WPF/C#：实现导航功能

前言

在WPF中使用导航功能可以使用Frame控件，这是比较基础的一种方法。前几天分享了wpfui中NavigationView的基本用法，但是如果真正在项目中使用起来，基础的用法是无法满足的。今天通过wpfui中的mvvm例子来说明在wpfui中如何通过依赖注入与MVVM模式使用导航功能。实践起来，我个人觉得这个例子中实现导航功能还是有点麻烦的，但我也不知道怎么能更优雅，也是学到了一些东西吧。

wpfui中MVVM例子的地址在：https://github.com/lepoco/wpfui/tree/main/src/Wpf.Ui.Demo.Mvvm

实现效果如下所示：

如果你对此感兴趣，可以继续阅读。

实践

使用依赖注入

将主窗体与主窗体的ViewModel与每个页面与每个页面的ViewModel都存入依赖注入容器中：

当然不只是窗体页面与ViewModel，也需要注册一些服务。

为了实现导航功能，使用了两个服务分别是NavigationService与PageService。

NavigationService在wpfui库中已经自带了，直接使用即可：

具体代码可自行研究，这里就不放了。

而PageService在wpfui中没有自带，需要自己定义，MVVM例子中的定义如下所示：

 public class PageService : IPageService
 {
     /// <summary>
     /// Service which provides the instances of pages.
     /// </summary>
     private readonly IServiceProvider _serviceProvider;

     /// <summary>
     /// Initializes a new instance of the <see cref="PageService"/> class and attaches the <see cref="IServiceProvider"/>.
     /// </summary>
     public PageService(IServiceProvider serviceProvider)
     {
         _serviceProvider = serviceProvider;
     }

     /// <inheritdoc />
     public T? GetPage<T>()
         where T : class
     {
         if (!typeof(FrameworkElement).IsAssignableFrom(typeof(T)))
         {
             throw new InvalidOperationException("The page should be a WPF control.");
         }

         return (T?)_serviceProvider.GetService(typeof(T));
     }

     /// <inheritdoc />
     public FrameworkElement? GetPage(Type pageType)
     {
         if (!typeof(FrameworkElement).IsAssignableFrom(pageType))
         {
             throw new InvalidOperationException("The page should be a WPF control.");
         }

         return _serviceProvider.GetService(pageType) as FrameworkElement;
     }
 }

现在已经将所有窗体、页面、ViewModels与相关服务都注册到容器中了。

ViewModel

在MainWindowViewModel中将页面存入一个属性中：

在非首页的ViewModel中实现INavigationAware接口：

View

MainWindow.cs如下所示：

 public partial class MainWindow : INavigationWindow
 {
     public ViewModels.MainWindowViewModel ViewModel { get; }

     public MainWindow(
         ViewModels.MainWindowViewModel viewModel,
         IPageService pageService,
         INavigationService navigationService
     )
     {
         ViewModel = viewModel;
         DataContext = this;

         Wpf.Ui.Appearance.SystemThemeWatcher.Watch(this);

         InitializeComponent();
         SetPageService(pageService);

         navigationService.SetNavigationControl(RootNavigation);
     }

     public INavigationView GetNavigation() => RootNavigation;

     public bool Navigate(Type pageType) => RootNavigation.Navigate(pageType);

     public void SetPageService(IPageService pageService) => RootNavigation.SetPageService(pageService);

     public void ShowWindow() => Show();

     public void CloseWindow() => Close();

     /// <summary>
     /// Raises the closed event.
     /// </summary>
     protected override void OnClosed(EventArgs e)
     {
         base.OnClosed(e);

         // Make sure that closing this window will begin the process of closing the application.
         Application.Current.Shutdown();
     }

     INavigationView INavigationWindow.GetNavigation()
     {
         throw new NotImplementedException();
     }

     public void SetServiceProvider(IServiceProvider serviceProvider)
     {
         throw new NotImplementedException();
     }
 }

首先实现了INavigationWindow接口。在构造函数中注入所需的依赖类。注意这里的RootNavigation其实就是页面中NavigationView的名称：

刚开始看这里没注意到，卡壳了很久。

因为你在代码中查看定义，它会转到这个地方：

没经验不知道是什么，但是这次过后，知道这是在Xaml中定义，由工具自动生成的代码了。

其他的页面改成了这样的写法：

 public partial class DashboardPage : INavigableView<DashboardViewModel>
 {
     public DashboardViewModel ViewModel { get; }
     public DashboardPage(DashboardViewModel  viewModel)
     {
         ViewModel = viewModel;
         this.DataContext = this;
         InitializeComponent();          
     }
 }

都实现了INavigableView<out T>接口：

显示主窗体与主页面

现在准备工作都做好了，下一步就是显示主窗体与主页面了。

在容器中我们也注入了这个：

ApplicationHostService如下所示：

    /// <summary>
    /// Managed host of the application.
    /// </summary>
    public class ApplicationHostService : IHostedService
    {
        private readonly IServiceProvider _serviceProvider;
        private INavigationWindow? _navigationWindow;

        public ApplicationHostService(IServiceProvider serviceProvider)
        {
            _serviceProvider = serviceProvider;
        }

        /// <summary>
        /// Triggered when the application host is ready to start the service.
        /// </summary>
        /// <param name="cancellationToken">Indicates that the start process has been aborted.</param>
        public async Task StartAsync(CancellationToken cancellationToken)
        {
            await HandleActivationAsync();
        }

        /// <summary>
        /// Triggered when the application host is performing a graceful shutdown.
        /// </summary>
        /// <param name="cancellationToken">Indicates that the shutdown process should no longer be graceful.</param>
        public async Task StopAsync(CancellationToken cancellationToken)
        {
            await Task.CompletedTask;
        }

        /// <summary>
        /// Creates main window during activation.
        /// </summary>
        private async Task HandleActivationAsync()
        {
            await Task.CompletedTask;

            if (!System.Windows.Application.Current.Windows.OfType<MainWindow>().Any())
            {
                _navigationWindow = (
                    _serviceProvider.GetService(typeof(INavigationWindow)) as INavigationWindow
                )!;
                _navigationWindow!.ShowWindow();

                _ = _navigationWindow.Navigate(typeof(DashboardPage));
            }

            await Task.CompletedTask;
        }
    }
}

在app.xaml中定义了程序启动与退出事件的处理程序：

 /// <summary>
 /// Occurs when the application is loading.
 /// </summary>
 private async void OnStartup(object sender, StartupEventArgs e)
 {
     await _host.StartAsync();
 }

 /// <summary>
 /// Occurs when the application is closing.
 /// </summary>
 private async void OnExit(object sender, ExitEventArgs e)
 {
     await _host.StopAsync();

     _host.Dispose();
 }