WPF：如何在视图中创建 ViewModel

Question

基本上，我有一个标记问题。我想出了一些解决方案，但我不禁觉得这应该更简单。与其把你引向我复杂的道路，我想我会分享最简单的实现，并询问你将如何解决它。

MainPage.xaml

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="6" />
        <ColumnDefinition />
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="6" />
        <ColumnDefinition />
        <!--Additional Columns-->
    </Grid.ColumnDefinitions>
    <!--Row Definitions-->
    <Label Grid.Row="0" Grid.Column="0" Content="Vin:" HorizontalAlignment="Right" />
    <ctrl:CommandTextBox Grid.Row="0" Grid.Column="2" Command="{Binding CreateVehicleCommand}" CommandParameter="{Binding Text, RelativeSource={RelativeSource Self}}" />
    <Label Grid.Row="0" Grid.Column="3" Content="Manufacturer:" HorizontalAlignment="Right" />
    <TextBox Grid.Row="0" Grid.Column="5" IsEnabled="False" Text="{Binding Vehicle.Manufacturer, Mode=OneWay}" />
    <!--Additional Read Only Values-->
</Grid>

鉴于上面的示例，鉴于创建车辆的命令在要创建的数据上下文（车辆）之外的约束，我如何将网格的内容放入视图中？

如果你想看看我的具体尝试，这个问题在这里UserControl's DependencyProperty is null when UserControl has a DataContext

Answer 1

在给定的情况下，如何将网格的内容放入视图中 限制创建车辆的命令在 要创建的DataContext(Vehicle)？

这感觉更像是一种竞争条件，而不是 MVVM 问题。我会先解决这个问题，然后再提出一个建议。

1. ViewModel 没有理由不能包含另一个视图模型作为参考，并且该参考绑定到使用 INotifyPropertyChanged 机制。
2. 或者您的 xaml（视图）页面包含对该页面（视图）不直接在其 DataContext 中使用的 ViewModel 的静态引用，但某个控件无法绑定到包含的数据上下文之外的该静态控制。

无论哪种方式，您都可以通过指向自身来获取数据或提供获取数据的备用管道来提供访问权限（在对您提供的另一篇帖子的回复中也提到过）。 p>

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或者您可以展平您的视图模型以包含更多信息并处理此恕我直言竞争条件，这样就不会出现这种情况，并且控件和网格可以以适当的格式访问信息。

我无法完全解决这个问题，因为您更清楚现在必须解决的设计目标和危险。

Answer 2

我想出了一些东西，我比较满意。这使我免于创建 100 多个复合 ViewModel，虽然它确实引入了一些不必要的复杂性，但它确实大大减少了我需要编写的复制/粘贴代码量。

VMFactoryViewModel.cs

public class CreatedViewModelEventArgs<T> : EventArgs where T : ViewModelBase
{
    public T ViewModel { get; private set; }

    public CreatedViewModelEventArgs(T viewModel)
    {
        ViewModel = viewModel;
    }
}

public class VMFactoryViewModel<T> : ViewModelBase where T : ViewModelBase
{
    private Func<string, T> _createViewModel;
    private RelayCommand<string> _createViewModelCommand;
    private readonly IDialogService _dialogService;

    /// <summary>
    /// Returns a command that creates the view model.
    /// </summary>
    public ICommand CreateViewModelCommand
    {
        get
        {
            if (_createViewModelCommand == null)
                _createViewModelCommand = new RelayCommand<string>(x => CreateViewModel(x));

            return _createViewModelCommand;
        }
    }

    public event EventHandler<CreatedViewModelEventArgs<T>> CreatedViewModel;

    private void OnCreatedViewModel(T viewModel)
    {
        var handler = CreatedViewModel;
        if (handler != null)
            handler(this, new CreatedViewModelEventArgs<T>(viewModel));
    }

    public VMFactoryViewModel(IDialogService dialogService, Func<string, T> createViewModel)
    {
        _dialogService = dialogService;
        _createViewModel = createViewModel;
    }

    private void CreateViewModel(string viewModelId)
    {
        try
        {
            OnCreatedViewModel(_createViewModel(viewModelId));
        }
        catch (Exception ex)
        {
            _dialogService.Show(ex.Message);
        }
    }
}

VMFactoryUserControl.cs

public class VMFactoryUserControl<T> : UserControl where T : ViewModelBase
{
    public static readonly DependencyProperty VMFactoryProperty = DependencyProperty.Register("VMFactory", typeof(VMFactoryViewModel<T>), typeof(VMFactoryUserControl<T>));

    public VMFactoryViewModel<T> VMFactory
    {
        get { return (VMFactoryViewModel<T>)GetValue(VMFactoryProperty); }
        set { SetValue(VMFactoryProperty, value); }
    }
}

GenericView.xaml

<ctrl:VMFactoryUserControl x:Class="GenericProject.View.GenericView"
                           x:TypeArguments="vm:GenericViewModel"
                           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                           xmlns:ctrl="clr-namespace:SomeProject.Controls;assembly=SomeProject.Controls"
                           xmlns:vm="clr-namespace:GenericProject.ViewModel">
    <Grid>
        <!-- Column Definitions -->
        <!-- Row Definitions -->
        <Label Grid.Row="0" Grid.Column="0" Content="Generic Id:" HorizontalAlignment="Right" />
        <ctrl:CommandTextBox Grid.Row="0" Grid.Column="2"
                             Command="{Binding VMFactory.CreateViewModelCommand, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"
                             CommandParameter="{Binding Text, RelativeSource={RelativeSource Self}}" />
        <Label Grid.Row="0" Grid.Column="3" Content="Generic Property:" HorizontalAlignment="Right" />
        <TextBox Grid.Row="0" Grid.Column="5" IsEnabled="False" Text="{Binding GenericProperty, Mode=OneWay}" />
        <!--Additional Read Only Values-->
    </Grid>
</ctrl:VMFactoryUserControl>

GenericView.xaml.cs

public partial class GenericView : VMFactoryUserControl<GenericViewModel>
{
    public GenericView()
    {
        InitializeComponent();
    }
}

MainPageViewModel.cs

public class MainPageViewModel : ViewModelBase
{
    private readonly IDialogService _dialogService;
    private GenericViewModel _generic;
    private readonly VMFactoryViewModel<GenericViewModel> _genericFactory;

    public GenericViewModel Generic
    {
        get { return _generic; }
        private set
        {
            if (_generic != value)
            {
                _generic = value;
                base.OnPropertyChanged("Generic");
            } 
        }
    }

    public VMFactoryViewModel<GenericViewModel> GenericFactory
    {
        get { return _genericFactory; }
    }

    private void OnGenericFactoryCreatedViewModel(object sender, CreatedViewModelEventArgs<GenericViewModel> e)
    {
        Generic = e.ViewModel;
    }

    public MainPageViewModel(IDialogService dialogService)
    {
        _dialogService = dialogService;

        _genericFactory = new VMFactoryViewModel<GenericViewModel>(_dialogService, x => new GenericViewModel(_dialogService, GetGeneric(x)));
        _genericFactory.CreatedViewModel += OnGenericFactoryCreatedViewModel;
    }

    private Generic GetGeneric(string genericId)
    {
        // Return some Generic model.
    }
}

MainPage.xaml

<Page x:Class="GenericProject.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:vw="clr-namespace:GenericProject.View">
    <StackPanel>
        <!-- Headers and Additional Content. -->
        <vw:EventView DataContext="{Binding Generic}"
                      VMFactory="{Binding DataContext.GenericFactory, RelativeSource={RelativeSource AncestorType={x:Type Page}}}" />
    </StackPanel>
</Page>