How to pass initialization parameters to UserControl in WPF (using MVVM) - wpf

Question

I have a UserControl called ActionsTreeView I built using MVVM practices where I have an IPluginsProvider interface that populates the data in my UserControl. I want to be able to provide an object implementating this IContentProvider interface as a parameter to initialize my UserControl's ViewModel.
Here is my approach so far, which isn't working. I am wondering if I'm going down the right path? I declare a DependencyProperty in my user control which is visible to my mainWindow where I want to instantiate this UserControl. This code just attempts to pass the PluginsProvider object to my UserControl which needs it to build its ViewModel.
My PluginProvider DependencyProperty setter in my UserControl never gets hit because my My PropertyChanged handler is always null in MainWindow.xaml.cs I think I have the code right, but not sure I'm going down the right road and what I'm missing to make this connection?
ActionsTreeView.xaml.cs
public partial class ActionsTreeView: UserControl
{
public static readonly DependencyProperty PluginProviderProperty = DependencyProperty.Register("PluginProvider", typeof(Models.IPluginsProvider), typeof(ActionsTreeView), new FrameworkPropertyMetadata(null, OnPluginProviderChanged));
private ViewModels.ActionsTreeViewModel vm;
public ActionsTreeView()
{
//Wire-up our ViewModel with the data provider and bind it to DataContext for our user control
//This is a Mock-up until I figure out a way to get the real provider here
Models.IPluginProvider pluginSource = new Models.MockPluginProvider();
vm = new ViewModels.ActionsTreeViewModel(pluginSource );
this.DataContext = vm;
InitializeComponent();
}
private static void OnPluginProviderChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
{
((ActionsTreeView)source).PluginProvider = (Models.IPluginsProvider)e.NewValue;
}
public Models.IPluginsProvider PluginProvider
{
get
{
return (Models.IPluginsProvider)GetValue(PluginProviderProperty);
}
set
{
SetValue(PluginProviderProperty, value);
vm.SetPluginSource(PluginProvider);
}
}...
MainWindow.xaml.cs
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public MainWindow()
{
InitializeComponent();
this.ActionProvider = new Models.PluginsProvider(Library.Action.AvailableActions);
}
private Models.IPluginsProvider _actionProvider;
public Models.IPluginsProvider ActionProvider
{
get { return _actionProvider; }
set
{
_actionProvider = value;
OnPropertyChanged("ActionProvider");
}
}
protected void OnPropertyChanged(string property)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) //HANDLER IS ALWAYS NULL
{
handler(this, new PropertyChangedEventArgs(property));
}
}
}
Using my UserControl in MainWindow.xaml
<Grid>
<UserControls:ActionsTreeView PluginProvider="{Binding ActionProvider}" />
</Grid>

Answer 1

I don't think you can pass a parameter in the ctor in xaml.
If you create control in code behind you can pass the parameter in the ctor(Param param)
Not sure if this fits in the MVVM model but I use it a lot in regular code behind
Use a frame in the XAML for a place to put the UserControl

Answer 2

Seems like you are missing the binding source
<Grid>
<UserControls:ActionsTreeView PluginProvider="{Binding ActionProvider, RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}" />
</Grid>
since your property ActionProvider is declared in MainWindow so during binding you are required to refer the same source unless you've set it as data context of the window
alternative to above you can also do the below if there is no other data context used in the MainWindow then you can use the original binding you have PluginProvider="{Binding ActionProvider}"
public MainWindow()
{
InitializeComponent();
this.ActionProvider = new Models.PluginsProvider(Library.Action.AvailableActions);
DataContext = this;
}
I've set the DataContext to this which will effectively resolve the value of ActionProvider in binding from the instance this
Extra
you may also choose to remove INotifyPropertyChanged from MainWindow as it is already DependencyObject and capable of property notification and declare a DependencyProperty for ActionProvider
eg
public Models.IPluginsProvider ActionProvider
{
get { return (Models.IPluginsProvider)GetValue(ActionProviderProperty); }
set { SetValue(ActionProviderProperty, value); }
}
// Using a DependencyProperty as the backing store for ActionProvider. This enables animation, styling, binding, etc...
public static readonly DependencyProperty ActionProviderProperty =
DependencyProperty.Register("ActionProvider", typeof(Models.IPluginsProvider), typeof(MainWindow), new PropertyMetadata(null));
so you don't need to worry for the notification change manually, you might be required to use this if the above solution does not work for you otherwise it is good to have.