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.