.NET Core 3 WPF MVVM框架 Prism系列之導航系統

本文將介紹如何在.NET Core3環境下使用MVVM框架Prism基於區域Region的導航系統在講解Prism導航系統之前，我們先來看看一個例子,我在之前的demo項目創建一個登錄界面：我們看到這裡是不是一開始想象到使用WPF帶有的導航系統，通過Frame和Page進行頁面跳轉，然後通過導航 ...

本文將介紹如何在.NET Core3環境下使用MVVM框架Prism基於區域Region的導航系統

在講解Prism導航系統之前，我們先來看看一個例子,我在之前的demo項目創建一個登錄界面：

我們看到這裡是不是一開始想象到使用WPF帶有的導航系統，通過Frame和Page進行頁面跳轉，然後通過導航日誌的GoBack和GoForward實現後退和前進，其實這是通過使用Prism的導航框架實現的,下麵我們來看看如何在Prism的MVVM模式下實現該功能

一.區域導航

我們在上一篇介紹了Prism的區域管理，而Prism的導航系統也是基於區域的，首先我們來看看如何在區域導航

1.註冊區域

LoginWindow.xaml：

<Window x:Class="PrismMetroSample.Shell.Views.Login.LoginWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:PrismMetroSample.Shell.Views.Login"
        xmlns:region="clr-namespace:PrismMetroSample.Infrastructure.Constants;assembly=PrismMetroSample.Infrastructure"
        mc:Ignorable="d"
        xmlns:prism="http://prismlibrary.com/"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
         Height="600" Width="400" prism:ViewModelLocator.AutoWireViewModel="True" ResizeMode="NoResize" WindowStartupLocation="CenterScreen"
        Icon="pack://application:,,,/PrismMetroSample.Infrastructure;Component/Assets/Photos/Home, homepage, menu.png" >
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Loaded">
            <i:InvokeCommandAction Command="{Binding LoginLoadingCommand}"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
    <Grid>
        <ContentControl prism:RegionManager.RegionName="{x:Static region:RegionNames.LoginContentRegion}" Margin="5"/>
    </Grid>
</Window>

2.註冊導航

App.cs：

  protected override void RegisterTypes(IContainerRegistry containerRegistry)
  {
        containerRegistry.Register<IMedicineSerivce, MedicineSerivce>();
        containerRegistry.Register<IPatientService, PatientService>();
        containerRegistry.Register<IUserService, UserService>();

        //註冊全局命令
        containerRegistry.RegisterSingleton<IApplicationCommands, ApplicationCommands>();
        containerRegistry.RegisterInstance<IFlyoutService>(Container.Resolve<FlyoutService>());

        //註冊導航
        containerRegistry.RegisterForNavigation<LoginMainContent>();
        containerRegistry.RegisterForNavigation<CreateAccount>();
  }

3.區域導航

LoginWindowViewModel.cs：

public class LoginWindowViewModel:BindableBase
{

    private readonly IRegionManager _regionManager;
    private readonly IUserService _userService;
    private DelegateCommand _loginLoadingCommand;
    public DelegateCommand LoginLoadingCommand =>
            _loginLoadingCommand ?? (_loginLoadingCommand = new DelegateCommand(ExecuteLoginLoadingCommand));

    void ExecuteLoginLoadingCommand()
    {
        //在LoginContentRegion區域導航到LoginMainContent
        _regionManager.RequestNavigate(RegionNames.LoginContentRegion, "LoginMainContent");

         Global.AllUsers = _userService.GetAllUsers();
    }

    public LoginWindowViewModel(IRegionManager regionManager, IUserService userService)
    {
          _regionManager = regionManager;
          _userService = userService;            
    }

}

LoginMainContentViewModel.cs：

public class LoginMainContentViewModel : BindableBase
{
    private readonly IRegionManager _regionManager;

    private DelegateCommand _createAccountCommand;
    public DelegateCommand CreateAccountCommand =>
            _createAccountCommand ?? (_createAccountCommand = new DelegateCommand(ExecuteCreateAccountCommand));

    //導航到CreateAccount
    void ExecuteCreateAccountCommand()
    {
         Navigate("CreateAccount");
    }

    private void Navigate(string navigatePath)
    {
         if (navigatePath != null)
              _regionManager.RequestNavigate(RegionNames.LoginContentRegion, navigatePath);
        
    }


    public LoginMainContentViewModel(IRegionManager regionManager)
    {
         _regionManager = regionManager;
    }

 }

效果如下：

這裡我們可以看到我們調用RegionMannager的RequestNavigate方法，其實這樣看不能很好的說明是基於區域的做法，如果將換成下麵的寫法可能更好理解一點：

   //在LoginContentRegion區域導航到LoginMainContent
  _regionManager.RequestNavigate(RegionNames.LoginContentRegion, "LoginMainContent");

換成

 //在LoginContentRegion區域導航到LoginMainContent
 IRegion region = _regionManager.Regions[RegionNames.LoginContentRegion];
 region.RequestNavigate("LoginMainContent");

其實RegionMannager的RequestNavigate源碼也是大概實現也是大概如此，就是去調Region的RequestNavigate的方法，而Region的導航是實現了一個INavigateAsync介面：

public interface INavigateAsync
{
   void RequestNavigate(Uri target, Action<NavigationResult> navigationCallback);
   void RequestNavigate(Uri target, Action<NavigationResult> navigationCallback, NavigationParameters navigationParameters);
    
}

我們可以看到有RequestNavigate方法三個形參：

target：表示將要導航的頁面Uri
navigationCallback:導航後的回調方法
navigationParameters:導航傳遞參數(下麵會詳解)

那麼我們將上述加上回調方法：

 //在LoginContentRegion區域導航到LoginMainContent
 IRegion region = _regionManager.Regions[RegionNames.LoginContentRegion];
 region.RequestNavigate("LoginMainContent", NavigationCompelted);

 private void NavigationCompelted(NavigationResult result)
 {
     if (result.Result==true)
     {
         MessageBox.Show("導航到LoginMainContent頁面成功");
     }
     else
     {
         MessageBox.Show("導航到LoginMainContent頁面失敗");
     }
 }

效果如下：

二.View和ViewModel參與導航過程

1.INavigationAware

我們經常在兩個頁面之間導航需要處理一些邏輯，例如，LoginMainContent頁面導航到CreateAccount頁面時候，LoginMainContent退出頁面的時刻要保存頁面數據，導航到CreateAccount頁面的時刻處理邏輯(例如獲取從LoginMainContent頁面的信息)，Prism的導航系統通過一個INavigationAware介面：

    public interface INavigationAware : Object
    {
        Void OnNavigatedTo(NavigationContext navigationContext);

        Boolean IsNavigationTarget(NavigationContext navigationContext);

        Void OnNavigatedFrom(NavigationContext navigationContext);
    }

OnNavigatedFrom：導航之前觸發,一般用於保存該頁面的數據
OnNavigatedTo：導航後目的頁面觸發，一般用於初始化或者接受上頁面的傳遞參數
IsNavigationTarget：True則重用該View實例，Flase則每一次導航到該頁面都會實例化一次

我們用代碼來演示這三個方法：

LoginMainContentViewModel.cs：

public class LoginMainContentViewModel : BindableBase, INavigationAware
{
     private readonly IRegionManager _regionManager;

     private DelegateCommand _createAccountCommand;
     public DelegateCommand CreateAccountCommand =>
            _createAccountCommand ?? (_createAccountCommand = new DelegateCommand(ExecuteCreateAccountCommand));

     void ExecuteCreateAccountCommand()
     {
         Navigate("CreateAccount");
     }

     private void Navigate(string navigatePath)
     {
         if (navigatePath != null)
            _regionManager.RequestNavigate(RegionNames.LoginContentRegion, navigatePath);
     }

     public LoginMainContentViewModel(IRegionManager regionManager)
     {
          _regionManager = regionManager;
     }

     public bool IsNavigationTarget(NavigationContext navigationContext)
     {            
          return true;
     }

     public void OnNavigatedFrom(NavigationContext navigationContext)
     {
          MessageBox.Show("退出了LoginMainContent");
     }

     public void OnNavigatedTo(NavigationContext navigationContext)
     {
          MessageBox.Show("從CreateAccount導航到LoginMainContent");
     }
 }

CreateAccountViewModel.cs:

public class CreateAccountViewModel : BindableBase,INavigationAware
{
     private DelegateCommand _loginMainContentCommand;
     public DelegateCommand LoginMainContentCommand =>
            _loginMainContentCommand ?? (_loginMainContentCommand = new DelegateCommand(ExecuteLoginMainContentCommand));

     void ExecuteLoginMainContentCommand()
     {
         Navigate("LoginMainContent");
     }

     public CreateAccountViewModel(IRegionManager regionManager)
     {
         _regionManager = regionManager;
     }

     private void Navigate(string navigatePath)
     {
        if (navigatePath != null)
            _regionManager.RequestNavigate(RegionNames.LoginContentRegion, navigatePath);
     }

     public bool IsNavigationTarget(NavigationContext navigationContext)
     {
         return true;
     }

     public void OnNavigatedFrom(NavigationContext navigationContext)
     {
         MessageBox.Show("退出了CreateAccount");
     }

     public void OnNavigatedTo(NavigationContext navigationContext)
     {
         MessageBox.Show("從LoginMainContent導航到CreateAccount");
     }

 }

效果如下：

修改IsNavigationTarget為false：

public class LoginMainContentViewModel : BindableBase, INavigationAware
{
     public bool IsNavigationTarget(NavigationContext navigationContext)
     {            
          return false;
     }
}

public class CreateAccountViewModel : BindableBase,INavigationAware
{
     public bool IsNavigationTarget(NavigationContext navigationContext)
     {
         return false;
     }
 }

效果如下:

我們會發現LoginMainContent和CreateAccount頁面的數據不見了，這是因為第二次導航到頁面的時候當IsNavigationTarget為false時，View將會重新實例化，導致ViewModel也重新載入,因此所有數據都清空了

2.IRegionMemberLifetime

同時，Prism還可以通過IRegionMemberLifetime介面的KeepAlive布爾屬性控制區域的視圖的生命周期,我們在上一篇關於區域管理器說到，當視圖添加到區域時候，像ContentControl這種單獨顯示一個活動視圖，可以通過Region的Activate和Deactivate方法激活和失效視圖，像ItemsControl這種可以同時顯示多個活動視圖的，可以通過Region的Add和Remove方法控制增加活動視圖和失效視圖，而當視圖的KeepAlive為false，Region的Activate另外一個視圖時，則該視圖的實例則會去除出區域，為什麼我們不在區域管理器講解該介面呢？因為當導航的時候，同樣的是在觸發了Region的Activate和Deactivate，當有IRegionMemberLifetime介面時則會觸發Region的Add和Remove方法，這裡可以去看下Prism的RegionMemberLifetimeBehavior源碼

我們將LoginMainContentViewModel實現IRegionMemberLifetime介面，並且把KeepAlive設置為false，同樣的將IsNavigationTarget設置為true

LoginMainContentViewModel.cs:

public class LoginMainContentViewModel : BindableBase, INavigationAware，IRegionMemberLifetime
{

     public bool KeepAlive => false;
     
     private readonly IRegionManager _regionManager;

     private DelegateCommand _createAccountCommand;
     public DelegateCommand CreateAccountCommand =>
            _createAccountCommand ?? (_createAccountCommand = new DelegateCommand(ExecuteCreateAccountCommand));

     void ExecuteCreateAccountCommand()
     {
         Navigate("CreateAccount");
     }

     private void Navigate(string navigatePath)
     {
         if (navigatePath != null)
            _regionManager.RequestNavigate(RegionNames.LoginContentRegion, navigatePath);
     }

     public LoginMainContentViewModel(IRegionManager regionManager)
     {
          _regionManager = regionManager;
     }

     public bool IsNavigationTarget(NavigationContext navigationContext)
     {            
          return true;
     }

     public void OnNavigatedFrom(NavigationContext navigationContext)
     {
          MessageBox.Show("退出了LoginMainContent");
     }

     public void OnNavigatedTo(NavigationContext navigationContext)
     {
          MessageBox.Show("從CreateAccount導航到LoginMainContent");
     }
 }

效果如下：

我們會發現跟沒實現IRegionMemberLifetime介面和IsNavigationTarget設置為false情況一樣，當KeepAlive為false時，通過斷點知道，重新導航回LoginMainContent頁面時不會觸發IsNavigationTarget方法，因此可以

知道判斷順序是：KeepAlive -->IsNavigationTarget

3.IConfirmNavigationRequest

Prism的導航系統還支持再導航前允許是否需要導航的交互需求，這裡我們在CreateAccount註冊完用戶後尋問是否需要導航回LoginMainContent頁面，代碼如下：

CreateAccountViewModel.cs:

public class CreateAccountViewModel : BindableBase, INavigationAware，IConfirmNavigationRequest
{
     private DelegateCommand _loginMainContentCommand;
     public DelegateCommand LoginMainContentCommand =>
            _loginMainContentCommand ?? (_loginMainContentCommand = new DelegateCommand(ExecuteLoginMainContentCommand));
    
     private DelegateCommand<object> _verityCommand;
     public DelegateCommand<object> VerityCommand =>
            _verityCommand ?? (_verityCommand = new DelegateCommand<object>(ExecuteVerityCommand));

     void ExecuteLoginMainContentCommand()
     {
         Navigate("LoginMainContent");
     }

     public CreateAccountViewModel(IRegionManager regionManager)
     {
         _regionManager = regionManager;
     }

     private void Navigate(string navigatePath)
     {
        if (navigatePath != null)
            _regionManager.RequestNavigate(RegionNames.LoginContentRegion, navigatePath);
     }

     public bool IsNavigationTarget(NavigationContext navigationContext)
     {
         return true;
     }

     public void OnNavigatedFrom(NavigationContext navigationContext)
     {
         MessageBox.Show("退出了CreateAccount");
     }

     public void OnNavigatedTo(NavigationContext navigationContext)
     {
         MessageBox.Show("從LoginMainContent導航到CreateAccount");
     }
    
     //註冊賬號
     void ExecuteVerityCommand(object parameter)
     {
         if (!VerityRegister(parameter))
         {
                return;
         }
         MessageBox.Show("註冊成功!");
         LoginMainContentCommand.Execute();
     }
    
     //導航前詢問
     public void ConfirmNavigationRequest(NavigationContext navigationContext, Action<bool> continuationCallback)
     {
         var result = false;
         if (MessageBox.Show("是否需要導航到LoginMainContent頁面?", "Naviagte?",MessageBoxButton.YesNo) ==MessageBoxResult.Yes)
         {
             result = true;
         }
         continuationCallback(result);
      }
 }

效果如下：

三.導航期間傳遞參數

Prism提供NavigationParameters類以幫助指定和檢索導航參數，在導航期間，可以通過訪問以下方法來傳遞導航參數：

INavigationAware介面的IsNavigationTarget，OnNavigatedFrom和OnNavigatedTo方法中IsNavigationTarget，OnNavigatedFrom和OnNavigatedTo中形參NavigationContext對象的NavigationParameters屬性
IConfirmNavigationRequest介面的ConfirmNavigationRequest形參NavigationContext對象的NavigationParameters屬性
區域導航的INavigateAsync介面的RequestNavigate方法賦值給其形參navigationParameters
導航日誌IRegionNavigationJournal介面CurrentEntry屬性的NavigationParameters類型的Parameters屬性(下麵會介紹導航日誌)

這裡我們CreateAccount頁面註冊完用戶後詢問是否需要用當前註冊用戶來作為登錄LoginId，來演示傳遞導航參數，代碼如下：

CreateAccountViewModel.cs(修改代碼部分):

private string _registeredLoginId;
public string RegisteredLoginId
{
    get { return _registeredLoginId; }
    set { SetProperty(ref _registeredLoginId, value); }
}

public bool IsUseRequest { get; set; }

void ExecuteVerityCommand(object parameter)
{
    if (!VerityRegister(parameter))
    {
        return;
    }
    this.IsUseRequest = true;
    MessageBox.Show("註冊成功!");
    LoginMainContentCommand.Execute();
}   

public void ConfirmNavigationRequest(NavigationContext navigationContext, Action<bool> continuationCallback)
{
    if (!string.IsNullOrEmpty(RegisteredLoginId) && this.IsUseRequest)
    {
         if (MessageBox.Show("是否需要用當前註冊的用戶登錄?", "Naviagte?", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
         {
              navigationContext.Parameters.Add("loginId", RegisteredLoginId);
         }
    }
    continuationCallback(true);
}

LoginMainContentViewModel.cs(修改代碼部分):

public void OnNavigatedTo(NavigationContext navigationContext)
{
     MessageBox.Show("從CreateAccount導航到LoginMainContent");
            
     var loginId= navigationContext.Parameters["loginId"] as string;
     if (loginId!=null)
     {
         this.CurrentUser = new User() { LoginId=loginId};
     }
            
 }

效果如下：

四.導航日誌

Prism導航系統同樣的和WPF導航系統一樣，都支持導航日誌，Prism是通過IRegionNavigationJournal介面來提供區域導航日誌功能，

    public interface IRegionNavigationJournal
    {
        bool CanGoBack { get; }

        bool CanGoForward { get; }

        IRegionNavigationJournalEntry CurrentEntry {get;}

        INavigateAsync NavigationTarget { get; set; }

        void GoBack();

        void GoForward();

        void RecordNavigation(IRegionNavigationJournalEntry entry, bool persistInHistory);

        void Clear();
    }

我們將在登錄界面接入導航日誌功能,代碼如下:

LoginMainContent.xaml(前進箭頭代碼部分):

<TextBlock Width="30" Height="30" HorizontalAlignment="Right" Text="&#xe624;" FontWeight="Bold" FontFamily="pack://application:,,,/PrismMetroSample.Infrastructure;Component/Assets/Fonts/#iconfont" FontSize="30" Margin="10" Visibility="{Binding IsCanExcute,Converter={StaticResource boolToVisibilityConverter}}">
      <i:Interaction.Triggers>
           <i:EventTrigger EventName="MouseLeftButtonDown">
                 <i:InvokeCommandAction Command="{Binding GoForwardCommand}"/>
            </i:EventTrigger>
      </i:Interaction.Triggers>
      <TextBlock.Style>
           <Style TargetType="TextBlock">
                <Style.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                          <Setter Property="Background" Value="#F9F9F9"/>
                    </Trigger>
                 </Style.Triggers>
            </Style>
      </TextBlock.Style>
 </TextBlock>

BoolToVisibilityConverter.cs:

public class BoolToVisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
          if (value==null)
          {
              return DependencyProperty.UnsetValue;
          }
          var isCanExcute = (bool)value;
          if (isCanExcute)
          {
              return Visibility.Visible;
          }
          else
          {
              return Visibility.Hidden;
          }
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
            throw new NotImplementedException();
    }
 }

LoginMainContentViewModel.cs(修改代碼部分):

IRegionNavigationJournal _journal;

private DelegateCommand<PasswordBox> _loginCommand;
public DelegateCommand<PasswordBox> LoginCommand =>
            _loginCommand ?? (_loginCommand = new DelegateCommand<PasswordBox>(ExecuteLoginCommand, CanExecuteGoForwardCommand));

private DelegateCommand _goForwardCommand;
public DelegateCommand GoForwardCommand =>
            _goForwardCommand ?? (_goForwardCommand = new DelegateCommand(ExecuteGoForwardCommand));

private void ExecuteGoForwardCommand()
{
    _journal.GoForward();
}

private bool CanExecuteGoForwardCommand(PasswordBox passwordBox)
{
    this.IsCanExcute=_journal != null && _journal.CanGoForward;
    return true;
}

public void OnNavigatedTo(NavigationContext navigationContext)
{
     //MessageBox.Show("從CreateAccount導航到LoginMainContent");
     _journal = navigationContext.NavigationService.Journal;

     var loginId= navigationContext.Parameters["loginId"] as string;
     if (loginId!=null)
     {
                this.CurrentUser = new User() { LoginId=loginId};
     }
     LoginCommand.RaiseCanExecuteChanged();
}

CreateAccountViewModel.cs(修改代碼部分):

IRegionNavigationJournal _journal;

private DelegateCommand _goBackCommand;
public DelegateCommand GoBackCommand =>
            _goBackCommand ?? (_goBackCommand = new DelegateCommand(ExecuteGoBackCommand));

void ExecuteGoBackCommand()
{
     _journal.GoBack();
}

 public void OnNavigatedTo(NavigationContext navigationContext)
 {
     //MessageBox.Show("從LoginMainContent導航到CreateAccount");
     _journal = navigationContext.NavigationService.Journal;
 }

效果如下：

選擇退出導航日誌

如果不打算將頁面在導航過程中不加入導航日誌，例如LoginMainContent頁面，可以通過實現IJournalAware並從PersistInHistory（）返回false

    public class LoginMainContentViewModel : IJournalAware
    {
        public bool PersistInHistory() => false;
    }

五.小結：

prism的導航系統可以跟wpf導航並行使用，這是prism官方文檔也支持的，因為prism的導航系統是基於區域的，不依賴於wpf，不過更推薦於單獨使用prism的導航系統，因為在MVVM模式下更靈活，支持依賴註入,通過區域管理器能夠更好的管理視圖View,更能適應複雜應用程式需求，wpf導航系統不支持依賴註入模式，也依賴於Frame元素，而且在導航過程中也是容易強依賴View部分，下一篇將會講解Prism的對話框服務

六.源碼

最後，附上整個demo的源代碼：PrismDemo源碼