消息:

Accessing State's value outside of being installed on a View. Thiswill result in a constant Binding of the initial value and will notupdate.

我从其他答案中看到@State 只能在结构中使用，并建议发布。但是我似乎无法弄清楚如何使用带有绑定(bind)的已发布属性而不是“安装在 View 上”

class ViewModel: ObservableObject {
    @Published var currentQuestion: String!

    private var questions = ["one", "two", "three"]
    private var index = 0

    init() {
        currentQuestion = questions[index]
    }

    func button(title: String) -> some View {
        Button(action: { [self] in
            if index < questions.count {
                index += 1
                currentQuestion = questions[index]
            } else {
                print("go to results view")
            }
        }) {
            Text(title)
        }
    }
}

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

    var body: some View {
        NavigationView {
            VStack {
                viewModel.button(title: viewModel.currentQuestion)
            }
        }
    }
}

struct ResultsView: View {
    var body: some View {
        Text("Results")
    }
}

任何关于采取何种方法的建议将不胜感激!

Answer 1

您可以尝试在 ViewModel 中使用所有逻辑:

class ViewModel: ObservableObject {
    @Published var currentQuestion: String!
    @Published var showResults = false

    private var questions = ["one", "two", "three"]
    private var index = 0

    init() {
        currentQuestion = questions[index]
    }
    
    func nextQuestion() {
        if index < questions.count {
            index += 1
            currentQuestion = questions[index]
        } else {
            showResults = true
        }
    }
}

和 View 中的 UI 组件(例如按钮):

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

    var body: some View {
        NavigationView {
            VStack {
                Text(viewModel.currentQuestion)
                Button(action: viewModel.nextQuestion) {
                    Text("Next")
                }
            }
            // if viewModel.showResults == true -> navigate to results
            .background(NavigationLink(destination: ResultsView(), isActive: $viewModel.showResults) { EmptyView() } .hidden())
        }
    }
}

在上面的示例中，您转到 ResultsView使用 NavigationLink但您可以轻松地用结果替换当前 View 。演示由您决定。