Insert text at caret from button in RichTextBox mvvm using wpf toolkit - mvvm

Question

I have selected text from a ListBox that I want to insert into a RichTextBox at the caret position. I can get the selected text to be inserted at the end of the text string.
I am not sure how to pass the RichTextBox caret position to my view model.
Here is part of my code for the project.
<Button x:Name="AddItemBtn" Content="Add Item" HorizontalAlignment="Left" Margin="417,10,0,0" VerticalAlignment="Top" Width="100" Command="{Binding AddItemBtn}" CommandParameter="{Binding ElementName=AddItemList,Path=SelectedItem}"/>
<wpftoolkit:RichTextBox Grid.Column="0" Text="{Binding TestText, UpdateSourceTrigger=PropertyChanged}" x:Name="MyEditor" ScrollViewer.VerticalScrollBarVisibility="Auto" Margin="0" Height="Auto" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" IsDocumentEnabled="True" AcceptsTab="True" AcceptsReturn="True" >
<wpftoolkit:RichTextBox.Resources>
<Style TargetType="{x:Type Paragraph}">
<Setter Property="Margin" Value="0" ></Setter>
<Setter Property="FontSize" Value="15"></Setter>
</Style>
</wpftoolkit:RichTextBox.Resources>
<wpftoolkit:RichTextBox.TextFormatter>
<wpftoolkit:PlainTextFormatter/>
</wpftoolkit:RichTextBox.TextFormatter>
</wpftoolkit:RichTextBox>
Here is the view model part.
private string _testText;
public string TestText
{
get
{
return _testText;
}
set
{
//_testText = _testText + value;
SetProperty(ref _testText, value);
}
}
public ICommand AddItemBtn
{
get;
set;
}
public void addItem(Tabbed selectedItem)
{
if (selectedItem != null)
{
MessageBox.Show(selectedItem.Command);
if (TestText != null)
{
TestText = TestText.ToString() + selectedItem.Command;
}
else
{
TestText = selectedItem.Command;
}
}
}
I tried a flowdocument but still could not get the parameters to pass correctly.

Answer 1

I like to put a function on the view model that is setup in the view code behind.
public class MainViewModel : ViewModelBase
{
public Func<int> GetCarrotPosition { get; set; }
//...
Looks like you can get the number of characters into the text string by getting the offset from the document start start to the current position
public MainWindow()
{
// InitializeComponent stuff..
var castedContext = (MainViewModel)DataContext;
castedContext.GetCarrotPosition = () =>
{
// Placing the cursor at the start of the text returns a value of 2, so I subtract 2 to get the current cursor location
return MyRichTextBox.CaretPosition.DocumentStart.GetOffsetToPosition(MyRichTextBox.CaretPosition) - 2;
};
//...
Finally, call the GetCarrotPosition() function in your command
var carrotPosition = GetCarrotPosition();
TestText.Insert(carrotPosition, selectedItem.Command);
Creating delegates on the view model that get wired up in the view code behind is the sexiest MVVM way of working with UI elements I know of.