Keeping the FlowDocument cursor centered vertically in the RichTextBox

Question

I am having a bit of trouble figuring this one out, hopefully someone can help.

I have a WPF project with a RichTextBox.

As I edit the text, I would like the cursor in document to always stay centered vertically.

For example as a push up or down while editing, rather than the cursor going up, I would like the text to come down. This should result in the impression of the cursor staying still.

Thanks so much.

Answer 1

Not sure if this is what you had in mind but here's a proof of concept for a RichTextBox that keeps the caret centered where the user places it (clicks in the box).

Though as Omkar said you'll need to add whitespace if the document has been scrolled to the start or the end you need to add white in order to allow the text to scroll.

<RichTextBox HorizontalAlignment="Left" Height="311" VerticalAlignment="Top" Width="509" PreviewKeyDown="HandleKeyDownEvent">
            <FlowDocument>
                <Paragraph Margin="0">
                    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla turpis sem, tincidunt id vestibulum venenatis, fermentum eget orci. Donec mollis neque ac leo tincidunt tempus. Pellentesque mollis, nunc sit amet fermentum rutrum, lectus augue ultrices nibh, at lacinia est est ut justo. Cras non quam eu enim vulputate porttitor eu sit amet lectus. Suspendisse potenti. Maecenas metus nunc, dapibus id dapibus rhoncus, semper quis leo. Pellentesque eget risus magna, dignissim aliquam diam. Morbi.
                </Paragraph>
            </FlowDocument>
        </RichTextBox>

In the code behind:

private void HandleKeyDownEvent(object sender, KeyEventArgs e)
        {
            RichTextBox rtb = sender as RichTextBox;
            if (rtb != null)
            {
                //text to scroll up relative to caret
                if (e.Key == Key.Down)
                {
                    Block paragraph;

                    //get the whitespace paragraph at end of documnent
                    paragraph = 
                            rtb.Document.Blocks
                                .Where(x => x.Name == "lastParagraph")
                                .FirstOrDefault();

                    // if there is no white space paragraph create it
                    if (paragraph == null)
                    {
                        paragraph = new Paragraph { Name = "lastParagraph", Margin = new Thickness(0) };

                        //add to the end of the document
                        rtb.Document.Blocks.InsertAfter(rtb.Document.Blocks.LastBlock, paragraph);
                    }

                    // if viewport larger than document, add whitespace content to fill view port
                    if (rtb.ExtentHeight < rtb.ViewportHeight)
                    {
                        Thickness margin = new Thickness() { Top = rtb.ViewportHeight - rtb.ExtentHeight };
                                margin.Bottom = rtb.ViewportHeight - rtb.ExtentHeight;
                                paragraph.Margin = margin;

                    }

                    // if the document has been scrolled to the end or doesn't fill the view port
                    if (rtb.VerticalOffset + rtb.ViewportHeight == rtb.ExtentHeight)
                    {
                        // and a line to the white paragraph
                        paragraph.ContentEnd.InsertLineBreak();   
                    }

                    //move the text up relative to caret
                    rtb.LineDown();
                }
                // text is to scroll download relative to caret
                if (e.Key == Key.Up)
                {
                    // get whitespace at start of document
                    Block paragraph;
                    paragraph =
                            rtb.Document.Blocks
                                .Where(x => x.Name == "firstParagraph")
                                .FirstOrDefault();

                    //if whitespace paragraph is null append a new one
                    if (paragraph == null)
                    {
                        paragraph = new Paragraph { Name = "firstParagraph", Margin = new Thickness(0) };
                        rtb.Document.Blocks.InsertBefore(rtb.Document.Blocks.FirstBlock, paragraph);
                    }

                    // up document is at top add white space 
                    if (rtb.VerticalOffset == 0.0)
                    {
                        paragraph.ContentStart.InsertLineBreak();
                    }

                    //move text one line down relative to caret
                    rtb.LineUp();
                }
            }
        }

EDIT: This approach seems to work. The line height is determined by using the difference between the top of one line to the next, avoiding the problem of having line break effecing the offset.

    <RichTextBox
        PreviewKeyDown="PreviewKeyDownHandler">
        <FlowDocument>
             <!-- Place content here -->
        </FlowDocument>
   </RichTextBox>

In the code behind:

    private void PreviewKeyDownHandler(object sender, KeyEventArgs e)
    {            
        RichTextBox rtb = sender as RichTextBox;
        if (rtb != null)
        {
            if (e.Key == Key.Down)
            {
                // if there is another line below current
                if (rtb.CaretPosition.GetLineStartPosition(0) != rtb.CaretPosition.GetLineStartPosition(1))
                {
                    // find the FlowDocumentView through reflection
                    FrameworkElement flowDocumentView = GetFlowDocument(rtb);

                    // get the content bounds of the current line 
                    Rect currentLineBounds = rtb.CaretPosition.GetCharacterRect(LogicalDirection.Forward);

                    // move the caret down to next line
                    EditingCommands.MoveDownByLine.Execute(null, rtb);

                    // get the content bounds of the new line
                    Rect nextLineBounds = rtb.CaretPosition.GetCharacterRect(LogicalDirection.Forward);

                    // get the offset the document
                    double currentDocumentOffset = flowDocumentView.Margin.Top;

                    // add the height of the previous line to the offset 
                    // the character rect of a line doesn't include the baseline offset so the actual height of line has to be determined
                    // from the difference in the offset between the tops of the character rects of the consecutive lines
                    flowDocumentView.Margin = new Thickness { Top = currentDocumentOffset + currentLineBounds.Top - nextLineBounds.Top };
                }

                // prevent default behavior
                e.Handled = true;
            }
            if (e.Key == Key.Up)
            {
                if (rtb.CaretPosition.GetLineStartPosition(0) != rtb.CaretPosition.GetLineStartPosition(-1))
                {
                    FrameworkElement flowDocumentView = GetFlowDocument(rtb);

                    Rect currentLineBounds = rtb.CaretPosition.GetCharacterRect(LogicalDirection.Forward);

                    EditingCommands.MoveUpByLine.Execute(null, rtb);

                    Rect nextLineBounds = rtb.CaretPosition.GetCharacterRect(LogicalDirection.Forward);

                    double currentDocumentOffset = flowDocumentView.Margin.Top;

                    flowDocumentView.Margin = new Thickness { Top = currentDocumentOffset + currentLineBounds.Top - nextLineBounds.Top };
                }

                e.Handled = true;
            }
        }
    }

    protected FrameworkElement GetFlowDocument(RichTextBox textBox)
    {
        FrameworkElement flowDocumentVisual =
          GetChildByTypeName(textBox, "FlowDocumentView") as FrameworkElement;

        return flowDocumentVisual;
    }

    protected DependencyObject GetChildByTypeName(DependencyObject dependencyObject, string name)
    {
        if (dependencyObject.GetType().Name == name)
        {
            return dependencyObject;
        }
        else
        {
            if (VisualTreeHelper.GetChildrenCount(dependencyObject) > 0)
            {
                int childCount = VisualTreeHelper.GetChildrenCount(dependencyObject);

                for (int idx = 0; idx < childCount; idx++)
                {
                    var dp = GetChildByTypeName(VisualTreeHelper.GetChild(dependencyObject, idx), name);
                    if (dp != null)
                        return dp;
                }

                return null;
            }
            else
            {
                return null;
            }
        }
    }