Binding from DependecyProperty to DataContext (ViewModel) in XAML

Question

Assume this situation:
I have created a new control ("MyControl") with DependencyProperty "SuperValue".
Now, in XAML i set "SuperValue" to "TestValue":

<local:MyControl SuperValue="TestValue" />

This control has a ViewModel (DataContext).
I want to pass value of DependencyProperty (in this example "TestValue") to property in ViewModel.

How can I do this?

Assume that ViewModel of my control do something calculations, for example: User inputs name of country, and control give him a time which is currently there.

The problem is: How can I provide the result of calculation? Assume that this is public property "Results" in ViewModel. I want to create a property like "TextBox.Text", "ListView.SelectedItem" which provides a part of ViewModel data "to outside".

For example TextBox and Text property:

<TextBox Text={Binding GiveMeTextValue} />

In this case DP "Text" provides to outside a ViewModel property which currently stores inputted text.

I want to use my control in the same way.

Answer 1

I don't know whether I get your question right: You want to set a static non-bound value in XAML to a DependencyProperty of the control and set a property on the control's DataContext to this static value? There is something wrong about your concept if you need to do this, why don't you provide this value on the ViewModel in an according field and bind the DP of the control to this field?

However, what you can do get what you want:

Define a PropertyChangedCallback when you register the DP:

// Dependency Property
public static readonly DependencyProperty TestProperty =
DependencyProperty.Register("Test", typeof(string),
typeof(MyControl), new FrameworkPropertyMetadata("123", new PropertyChangedCallback(OnTestChanged)));

In the OnTestChanged method, cast your DataContext to the type of your ViewModel and set the according value on the ViewModel to the new value:

private static void OnTestChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            MyControl c = d as MyControl;
            ViewModelType vm = c.DataContext as ViewModelType;
            vm.Property = e.New;
            Console.WriteLine(e.NewValue);
        }

Is that what you're asking for?

Answer 2

What about setting the MyDependencyProperty from the setter of property SomethingValueInDataContext.

EDIT

You can set the controls DependencyProperty where the control is used and not on its declaration. This will work (local is namespace where control resides) -

<Grid>
   <local:MyOwnControl MyDependencyProperty="{Binding Test}"/>
</Grid>

Same as like you can set the Width of the TextBox when you create an instance of it in xaml like this-

<TextBox Width="{Binding PropertyName}"/>

Answer 3

Notice, the root of your xaml is UserControl and not MyOwnControl. UserControl is the base class of MyOwnControl; your property is not defined in the base class. This is why you cannot reference MyDependencyProperty from within the root element of the UserControl.

Using your example, you can switch the binding and get your desired effect.

<UserControl 
    x:Class="namespace.MyOwnControl"
    x:Name="root">
    <UserControl.DataContext>
         <local:ControlViewModel  
             Test={Binding MyDependencyProperty, ElementName=root}" />
    </UserControl.DataContext>
</UserControl>

Answer 4

Since you are using a MVVM design paradigm all data should be relative to the ViewModel. So your DP should be set via the binding in your VM property.

If the test data is going to be used in Blend/VS designer you can check for that vs. Debug/Release... then do some sort of assignment to your property based off of that check for testing.

Answer 5

You could add a property to MyControl called InitialSuperValue that when set, sets the value of SuperValue. Then write some XAML like this:

<local:MyControl InitialSuperValue="TestValue" SuperValue="{Binding SuperValueInViewModel, Mode=OneWayToSource}" />