SwiftUI @EnvironmentObject error: may be missing as an ancestor of this view -- accessing object in the init()

Question

以下代码产生运行时错误：@EnvironmentObject 错误：作为此视图的祖先可能缺失。环境中的 tState 是一个@ObservedObject。

struct TEditorView: View {
    @EnvironmentObject private var tState: TState
    
    @State var name = ""
    
    init() {
        self._name = State(initialValue: tState.name)
    }
 
    var body: some View {
        ...
    }
}

XCode 12.0.1 iOS14

Answer 1

答案是显然不能在 init() 函数中访问环境对象。但是，ObservedObject 可以。所以我将代码更改为此并且它有效。为了简单起见，我将 TState 变成了一个我可以在任何地方访问的单例。在许多情况下，这可能会取代 @EnvironmentObject 的使用。

struct TEditorView: View {
    @ObservedObject private var tState = TState.shared
    //@EnvironmentObject private var tState: TState
    
    @State var name = ""
    
    init() {
        self._name = State(initialValue: tState.name)
    }
 
    var body: some View {
        ...
    }
}

Answer 2

此处的另一种方法可能是在构造函数中注入初始 TState 值，并使用 @EnvironmentObject 完全注入 do-away。然后在父视图中，您可以在创建视图时使用 @EnvironmentObject 值。

struct TEditorView: View {
    @State var name = ""
    
    init(tState: TState) {
        self._name = State(initialValue: tState.name)
    }
 
    var body: some View {
        ...
    }
}

struct ContentView: View {
    @EnvironmentObject private var tState: TState
    
    var body: some View {
        TEditorView(state: tState)
    }
}

或者如果 name 值是 two-way.

，则使用 @Binding 而不是 @State

一般来说，我还会质疑为什么在构造函数中需要 @EnvironmentObject。 @EnvironmentObject 的想法是它在所有视图中都表示相同，因此您应该只需要它 body.

如果您需要任何数据转换，应该在对象模型本身而不是视图中完成。

Answer 3

@State 应设置为 private 并且根据文档只能在 View body.

中访问

https://developer.apple.com/documentation/swiftui/state

应使用 ContentView().environmentObject(YourObservableObject)

设置 @EnvironmentObject

https://developer.apple.com/documentation/combine/observableobject https://developer.apple.com/documentation/swiftui/stateobject

下面是一些示例代码

import SwiftUI
class SampleOO: ObservableObject {
    @Published var name: String = "init name"
}
//ParentView
struct OOSample: View {
    //The first version of an @EnvironmentObject is an @ObservedObject or @StateObject
    //https://developer.apple.com/tutorials/swiftui/handling-user-input
    @ObservedObject var sampleOO: SampleOO = SampleOO()
    var body: some View {
        VStack{
            Button("change-name", action: {
                self.sampleOO.name = "OOSample"
            })
        
        Text("OOSample = " + sampleOO.name)
        //Doing this should fix your error code with no other workarounds
        ChildEO().environmentObject(sampleOO)
        SimpleChild(name: sampleOO.name)
        }
    }
}
//Can Display and Change name
struct ChildEO: View {
    @EnvironmentObject var sampleOO: SampleOO
    var body: some View {
        VStack{
            //Can change name
            Button("ChildEO change-name", action: {
                self.sampleOO.name = "ChildEO"
            })
            Text("ChildEO = " + sampleOO.name)
        }
    }
}
//Can only display name
struct SimpleChild: View {
    var name: String
    var body: some View {
        VStack{
            //Cannot change name
            Button("SimpleChild - change-name", action: {
                print("Can't change name")
                //self.name = "SimpleChild"
            })
            Text("SimpleChild = " + name)
        }
    }
}

struct OOSample_Previews: PreviewProvider {
    static var previews: some View {
        OOSample()
    }
}

SwiftUI @EnvironmentObject error: may be missing as an ancestor of this view -- accessing object in the init()

SwiftUI @EnvironmentObject error: may be missing as an ancestor of this view -- accessing object in the init()

swift

swiftui

environmentobject