Question

I need to get the values from my ComboBoxes that reside in an ObservableCollection being iterated through by an ItemsControl. And I need to store them in a separate data structure, at least I think I do. Sounds pretty straightforward? I hope it is, I’ve been stuck on it all day.

Here’s my XAML:

<Window.Resources>
    <Style x:Key="IVCell" TargetType="{x:Type ItemsControl}">
        <Setter Property="ItemTemplate">
            <Setter.Value>
                <DataTemplate>
                    <StackPanel>
                        <TextBlock Text="{Binding Variable.Name}"/>
                        <ComboBox x:Name="IVValues" ItemsSource="{Binding Values}"
                                  SelectedIndex="0"/>
                    </StackPanel>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>

...
        <ItemsControl Name="IndependentVariables" Style="{StaticResource IVCell}" ItemsSource="{Binding IVCollection}"/>

where IVCollection is defined here, in my ViewModel:

public class MainViewModel : ViewModelBase
{
    public ObservableCollection<ExternalClass> IVCollection { get; set; }

    ...
}

Before you ask what ExternalClass is, know that that is what contains Variable and Values.

Here’s the problem. I need to get the SelectedIndex of ComboBox (so I would be changing the current code SelectedIndex="0"). And if my ExternalClass were what needed to contain those SelectedIndex values, that would be easy, since I already have access to its fields (Variable and Value). But I need those int values in

public class MainViewModel : ViewModelBase
{
    public ObservableCollection<int> SelectedIndices { get; set; }

    ...
}

or something like that. I had made a quick wrapper class to contain both SelectedIndices and IVCollection, but obviously that won’t work, because ItemsControl wants an ObservableCollection, not a class with two ObservableCollections.

And I guess the real meat of the question here (I could be wrong, that’s why I’m asking) is how do I access a property that’s outside of the scope of the Style/DataTemplate I’m currently in (e.g. IVCell)? I can do it in the code-behind with no problem, like so,

    private void IVValues_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        for (var i = 0; i < IndependentVariables.Items.Count; i++)
        {
            var uiElement = IndependentVariables.ItemContainerGenerator.ContainerFromIndex(i);
            var cBox = FindVisualChild<ComboBox>(uiElement); //Homebrew method
            //cBox.SelectedIndex
        }
    }

but once again, I’m trying to stay MVVM friendly. I’ve even tried using event commands with MVVM Light, etc., but that merely postpones the problem. I need to solve it. Or circumvent it.

Answer 1

@sleiman and @Brandon
Neither of your answers were 100%, but they were very helpful nonetheless. I reached my solution using elements from both of your answers, so I hesitate to accept one over the other.

I used a combination of the wrapper class solution and the exposed property solution (along with a semi-unrelated but nonetheless necessary event-handling technique) to create the following:

public class IVWrapper : INotifyPropertyChanged
{
    public VariableValueList<double> IVCollection { get; set; }
    private int selectedIndex;
    public int SelectedIndex
    {
        get { return selectedIndex; }
        set
        {
            selectedIndex = value;
            OnPropertyChanged("SelectedIndex");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        var handler = PropertyChanged;

        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

and

public class MainViewModel : ViewModelBase
{

    public ObservableCollection<IVWrapper> IVWrapperCollection { get; set; }

    ...

    private void InitializeIVs()
    {
        IVWrapperCollection.Clear();
        foreach (var iv in ...)
        {
            var toBeAdded = new IVWrapper {IVCollection = iv};
            toBeAdded.PropertyChanged += (sender, args) => Foo();
            IVWrapperCollection.Add(toBeAdded);
        }
    }

    public void Foo()
    {
        //Doin stuffs
    }
}

This additional integration with my ViewModel allows me to call a function whenever my selectionIndex changes. Neat! And clean!

The corresponding changes in the XAML MainView:

                    <StackPanel>
                        <TextBlock Text="{Binding IVCollection.Variable.Name}"/>
                        <ComboBox ItemsSource="{Binding IVCollection.Values}"
                                  SelectedIndex="{Binding SelectedIndex}"/>
                    </StackPanel>

...

        <ItemsControl Name="IndependentVariables" Style="{StaticResource IVCell}" ItemsSource="{Binding IVWrapperCollection}"/>