访问父USerControl ViewModel，反之亦然

Question

Suppose, there is a user control named ParentUserControl in ParentView.xaml. In the code behind I have set an instance of ParentViewModel as it's DataContext. In ParentUserControl there's a ChildUserControl. How can I access ParentViewModel aka ParentUserControl's DataContext in childUserControl's view model and also in opposite way how can I access childviewmodel in parentViewModel.

ParentView.xaml

<UserControl x:Class="Test.ParentView"             
             mc:Ignorable="d"
             Name="ParentUserControl"
             >
    <UserControl.Resources>
        <ControlTemplate x:Key="ChildControlTemplate">
            <control:ChildUSerControl/>
        </ControlTemplate>
    </UserControl.Resources>

    <Grid>
        <Border Grid.Row="0" BorderBrush="{StaticResource BorderBrush}" BorderThickness="0,0,0,0">
           <ContentControl Name="ChildControlTemplate"  Width="Auto" ></ContentControl>
        </Border>
    </Grid>
</UserControl>

ParentView.xaml.cs

public ParentView()
{
    ParentViewModel parentViewModel = new ParentViewModel();
    this.DataContext = parentViewModel;
    ChildUSerControl childUserControl = new ChildUserControl();
    InitializeComponent();
    ChildControlTemplate.Content = childUserControl;
}

Answer 1

If you make ChildViewModel a property of ParentViewModel (as ChildVM in the example below), you can wire it up as the DataContext of ChildUserControl in the XAML. And while you're at it, you can create the ParentViewModel in the XAML without needing to do it in the code behind. Something like:

 <UserControl x:Class="Test.ParentView"             
                 xmlns:vm="clr-namespace:NamespaceForYourParentViewModel"
                 mc:Ignorable="d"
                 Name="ParentUserControl"
                 >
       <UserControl.DataContext>
          <vm:ParentViewModel />
        </UserControl.DataContext>
        <UserControl.Resources>
            <ControlTemplate x:Key="ChildControlTemplate">
                <control:ChildUSerControl DataContext="{Binding Path=ChildVM}/>
            </ControlTemplate>
        </UserControl.Resources>

Then your parent and child view models can communicate however you like in the C# - the ParentViewModel will be able to communicate with the child as it instantiates it. And the ParentViewModel could be passed to the Child View Model when it's instantiated.