I've got a dilemma regarding the DataContext. Let's inspect the following piece of XAML:
<Window xmlns:my="clr-namespace:MyNamespace.Controls"
... >
...
<my:MyControl Name="{Binding Prop1}" Value="{Binding Prop2}" />
</Window>
Obviously, the Window's code-behind contains something like:
DataContext = someViewModel;
Author's intentions are clear - he wants to bind MyControl's Name and Value to Window's DataContext's Prop1 and Prop2. And this will of course work. Unless. (dramatic pause)
Unless MyControl is a composite UserControl, which also wants to take advantage of short notation of bindings and sets its DataContext to its own viewmodel. Because then it will become clear, that the bindings in Window's XAML actually bind to MyControl's DataContext (previously inherited from Window's one) and now they will stop working (or worse, will keep working if MyControl's viewmodel actually contains properties named Prop1 and Prop21).
In this particular case solution is to bind in Window's code explicitly:
<Window x:Name="rootControl"
xmlns:my="clr-namespace:MyNamespace.Controls"
... >
...
<my:MyControl Name="{Binding ElementName=rootControl, Path=DataContext.Prop1}"
Value="{Binding ElementName=rootControl, Path=DataContext.Prop2}" />
</Window>
TL;DR If we're using short notation of bindings (when binding to DataContext) we may encounter quite tough to nail bugs resulting from bindings suddenly pointing to wrong DataContext.
My question is: how to use short binding notation without risk, that I'll bind to wrong DataContext? Of course I may use the short notation when I'm sure, that I'll be using inherited DataContext and long notation when I'm sure, that control will modify its DataContext. But that "I'm sure" will work only until first mistake, which will consume another hour of debugging.
Maybe I'm not following some MVVM rule? E.g. for example DataContext should be set only once on the top level and all composited controls should bind to something else?
1 I've gone through that, actually. The Window's DataContext contained a property named (say) Prop and the control replaced its DataContext with a class, which also contained a property Prop and everything worked fine. Problem appeared when I tried to use (unconsciously) the same pattern with non-matching property names.
By request:
Fragment of MyControl's code:
public string Name
{
get { return (string)GetValue(NameProperty); }
set { SetValue(NameProperty, value); }
}
// Using a DependencyProperty as the backing store for Name. This enables animation, styling, binding, etc...
public static readonly DependencyProperty NameProperty =
DependencyProperty.Register("Name", typeof(string), typeof(MyControl), new PropertyMetadata(null));
public int Value
{
get { return (int)GetValue(ValueProperty); }
set { SetValue(ValueProperty, value); }
}
// Using a DependencyProperty as the backing store for MyProperty. This enables animation, styling, binding, etc...
public static readonly DependencyProperty ValueProperty =
DependencyProperty.Register("Value", typeof(int), typeof(MyControl), new PropertyMetadata(0));
Window's viewmodel:
public class WindowViewmodel : INotifyPropertyChanged
{
// (...)
public string Prop1
{
get
{
return prop1;
}
set
{
prop1 = value;
OnPropertyChanged("Prop1");
}
}
public int Prop2
{
get
{
return prop2;
}
set
{
prop2 = value;
OnPropertyChanged("Prop2");
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
Now assume, that on changing of Name and Value dependency properties, MyControl generates some viewmodel and executes the code:
model = new MyControlViewModel(Name, Value);
this.DataContext = model;
And internal MyControl controls bind to this DataContext.
From now on, the original Name and Value bindings will no longer work.
Unless MyControl is a composite UserControl, which also wants to take advantage of short notation of bindings and sets its DataContext to its own viewmodel.
And that's where I stopped reading. This is, imho, a MVVM anti-pattern.
The reason for this is twofold. First, you screw with anybody who is using the control. "Hey," you say, "you can't bind your stinky VM to my beautiful UI. You have to use MY custom VM!" But what if your VM is hard to use, lacks logic or features needed by the overall application? What happens when, to use your UI, we have to translate our VM/models back and forth with your VM? Pain in the butt.
Second is that your custom control is UI. Its logic is UI logic, and so it is unnecessary to use a view model. It is better to expose DependencyProperties on your control and update your UI as necessary. That way anybody can bind to your UI and use it with any model or view model.
You can solve your problems by simply not using what you call a 'composite control. While I understand that you want to encapsulate some functionality in the associated view model, you don't need to set the view model to the UserControl.DataContext internally.
What I mean by this is that you can have a view model for any or all of your UserControls, but they're data classes, not UI classes, so keep them out of the view code. If you use this method of adding DataTemplates into Resources, then you won't need to set any DataContext properties at all:
<DataTemplate DataType="{x:Type ViewModels:YourUserControlViewModel}">
<Views:YourUserControl />
</DataTemplate>
The final difference is that you should add your view model for your UserControls as properties in a parent view model. This way, you still have no duplicated code (except maybe just a property declaration) and more importantly, you have no Binding problems from mixing DataContext values.
UPDATE >>>
When using this DataTemplate method of hooking up views and view models, you can display your view by Binding your view model property to the Content property of a ContentControl like this:
<ContentControl Content="{Binding YourViewModelProperty}" />
At run time, this ContentControl will be rendered as whatever view or UserControl that you defined in the DataTemplate of the relevant type for that property. Note that you shouldn't set the x:Key of the DataTemplate, otherwise you'd also need to set the ContentControl.ContentTemplate property and that can limit the possibilities afforded by this method.
For example, without setting the x:Key property on your DataTemplates, you could have a property of a base type and by setting it to different sub class, you can have different views for each from the one ContentControl. That is the basis of all of my views... I have one property of a base class view model data bound like this example and to change views, I just change the property to a new view model that is derived from the base class.
UPDATE 2 >>>
Here's the thing... you shouldn't have any 'proxy' object in your UserControls doing anything... it should all be done through properties. So just declare a DependencyProperty of the type of that object and supply it from the view model through data Binding. Doing it this way means that it will be easy to test the functionality of that class, whereas testing code behind views is not.
And finally, yes, it's perfectly fine doing this in MVVM:
<Controls:SomeUserControl DataContext="{Binding SomeViewModelProperty}" />
The overriding goal of MVVM is just to provide separation between the UI code and the view model code, so that we can easily test what's in the view models. That is why we try to remove as much functionality code from the views as possible.
within a usercontrol you should never set the datacontext to "this" or a new viewmodel. a developer/user of your MyUsercontrol expect that the datacontext inherit from top to bottom (from mainwindow to your myusercontrol).
your usercontrol xaml should use element binding
MyUserControl.xaml
<UserControl x:Name="uc">
<TextBlock Text="{Binding ElementName=uc, Path=Name}"/>
<TextBlock Text="{Binding ElementName=uc, Path=Value}"/>
this means your following code will work now in every situation
<Window xmlns:my="clr-namespace:MyNamespace.Controls">
<my:MyControl Name="{Binding Prop1}" Value="{Binding Prop2}" />
</Window>
the property Prop1 from Datacontext mainwindow is bound to the DP Name from your MyUsercontrol and the Textblock.Text within your MyUsercontrol is bound to the DP Name.
I've never met such a problem. It seems to be a little bit theoretical to me but maybe because of my approach to working with DataContext in WPF.
I minimize the explicit use DataContext property. I set it manually only for windows.
I have one dedicated method which is responsible for displaying new windows and it is the only one place where the DataContext property is set explicitly.
DataContext property for Windows is set to root ViewModel which contains child ViewModels, which contain...
I allow WPF to select which View should be used to display given a ViewModel by using DataTemplate
In my application I have a single ResourceDictionary which contains mappings between all ViewModels and Views.
Related
I am trying to find a better way to declare the ViewModel a UWP XAML Page uses.
At this moment,
I declare a ViewModel class ViewModelClass that contains my data properties.
Then I declare an instance of that class as StaticResource of the Page. I like calling those VIEW for consistency across all my Page designs.
Finally, I declare the Page's DataContext as a Binding to the StaticResource VIEW.
This yields a page that understands what data structure is in use and allows AutoComplete when working Bindings. Nice, though lots of lines of same-old-same-old code.
Only, it is not really suitable to ViewModels as the declared resource is a static resource. It is instantiated when the page is instantiated. Most pages will receive a ViewModel parameter upon NavigatedTo, which cannot be used to replace the static resource, because it is, well. static.
So I end up changing the Page's DataContext upon navigation from the initial reference to VIEW to the ViewModel instance I actually want to use.
Big caveat is to declare the back-reference to the page's DataContext when deep in the bowels of a Master-Detail situation is rather horrible. Imagine a collection whose display is in part depending on a Master's property.
How do you tie back robustly to the DataContext of the page from anywhere inside the page?
I have tried giving the page a Name (PAGE for simplicity) and then using ElementName=PAGE, Path=DataContext.someProperty. Ugly, plus you lose all information of the class represented by DataContext.
Another approach is to create a Wrapper around the actual ViewModel called StaticViewModel that has only one property: public ViewModel viewModel. Now I can declare the wrapper as a StaticResource, and tell the page's top-level FrameworkElement to use VIEW.viewModel as its DataContext. Works, and reliably, but sooooo ackward and cumbersome.
I would LOVE to implement a SOURCE class for bindings called PageDataContext that would do nothing else but to loop into the page and get the DataContext from there.
Imagine: {Binding someProperty, Source={PageDataContext}
How would I go about declaring said Source class for a UWP app???
I would LOVE to implement a SOURCE class for bindings called PageDataContext that would do nothing else but to loop into the page and get the DataContext from there. Imagine: {Binding someProperty, Source={PageDataContext}
For your requriment, you could implement your viewmodel in the page Resources and give it x:key. When you bind the property of viewmodel you could access this viewmodel with x:key Source={StaticResource ViewModel} for more please refer the following code.
ViewModel
public class ViewModel
{
public ViewModel()
{
Visibility = false;
}
public bool Visibility { get; set; }
}
Xaml
<Page.Resources>
<local:ViewModel x:Key="ViewModel" />
</Page.Resources>
<TextBlock
Width="100"
Height="44"
Text="{x:Bind Name}"
Visibility="{Binding Visibility, Source={StaticResource ViewModel}}" />
Well, no dice so far.
Working with all those options I found that the best one is a Wrapper class that can be assigned to a static resource which in turn is the basis for the page's DataContext. The wrapped instance of the actual ViewModel is then assigned during OnNavigatedTo() and used as the DataContext of the basic FrameworkElement of the page.
Brr, so much verbose code.
STILL WONDERING how to implement a different version of a source directive. Is there no way to declare one's own Source for a Data Binding? Is this stuff really hard-coded into the framework???
I am trying to learn MVVM and have come across a weird snag. I have a main menu with a drawer control that comes out and shows a menu:
In the main window where this drawer is, I have a ContentControl where I set its content with a Binding.
<ContentControl x:Name="MainWindowContentControl" Content="{Binding Path=WindowContent}"/>
This window's binding is set to a view model.
<Window.DataContext>
<viewmodels:MainWindowViewModel/>
</Window.DataContext>
and here is the ViewModel:
MainWindowViewModel.cs
public class MainWindowViewModel: ViewModelBase
{
private object _content;
public object WindowContent
{
get { return _content; }
set
{
_content = value;
RaisePropertyChanged(nameof(WindowContent));
}
}
public ICommand SetWindowContent { get; set; }
public MainWindowViewModel()
{
SetWindowContent = new ChangeWindowContentCommand(this);
}
}
So far up to this point, everything works fine. So for example, if I click "Recovery Operations", I get this:
RecoveryOperationsView.xaml
In "RecoveryOperationsView.xaml" (which is a UserControl) I also reference the view model from above like so..
<UserControl.DataContext>
<viewmodels:MainWindowViewModel/>
</UserControl.DataContext>
and have a button to call the command to change the Content property of the ContentControl from the main window..
<Button Grid.Row="2" Content="Restore Database" Width="150" Style="{StaticResource MaterialDesignFlatButton}" Command="{Binding SetWindowContent}" CommandParameter="DatabaseRecovery" >
In my class to process the commands, I change the content based off of the passed parameter using a switch statement like so
ChangeWindowContentCommand.cs
public class ChangeWindowContentCommand : ICommand
{
private MainWindowViewModel viewModel;
public ChangeWindowContentCommand(MainWindowViewModel vm)
{
this.viewModel = vm;
}
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
return true;
}
public void Execute(object parameter)
{
switch (parameter)
{
case "Home":
viewModel.WindowContent = new HomeView();
break;
case "RecoveryOps":
viewModel.WindowContent = new RecoveryOperationsView();
break;
case "DatabaseRecovery":
viewModel.WindowContent = new DatabaseRestoreView();
break;
}
}
}
However, this is where I get lost... If I click something within this new window, say "Restore Database" and inspect it with a breakpoint, I can see the property being changed but the actual ContentControl Content property doesnt change to the new UserControl I made... I can change the content with anything in the drawer, but if I try to click a button in the hosted Content of the ContentControl nothing changes. What am I missing?
It's hard to be 100% sure without having your project to test with, but I am fairly confident that at least one of the issues is that your UserControl and your MainWindow use different instances of the MainWindowViewModel. You do not need to instantiate the VM for the user control, as it will inherit the DataContext from the MainWindow. The way it works in WPF is that if any given UIElement does not have theDataContext assigned explicitly, it will inherit it from the first element up the logical tree that does has one assigned.
So, just delete this code, and it should solve at least that issue.
<UserControl.DataContext>
<viewmodels:MainWindowViewModel/>
</UserControl.DataContext>
And since you're learning WPF, I feel obligated to provide a couple other tips. Even though you're using a ViewModel, you are still mixing UI and logic by creating a very specific implementation of ICommand and assigning a UI element through your ViewModel. This breaks the MVVM pattern. I know MVVM takes a little time to understand, but once you do, it is very easy to use and maintain.
To solve your problem, I would suggest creating View Models for each of your user controls. Please see this answer, where I go into quite a bit of detail on the implementation.
For switching the different views, you have a couple of options. You can either use a TabControl, or if you want to use a command, you can have a single ContentControl bound to a property of MainWindowViewModel that is of type ViewModelBase. Let's call it CurrentViewModel. Then when the command fires, you assign the view model of the desired user control to that bound property. You will also need to utilize implicit data templates. The basic idea is that you create a template for each of the user control VM types, which would just contains an instance of the Views. When you assign the user control VM to the CurrentViewModel property, the binding will find those data templates and render the user control. For example:
<Window.Resources>
<DataTemplate DataType = "{x:Type viewmodels:RecoveryOperationsViewModel}">
<views:RecoveryOperationsView/>
</DataTemplate>
<!-- Now add a template for each of the views-->
</Window.Resources>
<ContentControl x:Name="MainWindowContentControl" Content="{Binding CurrentViewModel}"/>
See how this approach keeps UI and logic at an arm's length?
And lastly, consider creating a very generic implementation of ICommand to use in all your ViewModels rather than many specific implementations. I think most WPF programmers have more or less this exact RelayCommand implementation in their arsenal.
I have a problem designing my WPF application. I cannot get the Views to change dynamically. The code to call another View is contained in the Views themselves. (I am trying to implement the MVVM pattern. I do not want any code behind in the View xaml files other than assigning the DataContext. An exception is made in the xaml file of the MainWindow).
Basically, I have a Window that contains a UserControl. The UserControl is my View and it is connected to another class serving as ViewModel through Datacontext.
What I want to do is to dynamically change this View/ViewModel pairs contained in the Window.
My idea was define a static property in the ViewModel of the MainWindow and store the ViewModel of the current View in it. Then I planned to use DataTemplates to automatically load a new View whenever a new ViewModel is stored in the static property.
I decided to use a static property because the code to load another ViewModel is contained in the ViewModels itself and I needed a central point with access from everywhere.
So far so good. My initial View loads and displays correctly.
However, pressing a button in that View to load the next View fails although the new ViewModel is correctly assigned to the static property.
I tried several things.
I defined DataTriggers within the ContentControl to react to changes in the static property. No help.
Implementing INotifyProperty and DependencyProperty failed in the end because of the static nature of the property (or I did something wrong).
I just can’t get it to work.
Do you have any ideas why this would be?
Do you have an idea how I could solve my general problem of dynamically displaying Views without using a static property in my MainWindow. I believe this is causing problems and I have a notion that I am not using the most elegant method. (I do want to maintain the concept of each View holding the code to load any other View)
This is a code fragment from the MainWindow:
<UserControl>
<UserControl.Resources>
<DataTemplate DataType="{x:Type vm:StartViewModel}">
<v:StartView></v:StartView>
</DataTemplate>
<DataTemplate DataType="{x:Type vm:OverviewViewModel}">
<v:OverviewView></v:OverviewView>
</DataTemplate>
</UserControl.Resources>
<Grid>
<ContentControl Content="{Binding ActiveViewModel}">/ContentControl>
</Grid>
</UserControl>
Code behind:
DataContext = new MainViewModel();
MainViewModel contains the definition for the property ActiveViewModel. The constructor for the class is static. All ViewModels inherit from BaseViewModel class:
private static BaseViewModel activeViewModel;
static public BaseViewModel ActiveViewModel
{
get { return activeViewModel; }
set { activeViewModel = value; }
}
Thanks a lot for your help.
Bye,
Eskender
Say I have a WPF application (exe) that has this in the MainWindow.xaml:
<Grid>
<extraControls:MyMVVMUserControl MyDependencyProperty="{Binding Something}"/>
<extraControls:MyUserControl MyDependencyProperty="{Binding Something}" />
</Grid>
and my MainWindow.xaml.cs looks like this:
public MainWindow()
{
DataContext = new MainWindowVM();
InitializeComponent();
}
And my MainWindowVM.cs has a property setup for Something that notifies on property changed.
The user controls are made in a separate dll. As you may guess, MyMVVMUserControl has the DataContext set to a view model.
public MyMVVMUserControl()
{
DataContext = new MyMVVMUserControlVM();
InitializeComponent();
}
MyUserControl does not have a DataContext set in the code behind.
So the interesting thing is that they both have MyDependencyProperty setup exactly the same.
But the MVVM version does not work.
After digging in a bit, I found that the {Binding Something} in MainWindow.xaml is using the View Model setup for the MyMVVMUserControl as the DataContext (rather than the DataContext set in MainWindow.cs (set to MainWindowVM)).
And my question is why?
Why would WPF look inside the user control and use it's DataContext for a binding that is in the actual application?
(NOTE: I know I can get around this by setting the source in the binding, but I want others to be able to use my user controls. But with this issue, I now have a built-in "gotcha" for anyone I want to use my user controls.)
I think I understand you problem, and I'm gonna to give a solution that works for me (I had this problem before). The think is that seams that you are setting the DataContext for you MyMVVMUserControl in code behind, and then it take the bindings from that.
The solution I found for this, is to set the datacontext in code behind, but not at the user control. Set the datacontext for the UserControl's child item. For instance, supose this is the Xaml of your UserControl:
<UserControl ... x:Name="userControl">
<Grid x:Name="rootContainer">
...
</Grid>
</UserControl>
Then in the code behind set the rootContainer's data context, in this way all visual children can access to the control data context, and also the user control datacontext is empty.
...
rootContainer.DataContext = new UserControlViewModel();
...
Hope this may helps you to solve your issues...
You really shouldn't ever set the DataContext of a UserControl from inside the UserControl. By doing so, you are preventing any other DataContext from getting passed to the UserControl, which defeats one of WPF's biggest advantages of having separate UI and data layers.
WPF objects only inherit their DataContext from the parent object if the DataContext is not set to anything else. When your MyMVVMUserControl is being created, you are setting the DataContext to a new MyMVVMUserControlVM, which prevents the DataContext from being inherited from MainWindow.
So its normal that your MVVMUserControl would have it's DataContext set to your MyMVVMUserControlVM, because you set it explicitly in the UserControl's constructor.
This is by-design. UI objects in WPF/MVVM are only meant to be visual representations of the data layer, so it wouldn't make much sense to set the data layer and then try to bind your properties to something that is not on the data layer.
For example, take this line of code:
<UserControl DataContext="{Binding ClassA}" Content="{Binding Name}" />
This would bind the Content property to UserControl.DataContext.Name, which is ClassA.Name. It wouldn't make much sense if this would result in binding to UserControl.Parent.DataContext.Name, as the binding should refer to to the current object's DataContext, and not the Parent's DataContext.
So the only time I ever set the DataContext of a UserControl from inside the UserControl itself is if the UserControl is its own separate object that is never meant to interact with data from the rest of the application. Which so far has been never :)
Typically my UserControls are almost always one of two things:
Either a visual representation of a ViewModel (or Model), such as a CustomerUserControl for a CustomerViewModel, in which case I pass them the DataContext they need when they get used
For example,
<local:CustomerUserControl DataContext="{Binding SelectedCustomer}" />
or
<DataTemplate DataType="{x:Type local:CustomerModel}">
<local:CustomerUserControl />
</DataTemplate>
Or a self-sustained UI object that receives any external data it needs via custom DependencyProperties, and does any additional logic in the code-behind the control, such as a DatePicker control that has a SelectedDate dependency property, or a CalculatorUserControl with dependency properties for Equation and Value
<local:DatePickerUserControl SelectedDate="{Binding SomeDate}" />
<local:CalculatorUserControl Equation="{Binding SomeString}"
Value="{Binding SomeDouble}" />
In your case, it sounds like you should be using the first case, and should be passing a ViewModel into your UserControl containing the data it needs.
<extraControls:MyMVVMUserControl DataContext="{Binding MyMVVMUserControlVM}"
MyDependencyProperty="{Binding Something}">
or
<extraControls:MyMVVMUserControl MyDependencyProperty="{Binding Something}">
<extraControls:MyMVVMUserControl.DataContext>
<viewModels:MyMVVMUserControlVM />
</extraControls:MyMVVMUserControl.DataContext>
<extraControls:MyMVVMUserControl />
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.
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?
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}"/>
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>
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.
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}" />