Question

我正在开发我的第一个Xamarin应用程序。我试图显示我的第一页，但在base.ViewDidLoad（）上收到以下错误; ViewDidLoad（）中的行

已抛出MvvmCross.Platform.Exceptions.MvxException

无法从定位器MvxDefaultViewModelLocator构造和初始化类型iManage.ViewModels.LoginViewModel的ViewModel - 检查InnerException以获取更多信息

LoginView.cs：

public partial class LoginView : MvxViewController<LoginViewModel>
{
    public LoginView() : base("LoginView", null)
    {
    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
        txtUser.Layer.CornerRadius = 15;
        txtPassword.Layer.CornerRadius = 15;
        btnLogin.Layer.CornerRadius = 20;

        var set = this.CreateBindingSet<LoginView, LoginViewModel>();
        set.Bind(txtUser).To(vm => vm.Username);
        set.Bind(txtPassword).To(vm => vm.Password);
        set.Bind(btnLogin).To(vm => vm.LoginCommand);
        //set.Bind(btnLogin).To(vm => vm.AttemptLogin());
        set.Apply();
    }

    public override void DidReceiveMemoryWarning()
    {
        base.DidReceiveMemoryWarning();
        // Release any cached data, images, etc that aren't in use.
    }
}

LoginViewModel.cs：

public class LoginViewModel : BaseViewModel
{
    private readonly ILoginService _loginService;

    private readonly IDialogService _dialogService;

    public LoginViewModel(ILoginService loginService, IDialogService dialogService)
    {
        _loginService = loginService;
        _dialogService = dialogService;

        Username = "TestUser";
        Password = "YouCantSeeMe";
        IsLoading = false;
    }

    private string _username;
    public string Username
    {
        get
        {
            return _username;
        }

        set
        {
            SetProperty(ref _username, value);
            RaisePropertyChanged(() => Username);
        }
    }

    private string _password;
    public string Password
    {
        get
        {
            return _password;
        }

        set
        {
            SetProperty(ref _password, value);
            RaisePropertyChanged(() => Password);
        }
    }

    private bool _isLoading;

    public bool IsLoading
    {
        get
        {
            return _isLoading;
        }

        set
        {
            SetProperty(ref _isLoading, value);
        }
    }

    private IMvxCommand _loginCommand;
    public virtual IMvxCommand LoginCommand
    {
        get
        {
            _loginCommand = _loginCommand ?? new MvxCommand(AttemptLogin, CanExecuteLogin);
            return _loginCommand;
        }
    }

    private void AttemptLogin()
    {
        if (_loginService.Login(Username, Password))
        {
            ShowViewModel<DashboardEmpViewModel>();
        }
        else
        {
            _dialogService.Alert("We were unable to log you in!", "Login Failed", "OK");
        }
    }

    private bool CanExecuteLogin()
    {
        return (!string.IsNullOrEmpty(Username) || !string.IsNullOrWhiteSpace(Username))
               && (!string.IsNullOrEmpty(Password) || !string.IsNullOrWhiteSpace(Password));
    }
}

LoginService.cs：

public class LoginService : ILoginService
{
    /// <summary>Initializes a new instance of the <see cref="LoginService"/> class.</summary>
    public LoginService() // e.g. LoginService(IMyApiClient client)
    {
        // this constructor would most likely contain some form of API Client that performs
        // the message creation, sending and deals with the response from a remote API
    }

    /// <summary>
    /// Gets a value indicating whether the user is authenticated.
    /// </summary>
    public bool IsAuthenticated { get; private set; }

    /// <summary>Gets the error message.</summary>
    /// <value>The error message.</value>
    public string ErrorMessage { get; private set; }

    /// <summary>
    /// Attempts to log the user in using stored credentials if present
    /// </summary>
    /// <returns> <see langword="true"/> if the login is successful, false otherwise </returns>
    public bool Login()
    {
        // get the stored username from previous sessions
        // var username = Settings.UserName;
        // var username = _settingsService.GetValue<string>(Constants.UserNameKey);

        // force return of false just for demo purposes
        IsAuthenticated = false;
        return IsAuthenticated;
    }

    /// <summary>The login method to retrieve OAuth2 access tokens from an API. </summary>
    /// <param name="userName">The user Name (email address) </param>
    /// <param name="password">The users <paramref name="password"/>. </param>
    /// <param name="scope">The required scopes. </param>
    /// <returns>The <see cref="bool"/>. </returns>
    public bool Login(string userName, string password, string scope)
    {
        try
        {
            //IsAuthenticated = _apiClient.ExchangeUserCredentialsForTokens(userName, password, scope);
            return IsAuthenticated;
        }
        catch (ArgumentException argex)
        {
            ErrorMessage = argex.Message;
            IsAuthenticated = false;
            return IsAuthenticated;
        }
    }

    /// <summary>
    /// Logins the specified user name.
    /// </summary>
    /// <param name="userName">Name of the user.</param>
    /// <param name="password">The users password.</param>
    /// <returns></returns>
    public bool Login(string userName, string password)
    {
        // this simply returns true to mock a real login service call
        return true;
    }
}

Setup.cs：

public class Setup : MvxIosSetup
{
    public Setup(IMvxApplicationDelegate applicationDelegate, IMvxIosViewPresenter presenter) : base(applicationDelegate, presenter)
    {

    }

    protected override IMvxApplication CreateApp()
    {
        return new App();
    }
}

我已使用this url来实现它。

EDIT1：

EDIT2： 内部例外：

MvvmCross.Platform.Exceptions.MvxException：无法从定位器MvxDefaultViewModelLocator为类型iManage.ViewModels.LoginViewModel构造和初始化ViewModel - 检查InnerException以获取更多信息---＆gt; MvvmCross.Platform.Exceptions.MvxException：创建LoginViewModel类型的viewModel时出现问题---＆gt; MvvmCross.Platform.Exceptions.MvxIoCResolveException：创建iManage.ViewModels.LoginViewModel时，无法解析IDialogService类型的参数dialogService的参数在MvvmCross.Platform.IoC.MvxSimpleIoCContainer.GetIoCParameterValues（System.Type type，System.Reflection.ConstructorInfo firstConstructor）[0x00066] in＆lt; 6adc0d5857264558a9d45778a78ae02a＆gt ;:0 在MvvmCross.Platform.IoC.MvxSimpleIoCContainer.IoCConstruct（System.Type type）[0x0002c] in＆lt; 6adc0d5857264558a9d45778a78ae02a＆gt ;:0 在MvvmCross.Platform.Mvx.IocConstruct（System.Type t）[0x00006] in＆lt; 6adc0d5857264558a9d45778a78ae02a＆gt ;:0 在MvvmCross.Core.ViewModels.MvxDefaultViewModelLocator.Load（System.Type viewModelType，MvvmCross.Core.ViewModels.IMvxBundle parameterValues，MvvmCross.Core.ViewModels.IMvxBundle savedState）[0x00000] in：0 ---内部异常堆栈跟踪结束--- 在MvvmCross.Core.ViewModels.MvxDefaultViewModelLocator.Load（System.Type viewModelType，MvvmCross.Core.ViewModels.IMvxBundle parameterValues，MvvmCross.Core.ViewModels.IMvxBundle savedState）[0x00029] in：0 在MvvmCross.Core.ViewModels.MvxViewModelLoader.LoadViewModel（MvvmCross.Core.ViewModels.MvxViewModelRequest请求，MvvmCross.Core.ViewModels.IMvxBundle savedState）[0x00035] in：0 ---内部异常堆栈跟踪结束--- 在MvvmCross.Core.ViewModels.MvxViewModelLoader.LoadViewModel（MvvmCross.Core.ViewModels.MvxViewModelRequest请求，MvvmCross.Core.ViewModels.IMvxBundle savedState）[0x00068] in：0 在MvvmCross.iOS.Views.MvxViewControllerExtensionMethods.LoadViewModel（MvvmCross.iOS.Views.IMvxIosView iosView）[0x0005f] in＆lt; 6f99728979034e579bc72f6d53e5bc35＆gt;：0 在MvvmCross.Core.Views.MvxViewExtensionMethods.OnViewCreate（MvvmCross.Core.Views.IMvxView视图，System.Func`1 [TResult] viewModelLoader）[0x00012] in：0 在MvvmCross.iOS.Views.MvxViewControllerExtensionMethods.OnViewCreate（MvvmCross.iOS.Views.IMvxIosView iosView）[0x00001] in＆lt; 6f99728979034e579bc72f6d53e5bc35＆gt;：0 在MvvmCross.iOS.Views.MvxViewControllerAdapter.HandleViewDidLoadCalled（System.Object sender，System.EventArgs e）[0x00007] in＆lt; 6f99728979034e579bc72f6d53e5bc35＆gt ;:0 at at（wrapper delegate-invoke）：invoke_void_object_EventArgs（object，System.EventArgs）在MvvmCross.Platform.Core.MvxDelegateExtensionMethods.Raise（System.EventHandler eventHandler，System.Object sender）[0x00003] in＆lt; 6adc0d5857264558a9d45778a78ae02a＆gt ;:0 在MvvmCross.Platform.iOS.Views.MvxEventSourceViewController.ViewDidLoad（）[0x00006] in＆lt; 4467c42ffcc4478e847227b8e4af47fe＆gt;：0 在MvvmCross.iOS.Views.MvxViewController.ViewDidLoad（）[0x00001] in＆lt; 6f99728979034e579bc72f6d53e5bc35＆gt;：0 at iManage.iOS.Views.LoginView.ViewDidLoad（）[0x00001] in /Users/pankajsachdeva/Projects/iManage/iOS/Views/LoginView.cs:18 at at（wrapper managed-to-native）UIKit.UIApplication：UIApplicationMain（int，string []，intptr，intptr）在UIKit.UIApplication.Main（System.String [] args，System.IntPtr principal，System.IntPtr委托）[0x00005]在/ Users / builder / data / lanes / 5665 / 6857dfcc / source / xamarin-macios / src / UIKit中/UIApplication.cs:79 在/ Users / builder / data / lanes / 5665 / 6857dfcc / source / xamarin-macios / src / UIKit中的UIKit.UIApplication.Main（System.String [] args，System.String principalClassName，System.String delegateClassName）[0x00038] /UIApplication.cs:63 at iManage.iOS.Application.Main（System.String [] args）[0x00001] in /Users/pankajsachdeva/Projects/iManage/iOS/Main.cs:17

Answer 1

您必须在IoC容器中注册服务的实现，以便在需要时解析它们。一般来说，这可以在两个地方完成：

对于App.cs方法中Initialize的平台无关服务：

public class App : MvxApplication
{
   public override void Initialize()
   {
      ...
      Mvx.ConstructAndRegisterSingleton<IDialogService,DialogService>();
      ...
   }
}

（请注意，默认情况下应该这样做，因为默认的App类已经包含一个块，它会自动使用＆＃34; Service＆＃34;后缀）注册所有类型

对于Setup.cs方法中平台InitializeFirstChance中的特定于平台的服务，例如对于Windows，它将是：

public class Setup : MvxWindowsSetup
{
   protected override void InitializeFirstChance()
   {
       Mvx.ConstructAndRegisterSingleton<IDialogService,DialogService>();
       base.InitializeFirstChance();
   }
}

我猜测DialogService将取决于平台，因此第二种解决方案会更合适。

您可以使用LazyConstructAndRegisterSingleton确保仅在第一次实际需要时创建实例。您也可以使用RegisterType来注册类型，并在每次需要时创建一个新实例。

有关MvvmCross IoC的更多信息available in the documentation。

无法构造和初始化viewModel

1 个答案: