WPF: How to Create ViewModel Within the View - wpf

Question

Essentially, I have a markup issue. I have come up with a few solutions but I can't help but feel like this should be simpler. Rather than lead you down my convoluted path I thought I would share the simplest implementation and ask how you would address it.
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>
Given the example above, how can I get the Contents of the Grid into a View given the constraint that the Command to create the vehicle is outside of the DataContext to be created(Vehicle)?
If you do wish to look at my specific attempt, that question is here UserControl's DependencyProperty is null when UserControl has a DataContext

Answer 1

how can I get the Contents of the Grid into a View given the
constraint that the Command to create the vehicle is outside of the
DataContext to be created(Vehicle)?
This feels like a race condition more than an MVVM problem. I will address the issue first but make a secondary suggestion after.
There are no reasons in which a ViewModel cannot contain another viewmodel as a reference and that reference is bound to using the INotifyPropertyChanged mechanisim.
Or that your xaml (view) page contains a static reference to a ViewModel which the page (view) does not directly use in its DataContext, but that a certain control cannot bind to that static outside of the data context of the containing control.
Either way one can provide access (as also mentioned in the response to the other post you provided) by pointing to itself to get data or to provide an alternate plumbing which gets the data.
Or you can flatten your viewmodel to contain more information and handle this IMHO race condition so that this situation doesn't arise and the control as well as the grid can access information in a proper format.
I can't fully address the problem because you are more aware of the design goals and hazards which now must be worked around.

Answer 2

I've come up with something, I'm relatively happy with. This has saved me from creating 100s of composite ViewModel's and while it does introduce some unnecessary complexity it does dramatically reduce the amount copy/paste code I need to write.
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>