How do you pass a ViewModel to the constructor of a UserControl that is shown in a ContentPresenter? - wpf

Question

I have several UserControls that should display the same data. Each UserControl has a different layout of the data that is to be presented. The ContentPresenter can bind to any one of the UserControls by using a DataTemplate in my Resources and by binding the Content to a StyleViewModel. Each UserControl is associated with a ViewModel as defined in the DataType of the DataTemplate. The ViewModels associated with any given UserControl all inherit from the StyleViewModel. The UserControls should get their data from a SettingsViewModel. The UserControls appear in the main Window.
The problem is that I can't figure out how to make the data from the SettingsViewModel accessible to the UserControls.
Is it possible to pass a reference to a SettingsViewModel to the constructor of one of these UserControls that are displayed using a ContentPresenter?
Is there another way to easily switch between different views of the data (i.e. my UserControls) without using a ContentPresenter? If so, how would I make the data accessible to the UserControls?
The following is code from my SingleLineViewModel.cs:
public class SingleLineViewModel : StyleViewModel
{
public SingleLineViewModel() { }
}
The other ViewModels are similar. They are essentially empty classes that inherit from StyleViewModel, so that I can bind to a Style property which is of type StyleViewModel in my SettingsViewModel. The StyleViewModel is also an essentially empty class that inherits from ViewModelBase.
The following is code from my Resources.xaml:
<ResourceDictionary <!--other code here-->
xmlns:vm="clr-namespace:MyProject.ViewModel"
<!--other code here-->
<DataTemplate DataType="{x:Type vm:SingleLineViewModel}">
<vw:ucSingleLine/>
</DataTemplate>
<DataTemplate DataType="{x:Type vm:SeparateLinesViewModel}">
<vw:ucSeparateLines/>
</DataTemplate>
<!--other code here-->
</ResourceDictionary>
The following is code from SettingsViewModel.cs:
public class SettingsViewModel : ViewModelBase
{
// other code here
private StyleViewModel _style;
public StyleViewModel Style
{
get { return _style; }
set
{
if (value != _style && value != null)
{
_style = value;
OnPropertyChanged("Style");
}
}
}
// other code here
public SettingsViewModel()
{
_style = new SingleLineViewModel();
}
// other code here
}
The following is code from my MainView.xaml:
<ContentPresenter Name="MainContent" Content="{Binding SettingsVM.Style}"/>

Answer 1

You might find that you are trying to do much at once. Consider how you might test this scenario? Or how would you walk the data in a Debugger to check its state? Good practice recommends that your data is separate from your UI elements. An MVVM pattern such as you are trying to use normally provides the view models to help transition the data from it's simple data into forms that the UI can use.
With that in mind, I would recommend that you try do develop a ViewModel tier that presents all the data without the UI holding it together, i.e. instead of trying to inject the additional SettingsViewModel into your controls you should make your viewmodels hold everything they need.
It looks like you are off to a good start, your SettingsViewModel lets you get hold of a Style, but your style doesn't seem to have any data. So why not pass it in the constructor.
public SettingsViewModel()
{
_style = new SingleLineViewModel(WhatINeedForStyle);
}