How is DataContext inherited in Views which contain Views?

Question

I'm rebuilding Josh Smith's WPF CommandSink example and there are a few things that I don't understand about his databinding, especially about how datacontext is inherited when a view is contained in another view which is contained in a window which has a datacontext.

• all databinding is declared in the XAML files, there is absolutely no code behind the Window or either of the Views (nice)
• the top Window defines its DataContext as CommunityViewModel and simply displays the CommunityView

• Question: so now in the CommunityViewModel, what does the jas:CommandSinkBinding.CommandSink="{Binding}" do actually? "CommandSink" is an attached property so is this "attaching" the inherited Binding that comes from DemoWindow as the value of the attached property called "CommandSink" on the CommandSinkBinding object?

• Question: also, PersonView doesn't seem to have a DataContext yet it has lines such as <TextBlock Text="{Binding Name}" Width="60" /> which assume that a binding is set. So does PersonView automatically get its binding from the line in CommunityView ItemsSource="{Binding People}"?

Thanks for any clarification here.

DemoWindow.xaml:

<Window 
  x:Class="VMCommanding.DemoWindow"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:view="clr-namespace:VMCommanding.View"
  xmlns:vm="clr-namespace:VMCommanding.ViewModel"
  FontSize="13"
  ResizeMode="NoResize"
  SizeToContent="WidthAndHeight" 
  Title="ViewModel Commanding Demo"   
  WindowStartupLocation="CenterScreen"
  >
  <Window.DataContext>
    <vm:CommunityViewModel />
  </Window.DataContext>

  <Window.Content>
    <view:CommunityView />
  </Window.Content>
</Window>

CommunityView.xaml:

<UserControl 
  x:Class="VMCommanding.View.CommunityView"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:jas="clr-namespace:VMCommanding"
  xmlns:view="clr-namespace:VMCommanding.View"
  xmlns:vm="clr-namespace:VMCommanding.ViewModel"  
  jas:CommandSinkBinding.CommandSink="{Binding}"
  >
    <UserControl.CommandBindings>
        <jas:CommandSinkBinding Command="vm:CommunityViewModel.KillAllMembersCommand" />
    </UserControl.CommandBindings>

    <DockPanel Margin="4">
        <ItemsControl
      DockPanel.Dock="Bottom" ItemsSource="{Binding People}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <view:PersonView />
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
        <Button 
      Command="vm:CommunityViewModel.KillAllMembersCommand"
      Content="Kill All"
      Margin="0,0,0,8"
      />
    </DockPanel>
</UserControl>

PersonView.xml:

<UserControl 
  x:Class="VMCommanding.View.PersonView"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:jas="clr-namespace:VMCommanding"
  xmlns:vm="clr-namespace:VMCommanding.ViewModel"
  jas:CommandSinkBinding.CommandSink="{Binding}"
  >  
  <UserControl.CommandBindings>
    <jas:CommandSinkBinding Command="vm:PersonViewModel.DieCommand" />
    <jas:CommandSinkBinding Command="vm:PersonViewModel.SpeakCommand" />
  </UserControl.CommandBindings>

  <UserControl.Resources>
    <Style TargetType="{x:Type TextBlock}">
      <Setter Property="Margin" Value="0,0,6,0" />
      <Style.Triggers>
        <DataTrigger Binding="{Binding CanDie}" Value="False">
          <Setter Property="Foreground" Value="#88000000" />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </UserControl.Resources>

  <StackPanel Margin="2" Orientation="Horizontal">
    <TextBlock Text="Name:" FontWeight="Bold" />
    <TextBlock Text="{Binding Name}" Width="60" />
    <TextBlock Text="Age:" FontWeight="Bold" />
    <TextBlock Text="{Binding Age}" Width="40" />
    <Button 
      Command="vm:PersonViewModel.SpeakCommand"
      CommandParameter="Howdy partner!"
      Content="Speak"
      Margin="0,0,6,0"
      Width="60"
      />
    <Button
      Command="vm:PersonViewModel.DieCommand"
      Content="Die"
      Width="60"
      />
  </StackPanel>
</UserControl>

Answer 1

If you set the ItemsSource of an ItemsControl, the DataContexts of the items in that control are directly mapped to the items of the ItemsSource.