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WPF中在MVVM模式下实现导航功能

一、利用TabControl

使用场景：项目小，不用考虑内存开销的问题。

实现方式1-手动指定ViewModel

1. 分别定义3个UserControl作为View用于演示

 <UserControl...><Grid><StackPanel Orientation="Vertical"><TextBlockHorizontalAlignment="Center"VerticalAlignment="Top"Text="Page 1" /><TextBlockd:Text="Page 1"FontSize="50"Text="{Binding PageMessage}" /></StackPanel></Grid></UserControl>

2. 分别定义ViewModel

 public abstract class PageViewModelBase {public string? Header { get; set; }}public class MainViewModel {public List<PageViewModelBase> ViewModels { get; }public MainViewModel(Page1ViewModel p1, Page2ViewModel p2, Page3ViewModel p3){ViewModels = new List<PageViewModelBase> { p1, p2, p3 };}}public class Page1ViewModel : PageViewModelBase{public Page1ViewModel() => Header = "Page 1";public string PageMessage { get; set; } = "Hello, Page 1";}public class Page2ViewModel : PageViewModelBase{public Page2ViewModel() => Header = "Page 2";public string PageMessage { get; set; } = "Hello, Page 2";}public class Page3ViewModel : PageViewModelBase{public Page3ViewModel() => Header = "Page 3";public string PageMessage { get; set; } = "Hello, Page 3";}

3. 在MainWindow上定义Tabcontrol

 <Window...><Grid><TabControl ItemsSource="{Binding ViewModels}"><TabItem Header="Pag1"><view:Page1><view:Page1.DataContext><local:Page1ViewModel /></view:Page1.DataContext></view:Page1></TabItem><TabItem Header="Pag2"><view:Page1><view:Page1.DataContext><local:Page2ViewModel /></view:Page1.DataContext></view:Page1></TabItem><TabItem Header="Pag3"><view:Page1><view:Page1.DataContext><local:Page3ViewModel /></view:Page1.DataContext></view:Page1></TabItem></TabControl></Grid></Window>

这种方式需要手动指定每个View的ViewModel

实现方式2-利用ItemTemplate

1. 在MainViewModel中声明一个ViewModel列表

public class MainViewModel 
{public List<PageViewModelBase> ViewModels { get; }public MainViewModel(Page1ViewModel p1, Page2ViewModel p2, Page3ViewModel p3){ViewModels = new List<PageViewModelBase> { p1, p2, p3 };}
}

2. 在MainWindow中为TabControl指定ItemTemplate，上一步声明的ViewModel列表作为 TabControl 的 ItemsSource；为 TabControl.Resources 添 加多个 DataTemplate，指定 VM 对应什么样的 Page

<Window d:DataContext="{d:DesignInstance Type=local:MainViewModel}"....><Grid><TabControl ItemsSource="{Binding ViewModels}"><TabControl.ItemTemplate><DataTemplate><TextBlock Text="{Binding Header}"/></DataTemplate></TabControl.ItemTemplate><TabControl.Resources><DataTemplate DataType="{x:Type local:Page1ViewModel}"><view:Page1/></DataTemplate><DataTemplate DataType="{x:Type local:Page2ViewModel}"><view:Page2/></DataTemplate><DataTemplate DataType="{x:Type local:Page3ViewModel}"><view:Page3/></DataTemplate></TabControl.Resources>   </TabControl></Grid>
</Window>

这样的好处是自动会为不同的View绑定了相应的ViewModel。

小技巧：在xaml中加上了d:DataContext="{d:DesignInstance Type=local:MainViewModel}，这样在写Binding的时候就有了智能提示。

以上两种方式均可结合依赖注入的方式来实现

二、自定义NavigationService服务

1. 实现一个NavigationService服务，并作为单例

 class NavigationService{//设置一个单例服务public static NavigationService Instance { get; private set; } = new NavigationService();//声明一个事件，当更改CurrentViewModel时触发public event Action? CurrentViewModelChanged;//设置一个当前VM的属性，并在属性改变时触发CurrentViewModelChangedprivate ViewModelBase? currentViewModel;public ViewModelBase? CurrentViewModel{get => currentViewModel;set{currentViewModel = value;CurrentViewModelChanged?.Invoke();}}//页面导航方法，给CurrentViewModel赋值，触发CurrentViewModelChanged事件public void NavigateTo(ViewModelBase viewModel)=>CurrentViewModel = viewModel;}

2. 设置MainViewModel中的CurrentViewModel属性

 public class ViewModelBase : ObservableObject{}public partial class MainViewModel : ViewModelBase{[ObservableProperty]private ViewModelBase? currentViewModel;//当前的VMpublic MainViewModel(){//为事件绑定委托方法，设置CurrentVM和NavigationService中的CurrentVM保持一致NavigationService.Instance.CurrentViewModelChanged += () =>{CurrentViewModel = NavigationService.Instance.CurrentViewModel;};//调用导航方法NavigationService.Instance.NavigateTo(new LoginViewModel());}}

其他两个ViewModel分别为

 public partial class LoginViewModel : ViewModelBase{[ObservableProperty]string? userName = "Sean";[RelayCommand]void Login(){NavigationService.Instance.NavigateTo(new HomeViewModel());}}public partial class HomeViewModel : ViewModelBase{[ObservableProperty]string? userName;[RelayCommand]void Logout(){NavigationService.Instance.NavigateTo(new LoginViewModel());}}

3. 使用ContentControl作为MainWindow上不同页面载体显示内容，并借助DataTemplate来实现View和ViewModel的映射

 <Window ...><ContentControl Content="{Binding CurrentViewModel}"><ContentControl.Resources><DataTemplate DataType="{x:Type vm:LoginViewModel}"><view:Login /></DataTemplate><DataTemplate DataType="{x:Type vm:HomeViewModel}"><view:Home /></DataTemplate></ContentControl.Resources></ContentControl></Window>

在ContentControl.Resources中设置DataTemplate，根据DataType自动选择相应的VM，这样做的好处是会自动将View和VM进行了绑定。

改进

1. 单例方式可以采用依赖注入的方式来实现
2. 在NavigationService服务中，可以改进页面导航的方法

public void NavigateTo<T>() where T : ViewModelBase=> CurrentViewModel = App.Current.Services.GetService<T>();//在调用导航方法时可以使用
navigationService.NavigateTo<HomeViewModel>();

三、借助ValueConverter

实现上一章节的功能，这种方法本质上是通过View来自动绑定VM。

1. 定义Page的枚举

 public enum ApplicationPage{Empty,Login,Home}

2. 定义各ViewModel

 public class ViewModelBase : ObservableObject{}public partial class MainViewModel : ViewModelBase{//MainViewModel中的CurrentPage是一个枚举类型[ObservableProperty]ApplicationPage currentPage;public MainViewModel(){CurrentPage = ApplicationPage.Login;}}public partial class LoginViewModel : ViewModelBase{public string UserName { get; set; } = "AngelSix";[RelayCommand]void Login(){var mainVM= App.Current.MainWindow.DataContext as MainViewModel;mainVM!.CurrentPage = ApplicationPage.Home;}}public partial class HomeViewModel : ViewModelBase{[RelayCommand]void Logout(){var mainVM = App.Current.MainWindow.DataContext as MainViewModel;mainVM!.CurrentPage = ApplicationPage.Login;}}

3. 定义Page基类和各个Page

   这种方法本质上是通过View来自动绑定VM，所以在此处使用泛型

 public abstract class BasePage<VM> : UserControl where VM : ViewModelBase, new(){public BasePage(){DataContext = new VM();}}

• 实现Home页面

将Home.xaml.cs中的继承删掉，以为它和Home.xaml相互为分部类，只在一个分部类上实现继承就可以。

 <local:BasePage x:TypeArguments="vm:HomeViewModel"...><!--x:TypeArguments指定泛型--><Grid><TextBlock HorizontalAlignment="Center"VerticalAlignment="Center"Text="Home"FontSize="32" /><Button Margin="10" Grid.Row="1"HorizontalAlignment="Right"VerticalAlignment="Bottom"Content="Logout"Command="{Binding LogoutCommand}" /></Grid></local:BasePage>

• 实现Login页面

方法和实现Home页面方法相同

 <local:BasePagex:TypeArguments="vm:LoginViewModel" ...><Grid><BorderPadding="10"HorizontalAlignment="Center"VerticalAlignment="Center"BorderBrush="LightGray"BorderThickness="1"CornerRadius="10"><StackPanel Width="300"><TextBlock HorizontalAlignment="Center" FontSize="28">Login</TextBlock><Separator Margin="0,10" /><TextBlock>User name:</TextBlock><TextBoxMargin="0,10"InputMethod.IsInputMethodEnabled="False"Text="{Binding UserName}" /><TextBlock>Password:</TextBlock><PasswordBox Margin="0,10" Password="123456" /><Button Command="{Binding LoginCommand}" Content="Login" /></StackPanel></Border></Grid></local:BasePage>

4. 定义PageViewConverter

   public class PageViewConverter : IValueConverter
   {public object Convert(object value, Type targetType, object parameter, CultureInfo culture){switch ((ApplicationPage)value){case ApplicationPage.Empty:return new TextBlock { Text = "404 Not Found" };case ApplicationPage.Login:return new Login();case ApplicationPage.Home:return new Home();default:throw new ArgumentException("Invalid value passed to ApplicationPageViewConverter");}}public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture){throw new NotImplementedException();}
   }
   

5. 完成MainWindow

 <Window ...><Window.DataContext><local:MainViewModel/></Window.DataContext><Window.Resources><share:PageViewConverter x:Key="pageConv"/></Window.Resources><ContentControl Content="{Binding CurrentPage,Converter={StaticResource pageConv}}"/></Window>

改进

1. 可以结合依赖注入的方式来实现

2. 导航方法可以封装为一个NavigationService服务

 //封装服务class NavigationService{public static NavigationService Instance { get; } = new NavigationService();private MainViewModel mainVM;public void Navigate(ApplicationPage page){if (mainVM == null){mainVM = (MainViewModel)App.Current.MainWindow.DataContext;}mainVM.CurrentPage = page;}}//原来的方式void Logout(){var mainVM = App.Current.MainWindow.DataContext as MainViewModel;mainVM!.CurrentPage = ApplicationPage.Login;}//使用封装好的服务void Login(){NavigationService.Instance.Navigate(ApplicationPage.Login);}

四、使用Frame和NavigationService

实现上一章节功能，本质上是使用依赖注入的方式将View和ViewModel进行绑定，并利用Frame的自带的Navigate方法进行导航

1. 定义ViewModel

 public class ViewModelBase : ObservableObject{}public partial class MainWindowViewModel : ViewModelBase{private readonly NavigationService navigationService;//依赖注入public MainWindowViewModel(NavigationService navigationService){this.navigationService = navigationService;}[RelayCommand]void Loaded(){  //navigationService实现的导航方法navigationService.Navigate<LoginViewModel>();}}public partial class HomeViewModel : ViewModelBase{[ObservableProperty]string? userName;}public partial class LoginViewModel : ViewModelBase{private readonly NavigationService navigationService;//依赖注入public string UserName { get; set; } = "Sergio";public LoginViewModel(NavigationService navigationService){this.navigationService = navigationService;}[RelayCommand]void Login(){   //navigationService实现的导航方法，此处进行了传参navigationService.Navigate<HomeViewModel>(new Dictionary<string, object?>{[nameof(HomeViewModel.UserName)] = UserName});}}

2. 定义View

主窗口，使用Behaviors实现mvvm模式

<Windowxmlns:b="http://schemas.microsoft.com/xaml/behaviors"><b:Interaction.Triggers><b:EventTrigger><b:InvokeCommandAction Command="{Binding LoadedCommand}" /></b:EventTrigger></b:Interaction.Triggers>
</Window>

主窗口后台类

public partial class MainWindow : Window
{public MainWindow(MainWindowViewModel viewModel,Frame frame){InitializeComponent();DataContext = viewModel;AddChild(frame);}
}

其他View

<!--使用Page来承载内容-->
<Page ...><Grid><TextBlock HorizontalAlignment="Center"VerticalAlignment="Center"d:Text="Hello, world!"Text="{Binding UserName, StringFormat='Hello, {0}!'}"FontSize="32" /></Grid>
</Page><Page ...><Grid><Border Padding="10"HorizontalAlignment="Center"VerticalAlignment="Center"BorderThickness="1"CornerRadius="10"BorderBrush="LightGray"><StackPanel Width="300"><TextBlock HorizontalAlignment="Center" FontSize="28">Login</TextBlock><Separator Margin="0,10" /><TextBlock>User name:</TextBlock><TextBox Margin="0,10" Text="{Binding UserName}" InputMethod.IsInputMethodEnabled="False" /><TextBlock>Password:</TextBlock><PasswordBox Margin="0,10" Password="123456" /><Button Content="Login" Command="{Binding LoginCommand}" /></StackPanel></Border></Grid>
</Page>

在后台类中使用依赖注入的方式定义DataContext

public Home(HomeViewModel viewModel)
{InitializeComponent();DataContext = viewModel;
}public Login(LoginViewModel viewModel)
{InitializeComponent();DataContext = viewModel;
}

3. 实现NavigationService

 public class NavigationService{//注册了单例的Frameprivate readonly Frame? mainFrame;public NavigationService(Frame? frame){mainFrame = frame;//要使用LoadCompleted事件mainFrame.LoadCompleted += MainFrame_LoadCompleted;}private void MainFrame_LoadCompleted(object sender, System.Windows.Navigation.NavigationEventArgs e){if (e.ExtraData is not Dictionary<string,object?> extraData){return;}if ((mainFrame?.Content as FrameworkElement)?.DataContext is not ViewModelBase vm){return;}foreach (var item in extraData){//为每个属性赋值vm.GetType().GetProperty(item.Key)?.SetValue(vm, item.Value);}}//根据VM类型查找View，要注意VM和View的命名规范private Type? FindView<T>(){return Assembly.GetAssembly(typeof(T))?.GetTypes().FirstOrDefault(x => x.Name == typeof(T).Name.Replace("ViewModel", ""));}public void Navigate<T>(Dictionary<string,object?>? extraData=null) where T:ViewModelBase{var viewType = FindView<T>();if (viewType is null)return;var page = App.Current.Services.GetService(viewType) as Page;//利用Frame的Navigate方法进行导航和传参mainFrame?.Navigate(page,extraData);}   }

4. 注册需要的类，此案例在App.cs中进行注册

 public partial class App : Application{public IServiceProvider Services { get; }public static new App Current => (App)Application.Current;public App(){var container = new ServiceCollection();container.AddSingleton(_ => new Frame { NavigationUIVisibility = NavigationUIVisibility.Hidden });container.AddSingleton<MainWindow>();container.AddSingleton<MainWindowViewModel>();container.AddTransient<Login>();container.AddTransient<Home>();container.AddTransient<LoginViewModel>();container.AddTransient<HomeViewModel>();container.AddSingleton<NavigationService>();Services = container.BuildServiceProvider();}protected override void OnStartup(StartupEventArgs e){base.OnStartup(e);MainWindow = Services.GetRequiredService<MainWindow>();MainWindow.Show();}}