使用Xcode 12访问SwiftUI中的ViewModel字段: [英] Accessing ViewModel field in SwiftUI using Xcode 12: "Accessing State's value outside of being installed on a View"

问题描述

我认为此错误消息是Xcode 12中SwiftUI的新增功能，因为它在Google中获得了0次匹配，而消息本身却是相当通用的:

I think this error message is new to SwiftUI in Xcode 12 since it gave 0 hits in Google while the message itself is fairly generic:

在安装到View之外访问State的值.这将导致初始值的恒定绑定，并且不会更新.

Accessing State's value outside of being installed on a View. This will result in a constant Binding of the initial value and will not update.

我有以下代码(删除了一些绒毛):

I have the following code (removed some fluff):

public struct ContentView: View {
    @ObservedObject var model: RootViewModel

    public var body: some View {
        VStack(alignment: .center, content: {
            Picker(selection: model.$amount, label: Text("Amount")) {
                Text("€1").tag(1)
                Text("€2").tag(2)
                Text("€5").tag(5)
                Text("€10").tag(10)
            }.pickerStyle(SegmentedPickerStyle())
            Text("Donating: €\(model.amount)").font(.largeTitle)
        }).padding(.all, 20.0)
    }
}

public class RootViewModel: ObservableObject {
    @State public var amount: Int = 1
}

我曾经在 ContentView 中拥有 field 的权限，并且一切正常.现在用户界面不再更新，而是收到了运行时警告.

I used to have the field right in the ContentView and that worked alright. Now the UI does not update anymore and I got that run-time warning instead.

推荐答案

由于@Andrew的回答，我弄清楚了如何使其再次工作.首先，将 @State 更改为 @Published :

Thanks to @Andrew's answer I figured out how to make it work again. First you change the @State to @Published:

    @Published public var amount: Int = 1

接下来，您需要更改将 Picker 绑定到数据的方式:

Next, you need to change how you Picker is bound to the data:

            Picker(selection: $model.amount, label: Text("Amount")) {
                Text("€1").tag(1)
                Text("€2").tag(2)
                Text("€5").tag(5)
                Text("€10").tag(10)
            }.pickerStyle(SegmentedPickerStyle())

所以我们从 model.$ amount 转到了 $ model.amount .

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