Trying to reset the cursor position in a WPF Rich Text Box control - wpf

Question

I have a WPF Rich Text Box control where I am replacing the text with new text of the same length and I simply want to get the original caret position before I replace the text, replace the text and then reset the caret position.
I think that the problem is that it uses a TextPointer that is tied to the RichTextBox and is reset when I clear the original text so that when reapply it the position has changed. What I actually want to do is to reset it to either the character position index in the original text or I am happy to deal with Lined and Columns.
I have scoured the internet for an answer to what should be a very simple problem and nothing seems to answer it. What I really want is an X/Y coordinate or anything that will help.

Answer 1

You can use GetOffsetToPosition() and GetPositionAtOffset() to save the relative position of the Caret and to restore it.
In other words, assuming that the RichTextBox initializes like this:
RichTextBox rtb;
int paragraphIndex = -1;
int indexInParagraph;
public MainWindow()
{
InitializeComponent();
rtb = new RichTextBox();
rtb.Document = new FlowDocument();
Paragraph para = new Paragraph(new Run("some text some text some text."));
rtb.Document.Blocks.Add(para);
// sets the caret at a specific (random) position in the paragraph:
rtb.CaretPosition = para.ContentStart.GetPositionAtOffset(5);
this.Content = rtb;
}
Note the three private fields in the class.
You should save the caret's paragraph index and caret index in the paragraph, before you replace the text:
public void SaveCaretState()
{
//enumerate and get the paragraph index
paragraphIndex = -1;
foreach (var p in rtb.Document.Blocks)
{
paragraphIndex++;
if (p == rtb.CaretPosition.Paragraph)
break;
}
//get index relative to the start of the paragraph:
indexInParagraph = rtb.CaretPosition.Paragraph.ElementStart.GetOffsetToPosition(rtb.CaretPosition);
}
and restore it whenever you liked:
public void RestoreCaretState(MouseEventArgs e)
{
// you might need to insure some conditions here (paragraph should exist and ...)
Paragraph para = rtb.Document.Blocks.ElementAt(paragraphIndex) as Paragraph;
rtb.CaretPosition = para.ElementStart.GetPositionAtOffset(indexInParagraph);
}
Please note that its a simple example and there might be other Blocks in RichTextBox.Document. However, the idea and implementation is not that much different.