EnvironmentVariables在SwiftUI中将变量从一个视图传递到另一个视图时不工作

Question

我发现了一些类似的例子，说明如何在SwiftUI中的多个视图之间传递变量：

1. 使用Swift进行黑客攻击--如何使用@EnvironmentObject在视图之间共享数据
2. 如何在SwiftUI中将变量从一个视图传递到另一个视图

我正在尝试遵循这些示例，使用EnvironmentVariables并修改ContentView，在SceneDelegate中首先定义它。但是，在尝试这两个示例时，我会得到“编译失败：'ContentView_Previews‘不是’Environment‘的成员类型’”的错误。我正在使用Xcode版本11.3.1。

下面是如何在SwiftUI中将变量从一个视图传递到另一个视图中给出的示例，下面是ContentView中包含的代码：

class SourceOfTruth: ObservableObject{
    @Published var count = 0
}

struct ContentView: View {
    @EnvironmentObject var truth: SourceOfTruth
    var body: some View {
        VStack {
            FirstView()
            SecondView()
        }
    }
}

struct FirstView: View {
    @EnvironmentObject var truth: SourceOfTruth
    var body: some View {
       VStack{
        Text("\(self.truth.count)")
           Button(action:
            {self.truth.count = self.truth.count-10})
           {
               Text("-")
           }
       }
    }
}

struct SecondView: View {
    @EnvironmentObject var truth: SourceOfTruth
    var body: some View {
        Button(action: {self.truth.count = 0}) {
            Text("Reset")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environmentObject(SourceOfTruth())
    }
}

..。以下是SceneDelegate的内容：

import UIKit
import SwiftUI

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?
    var truth = SourceOfTruth() // <- Added

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    //    let contentView = ContentView()
        if let windowScene = scene as? UIWindowScene {
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = UIHostingController(rootView: ContentView().environmentObject(SourceOfTruth())) // <- Modified
            self.window = window
            window.makeKeyAndVisible()
        }
    }
    func sceneDidDisconnect(_ scene: UIScene) {
    }
    func sceneDidBecomeActive(_ scene: UIScene) {
    }
    func sceneWillResignActive(_ scene: UIScene) {
    }
    func sceneWillEnterForeground(_ scene: UIScene) {
    }
    func sceneDidEnterBackground(_ scene: UIScene) {
    }
}

Answer 1

我不依赖Xcode版本，这也不是一个问题。您必须以与在ContentView中相同的方式在ContentView_Previews中设置.environmentObject，提供.environmentObject，如下例所示

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environmentObject(_Your_object_here())
    }
}