state - “在 View 更新期间修改状态，这将导致未定义的行为。”在文本字段(SwiftUI)上键入时出错

Question

我有两个文本字段，分别分配给:

@State  private var emailAddress: String = ""
@State  private var password: String = ""

现在，每当我在上面输入内容时，该应用似乎都卡住了，并给了我这个错误:

'Modifying state during view update, this will cause undefined behavior.'

我有一个 StartView():

class UserSettings: ObservableObject {

var didChange = PassthroughSubject<Void, Never>()

@Published var loggedIn : Bool = false {
     didSet {
         didChange.send(())
     }
 }
}

struct StartView: View {
@EnvironmentObject var settings: UserSettings

var body: some View {
    if settings.loggedIn {
        return AnyView(TabbarView())
    }else {
        return AnyView(ContentView())
    }
}

}

我创建了一个具有 loggedIn bool值的UserSettings的ObservableObject类。当用户点击 LogInView()中的“登录”按钮时，此 bool 值将变为 true并出现一个新 View ( TabbarView())

这是LogInView():

struct LogInView: View {
@EnvironmentObject var settings: UserSettings

@State  private var emailAddress: String = ""
@State  private var password: String = ""

var body: some View {
GeometryReader { geometry in
        VStack (alignment: .center){
            HStack {
                Image("2")
                .resizable()
                .frame(width: 20, height: 20)
                Text("Social App")
                    .font(.system(size: 12))

            }.padding(.top, 30)
                .padding(.bottom, 10)

            Text("Log In to Your Account")
                .font(.title)
                .font(.system(size: 14, weight: .bold, design: Font.Design.default))
                .padding(.bottom, 50)

            TextField("Email", text: self.$emailAddress)
                .frame(width: geometry.size.width - 45, height: 50)
                .textContentType(.emailAddress)
                .padding(EdgeInsets(top: 0, leading: 5, bottom: 0, trailing: 0))
                .accentColor(.red)
                .background(Color(red: 242 / 255, green: 242 / 255, blue: 242 / 255))
                .cornerRadius(5)


            TextField("Password", text: self.$password)
                .frame(width: geometry.size.width - 45, height: 50)
                .padding(EdgeInsets(top: 0, leading: 5, bottom: 0, trailing: 0))
                .foregroundColor(.gray)
                .background(Color(red: 242 / 255, green: 242 / 255, blue: 242 / 255))
                .textContentType(.password)
                .cornerRadius(5)

             Button(action: {
                self.settings.loggedIn = true
             }) {
                    HStack {
                        Text("Log In")
                    }
                        .padding()
                        .frame(width: geometry.size.width - 40, height: 40)
                        .foregroundColor(Color.white)
                        .background(Color.blue)
                        .cornerRadius(5)
                }
                 .padding(.bottom, 40)

            Divider()

            Button(action: {
                    print("Take to forget password VC")
                    }) {
                    Text("Forgot your password?")
            }

            Spacer()

            }
            .padding(.bottom, 90)
        }
}
}

我知道如果在修改状态(在文本字段中键入)时更新 View ，则会出现此错误。但是我不会在“登录”屏幕中的任何位置更新 View 。那么为什么会发生此错误。帮助将不胜感激!

Answer 1

这对我有用，您甚至不需要导入Combine!当您使用@Published时，SwiftUI将自动合成objectWillChange主题，并且只要该属性发生突变，就会调用send。如果需要，您仍然可以手动调用.send()，但是在大多数情况下，您不需要这样做。

class UserSettings: ObservableObject {
    @Published var loggedIn : Bool = false
}

Beta 5发行说明的摘录:

You can manually conform to ObservableObject by defining an objectWillChange publisher that emits before the object changes. However, by default, ObservableObject automatically synthesizes objectWillChange and emits before any @Published properties change.

这是对我来说很好的完整代码(iPhone Xr和真实设备，iPad 6th Gen):

window.rootViewController = UIHostingController(rootView: ContentView().environmentObject(UserSettings()))

import SwiftUI

struct ContentView: View {
    var body: some View {
        StartView()
    }
}

class UserSettings: ObservableObject {
    @Published var loggedIn : Bool = false
}

struct StartView: View {
    @EnvironmentObject var settings: UserSettings

    var body: some View {
        if settings.loggedIn {
            return AnyView(Text("LOGGED IN"))
        } else {
            return AnyView(LogInView())
        }
    }
}

struct LogInView: View {
    @EnvironmentObject var settings: UserSettings


    @State  private var emailAddress: String = ""
    @State  private var password: String = ""

    var body: some View {
        GeometryReader { geometry in
            VStack (alignment: .center){
                HStack {
                    Image(systemName: "2.circle.fill")
                        .resizable()
                        .frame(width: 20, height: 20)
                    Text("Social App")
                        .font(.system(size: 12))

                }.padding(.top, 30)
                    .padding(.bottom, 10)

                Text("Log In to Your Account")
                    .font(.title)
                    .font(.system(size: 14, weight: .bold, design: Font.Design.default))
                    .padding(.bottom, 50)

                TextField("Email", text: self.$emailAddress)
                    .frame(width: geometry.size.width - 45, height: 50)
                    .textContentType(.emailAddress)
                    .padding(EdgeInsets(top: 0, leading: 5, bottom: 0, trailing: 0))
                    .accentColor(.red)
                    .background(Color(red: 242 / 255, green: 242 / 255, blue: 242 / 255))
                    .cornerRadius(5)


                TextField("Password", text: self.$password)
                    .frame(width: geometry.size.width - 45, height: 50)
                    .padding(EdgeInsets(top: 0, leading: 5, bottom: 0, trailing: 0))
                    .foregroundColor(.gray)
                    .background(Color(red: 242 / 255, green: 242 / 255, blue: 242 / 255))
                    .textContentType(.password)
                    .cornerRadius(5)

                Button(action: {
                    self.settings.loggedIn = true
                }) {
                    HStack {
                        Text("Log In")
                    }
                    .padding()
                    .frame(width: geometry.size.width - 40, height: 40)
                    .foregroundColor(Color.white)
                    .background(Color.blue)
                    .cornerRadius(5)
                }
                .padding(.bottom, 40)

                Divider()

                Button(action: {
                    print("Take to forget password VC")
                }) {
                    Text("Forgot your password?")
                }

                Spacer()

            }
            .padding(.bottom, 90)
        }
    }
}