Question

I speak about josh smith article.

can anyone show me please how the CustomerView.xaml specifically this:j

 <TextBox 
  x:Name="firstNameTxt"
  Grid.Row="2" Grid.Column="2" 
  Text="{Binding Path=FirstName, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" 
  Validation.ErrorTemplate="{x:Null}"
  />

Why is there a Binding to FirstName which is public property in the CustomerViewModel.

There is a datacontext set for the MainViewModel, but not for the CustomerViewModel, so why does the binding work ???

Answer 1

Check out the ResourceDictionary in MainWindowResources.xaml. Josh uses the following code to describe what View should be used if an instance of CustomerViewModel is shown in the main window:

   <DataTemplate DataType="{x:Type vm:CustomerViewModel}">
        <vw:CustomerView />
   </DataTemplate>

We’ve described that when our DataType is of Type CustomerViewModel, we’ll create a new instance of the CustomerView. WPF takes care of the DataContext and creation when it sees the CustomerViewModel type.

From the rest of the article:

Applying a View to a ViewModel
MainWindowViewModel indirectly adds
and removes Workspace ViewModel
objects to and from the main window’s
Tab Control. By relying on data
binding, the Content property of a
TabItem receives a
ViewModelBase-derived object to
display. ViewModelBase is not a UI
element, so it has no inherent support
for rendering itself. By default, in
WPF a non-visual object is rendered by
displaying the results of a call to
its ToString method in a TextBlock.
That clearly is not what you need,
unless your users have a burning
desire to see the type name of our
ViewModel classes! You can easily tell
WPF how to render a ViewModel object
by using typed DataTemplates. A typed
DataTemplate does not have an x:Key
value assigned to it, but it does have
its DataType property set to an
instance of the Type class. If WPF
tries to render one of your ViewModel
objects, it will check to see if the
resource system has a typed
DataTemplate in scope whose DataType
is the same as (or a base class of)
the type of your ViewModel object. If
it finds one, it uses that template to
render the ViewModel object referenced
by the tab item’s Content property.
The MainWindowResources.xaml file has
a Resource Dictionary. That dictionary
is added to the main window’s resource
hierarchy, which means that the
resources it contains are in the
window’s resource scope. When a tab
item’s content is set to a ViewModel
object, a typed DataTemplate from this
dictionary supplies a view (that is, a
user control) to render it, as shown
in Figure 10.

The DataContext for the MainViewModel in App.xaml.cs serves as a starting point for our application.