如何在 Xamarin.Forms 中按照 MVVM 架构进行导航而不使用 PRISM 或任何导航服务等任何框架？

Question

我有一个 LoginPage.xaml 继承了 PageBase.xaml。 LoginViewModel 是 LoginPage 的 ViewModel，它继承了 BaseViewModel . BaseViewModel 定义了 Func 委托 - OnModalNavigationRequest 由 PageBase[=87 订阅=] 在它的 OnAppearing 方法中，HandleModalNavigationRequest 方法中提供了实现。在 LoginPage.xaml 上单击 登录按钮 时， 的 OnSubmit 方法LoginViewModel，调用通过命令绑定，在成功验证后，调用 BaseViewModel 的 NavigateToModal 方法。 NavigateToModel BaseViewModel 方法调用 HandleModalNavigationRequest 方法 PageBase Func 委托 OnModalNavigationRequest 检查它是否不是 null.

我的问题是 OnModalNavigationRequest always coming as null，说明没有订阅者。这意味着 PageBase 的 OnAppearing 方法没有被调用，这意味着 PageBase 没有被实例化。如何解决这个问题？

我正在发布正在发生的一系列事件的代码：

免责声明：代码帮助已取自 here

App.xaml.cs

    protected override void OnStart()
    {
        // Handle when your app starts
        var properties = Current.Properties;

        if (properties.ContainsKey("Username"))
        {
            if (properties["Username"] != null)
            {
                var loggedInUser = (string)properties["Username"];
                MainPage = new MainPage();
            }
            else
            {
                MainPage = new LoginPage();
            }
        }
        else
        {
            MainPage = new LoginPage();
        }
    }

IView

public interface IView
{
}

public interface IView<TViewModel> : IView where TViewModel : BaseViewModel
{
    TViewModel ViewModel { get; set; }
}

BaseViewModel

    public class BaseViewModel : INotifyPropertyChanged
    {
       private string title;
       public string Title
       {
         get { return GetProperty<string>("title"); }
         set { SetProperty(value, "title"); }
       }

       protected bool SetProperty<T>(T value,
        [CallerMemberName] string propertyName = "",
        Action onChanged = null)
       {
         if (!properties.ContainsKey(propertyName))
         {
            properties.Add(propertyName, default(T));
         }

         var oldValue = GetProperty<T>(propertyName);
         if (EqualityComparer<T>.Default.Equals(oldValue, value))
            return false;

         properties[propertyName] = value;
         onChanged?.Invoke();
         OnPropertyChanged(propertyName);
         return true;
       }

       protected T GetProperty<T>([CallerMemberName] string propertyName = null)
       {
         if (!properties.ContainsKey(propertyName))
         {
            return default(T);
         }
         else
         {
            return (T)properties[propertyName];
         }
       }

       public Func<BaseViewModel, Task> OnModalNavigationRequest { get; set; }

       public async Task NavigateToModal<TViewModel>(TViewModel targetViewModel) where TViewModel : BaseViewModel
      {
        await OnModalNavigationRequest?.Invoke(targetViewModel);
      }
    }

ViewResolver

    internal static class ViewResolver
    {
        public static Page GetViewFor<TargetViewModel>(TargetViewModel targetViewModel) where 
TargetViewModel : BaseViewModel, new()
        {
            var targetViewName = targetViewModel.GetType().Name.Replace("ViewModel", "Page");
            var definedTypes = targetViewModel.GetType().GetTypeInfo().Assembly.DefinedTypes;
            var targetType = definedTypes.FirstOrDefault(t => t.Name == targetViewName);
            return Activator.CreateInstance(targetType.AsType()) as Page;
        }
    }

PageBase.xaml

    <?xml version="1.0" encoding="utf-8" ?>
    <ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:d="http://xamarin.com/schemas/2014/forms/design"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         mc:Ignorable="d"
         x:Class="IPMS.Views.Shared.PageBase">
    </ContentPage>

PageBase.xaml.cs

    public partial class PageBase<TViewModel> : ContentPage, IView<TViewModel> where TViewModel : BaseViewModel, new()
   {
    public TViewModel ViewModel
    {
        get
        {
            return GetValue(BindingContextProperty) as TViewModel;
        }
        set
        {
            SetValue(BindingContextProperty, value);
        }
    }

    #region <Events>

    protected override void OnAppearing()
    {
        base.OnAppearing();

        ViewModel.OnModalNavigationRequest = HandleModalNavigationRequest;
    }

    protected override void OnDisappearing()
    {
        base.OnDisappearing();

        ViewModel.OnModalNavigationRequest = null;
    }
    #endregion

    async Task HandleModalNavigationRequest(BaseViewModel targetViewModel)
    {
        var targetView = ViewResolver.GetViewFor(targetViewModel);
        targetView.BindingContext = targetViewModel;
        await Navigation.PushModalAsync(new NavigationPage(targetView));
    }
}

LoginPage.xaml

    <?xml version="1.0" encoding="utf-8" ?>
    <views:PageBase xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:views="clr-namespace:IPMS.Views.Shared;assembly=IPMS"
         x:Class="IPMS.Views.Shared.LoginPage"
         Title="{Binding Title}">

         <ContentPage.Resources>
           <ResourceDictionary>
             // Resources goes here
           </ResourceDictionary>
         </ContentPage.Resources>

         <ContentPage.Content>
           <StackLayout>
                <Button Command="{Binding SubmitCommand}" Text="Login" TextColor="White"  
                FontAttributes="Bold" FontSize="Large" HorizontalOptions="FillAndExpand" />
            </StackLayout>
         </ContentPage.Content>
    </views:PageBase>

LoginPage.xaml.cs

    public partial class LoginPage : PageBase
    {
        LoginViewModel ViewModel => BindingContext as LoginViewModel;

        public LoginPage() : base()
        {
            BindingContext = new LoginViewModel();

            InitializeComponent();
        }
    }

LoginViewModel.cs

        public class LoginViewModel : BaseViewModel
        {
            public Command SubmitCommand { protected set; get; }

            public LoginViewModel() : base()
            {
                Title = "Login";
                SubmitCommand = new Command(async () => await OnSubmit());
            }

            public async Task OnSubmit()
            {
                await NavigateToModal<ItemsViewModel>(new ItemsViewModel());
            }
        }

Answer 1

成功了

1. 在LoginPage.xaml中添加x:TypeArguments如下图：

   <views:PageBase xmlns="http://xamarin.com/schemas/2014/forms"
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
        xmlns:views="clr-namespace:IPMS.Views.Shared;assembly=IPMS"
        xmlns:ViewModel="clr-namespace:IPMS.ViewModels.Shared;assembly=IPMS"
        x:Class="IPMS.Views.Shared.LoginPage"
        x:TypeArguments="ViewModel:LoginViewModel"
        BackgroundImageSource="loginbg.9.jpg"
        Title="{Binding Title}">
   </views:PageBase>
   

2. 改变LoginPage.xaml.cs如下图：

   public partial class LoginPage : PageBase<LoginViewModel>
   {
       public LoginPage() : base()
       {
           BindingContext = new LoginViewModel();
   
           InitializeComponent();
       }
   }