处理WPF MVVM中ContentControl之间导航的更好方法？

Question

I am currently developing a C# WPF application and am trying to follow the MVVM design pattern.

The way it is working now, is I am using a ContentControl in my main window and binding it to the CurrentViewModel , and declaring in App.xaml my datatemplates. When I want to change the current view in the main window, all I have to do is change the CurrentViewModel property in the main window's view model, which works well. Also, in order to not have a direct reference of a view model (by doing new blablaViewModel() in a view model), I have a singleton FlowManager class that I call in the ICommand function, and the instantiation is done in that class rather than the view model.

The problem with this approach, is that for each view I add to my application, I have to add a datatemplate in App.xaml , an enum entry in my FlowManager class and a new case in my switch() in the ChangePage() function, a new ICommand in my MainViewModel , on top of adding the code for the actual view and creating it's own view model.

Here is an example of how I handle the flow of my application:

In MainWindow.xaml , I have the following layout:

<Window x:Class="EveExcelMineralUpdater.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewModels="clr-namespace:EveExcelMineralUpdater.ViewModels"
        Title="MainWindow" Height="720" Width="1280">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="15*"/>
            <ColumnDefinition Width="auto"/>
            <ColumnDefinition Width="85*"/>
        </Grid.ColumnDefinitions>

        <StackPanel Grid.Column="0">
            <Button>MarketStat Request</Button>
            <Button Command="{Binding ChangeToQuickLookCommand}">QuickLook Request</Button>
            <Button>History Request</Button>
            <Button>Route Request</Button>
            <Button>Settings</Button>
        </StackPanel>

        <Separator Grid.Column="1" Style="{StaticResource {x:Static ToolBar.SeparatorStyleKey}}" />

        <ContentControl Grid.Column="2" Content="{Binding CurrentViewModel}" />
    </Grid>
</Window>

In App.xaml.cs I start the application by creating the main window and settings its DataContext and MainViewModel property:

MainWindow mainWindow = new MainWindow();
MainViewModel mainViewModel = new MainViewModel();
mainWindow.DataContext = mainViewModel;
mainWindow.ViewModel = mainViewModel;

FlowManager.Instance.AppWindow = mainWindow;

mainWindow.Show();

In MainViewModel.cs , I handle a button request to change the CurrentView property with an ICommand like follows:

private void ChangeToQuickLook(object param)
{
    FlowManager.Instance.ChangePage(FlowManager.Pages.QuickLook);
}
...
public ICommand ChangeToQuickLookCommand
{
    get { return new RelayCommand(ChangeToQuickLook); }
}

In FlowManager.cs , I have an enum that lists all the pages (views) in my application, and the actual ChangePage() function that will change the CurrentViewModel property in my MainViewModel :

// Only one view is implemented for now, the rest are empty for now
public void ChangePage(Pages page)
{
    IViewModel newViewModel = null;

    switch (page)
    {
        case Pages.MarketStat:
            break;
        case Pages.QuickLook:
            newViewModel = new QuickLookRequestViewModel();
            break;
        case Pages.History:
            break;
        case Pages.Route:
            break;
        case Pages.Settings:
            break;
    }

    AppWindow.ViewModel.CurrentViewModel = newViewModel;
}
...
public enum Pages
{
    MarketStat,
    QuickLook,
    History,
    Route,
    Settings
}

Finally, in App.xaml , I have the list of all my datatemplates for all my views:

<Application x:Class="EveExcelMineralUpdater.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:viewModels="clr-namespace:EveExcelMineralUpdater.ViewModels"
             xmlns:views="clr-namespace:EveExcelMineralUpdater.Views"
             Startup="App_OnStartup">
    <Application.Resources>
        <!-- Pages DataTemplates -->
        <DataTemplate DataType="{x:Type viewModels:QuickLookRequestViewModel}">
            <views:QuickLookRequestView />
        </DataTemplate>
    </Application.Resources>
</Application>

Like I said, this works well, but I can see some scalability problems as I have to modify several parts of my code in order to add a view in the application. Is there a better way to do this without the use of any frameworks?

Answer 1

After looking at @WojciechKulik comment, I have come up with the following change in my FlowManager.cs class:

public class FlowManager
{
    private static FlowManager _instance;

    private MainWindow _mainWindow;
    private ICollection<IViewModel> _viewModels; 

    private FlowManager()
    {
        ViewModels = new List<IViewModel>();
    }

    public void ChangePage<TViewModel>() where TViewModel : IViewModel, new()
    {
        // If we are already on the same page as the button click, we don't change anything
        if (AppWindow.ViewModel.CurrentViewModel == null || 
            AppWindow.ViewModel.CurrentViewModel.GetType() != typeof(TViewModel))
        {
            foreach (IViewModel viewModel in ViewModels)
            {
                // If an instance of the viewmodel already exists, we switch to that one
                if (viewModel.GetType() == typeof(TViewModel))
                {
                    AppWindow.ViewModel.CurrentViewModel = viewModel;
                    return;
                }
            }

            // Else, we create a new instance of the viewmodel
            TViewModel newViewModel = new TViewModel();
            AppWindow.ViewModel.CurrentViewModel = newViewModel;
            ViewModels.Add(newViewModel);
        }
    }

    public static FlowManager Instance
    {
        get
        {
            if (_instance == null)
            {
                _instance = new FlowManager();
            }

            return _instance;
        }
    }

    public MainWindow AppWindow { get; set; }

    public ICollection<IViewModel> ViewModels { get; private set; }
}

This way, I add support for keeping the state of each of my view's viewmodel, and I got rid of my enum with an entry for each view I have in my application by using the power of Generics and reflection.

I will update this answer if I find other ways to reduce the number of places I have to modify for each view I want to add to the application.