SwiftUI MVVM如何在函数后显示警报

我正在使用MVVM架构创建一个用户登录页面，如果文本字段为空或用户名/密码错误，我希望显示一个警报页面。因此，在登录单击和API调用之后，如果登录失败，我想显示一个警报，但是我尝试更改状态，但仍然没有显示警报

登录视图

struct LoginView: View {
    @ObservedObject var user: User
    @ObservedObject var viewModel = LoginViewModel()
    @State var inputUser: String = ""
    @State var inputPass: String = ""
    @State private var showingAlert = true
    
    var body: some View {
        ScrollView {
            VStack(alignment: .leading) {
                Group {
                    Text("Login")
                    CustomTextField(placeHolder: "Username", value: $viewModel.user)
                    CustomTextField(placeHolder: "Password", value: $viewModel.pass)
                    Button(action: {
                        self.viewModel.login { (isSuccess) in
                            if isSuccess {
                                self.user.tokenIsActive = true
                            } else {
                                // if login failed, show alert, but alert not showing
                                self.showingAlert = true
                            }
                        }
                    }) {
                        Text("Sign In")
                    }.buttonStyle(PrimaryButtonStyle())
                    .alert(isPresented: $showingAlert) {
                        Alert(title: Text("Error"), message: Text("Invalid username / password"), dismissButton: .default(Text("Got it!")))
                    }
                }
            }
        }
    }
}

LoginView模型

class LoginViewModel: ObservableObject, LoginService {
    var apiSession: APIService
    
    @Published var user = ""
    @Published var pass = ""
    
    @Published var accessToken: String?
    @Published var refreshToken: String?
    @Published var showingAlert = true
    
    var cancellables = Set<AnyCancellable>()
    
    init(apiSession: APIService = APISession()) {
        self.apiSession = apiSession
    }
    
    func login(_ completion: @escaping ((Bool)->Void)) {
        let cancellable = self.loginUser(user: user, pass: pass)
            .sink(receiveCompletion: { result in
                switch result {
                case .failure(let error):
                    print("Handle error: \(error)")
                case .finished:
                    break
                }
                
            }) { (result) in
                completion(true)
                self.accessToken = result.accessToken
                self.refreshToken = result.refreshToken
                UserDefaults.standard.set(self.accessToken, forKey: "AccessToken")
                UserDefaults.standard.set(self.refreshToken, forKey: "RefreshToken")
        }
        cancellables.insert(cancellable)
    }
}

classloginviewmodel:observeObject，LoginService{
apiSession:APIService
@已发布的var user=“”
@已发布的var pass=“”
@已发布的var accessToken:字符串？
@已发布的var刷新令牌：字符串？
@已发布变量showingAlert=true
var cancelables=Set（）
初始化（apiSession:APIService=apiSession（））{
self.apiSession=apiSession
}
func登录（uu完成：@escaping（（Bool）->Void））{
让Cancelable=self.loginUser（用户：用户，通过：通过）
.sink（receiveCompletion:{结果为
切换结果{
案例。失败（let错误）：
打印（“句柄错误：\（错误）”）
案例。完成：
打破
}
})(结果)
完成（真）
self.accessToken=result.accessToken
self.refreshtToken=result.refreshtToken
UserDefaults.standard.set（self.accessToken，forKey:“accessToken”）
UserDefaults.standard.set（self.refreshtToken，forKey:“refreshtToken”）
}
可取消。插入（可取消）
}
}

---

如何显示警报？提前感谢大家。

如果我正确理解了您的流程，问题在于登录逻辑

let cancellable = self.loginUser(user: user, pass: pass)
    .receive(on: DispatchQueue.main)        // << add also this one !!
    .sink(receiveCompletion: { result in
        switch result {
        case .failure(let error):
            print("Handle error: \(error)")
            completion(false)               // << here !!
        case .finished:
            break
        }
        
    }) { (result) in
        self.accessToken = result.accessToken
        self.refreshToken = result.refreshToken
        UserDefaults.standard.set(self.accessToken, forKey: "AccessToken")
        UserDefaults.standard.set(self.refreshToken, forKey: "RefreshToken")
        completion(true) // obviously should be called here !!
                         // and if result is provided always, ie. for 
                         // failed login as well, then conditional
                         // completion should be called.
}

let cancelable=self.logiuser（用户：用户，通过：通过）
.receive（on:DispatchQueue.main）//非常感谢您的回答。我对Swift很陌生，你能举例说明“有条件完成”是如何实施的吗？