无法以编程方式获取和设置 RichTextBox.CaretPosition

Question

I have tried several attempts for this, but nothing has worked so far.

Here is what I have for a normal TextBox (this works like a charm, but as I need to do some formatting I need it to work for the RichTextBox, too):

//this works:
//Save the actual CaretIndex for later
safeActualCaretPositionInTextBox = txtFormulaString.CaretIndex;
//Insert the (user-)chosen text by another control
txtFormulaString.Text = txtFormulaString.Text.Insert(txtFormulaString.CaretIndex, function.PlainFunction);
//set focus back to TextBox
txtFormulaString.Focus();
//set CaretIndex at needed position (as I insert some formula (i. e. COS()) I need the CaretIndex n position(s) to the left)
txtFormulaString.CaretIndex = safeActualCaretPositionInTextBox + function.PlainFunction.Length;
txtFormulaString.CaretIndex -= function.CaretOffset;
txtFormulaString.SelectionLength = 0;

I have tried this ( RichTextBox.CaretPosition Property ) and currently have this:

safeActualCaretPositionInRichTextBox = rtbFormulaString.CaretPosition.GetInsertionPosition(LogicalDirection.Forward);
rtbFormulaString.CaretPosition.GetInsertionPosition(LogicalDirection.Forward).InsertTextInRun(function.PlainFunction);
rtbFormulaString.CaretPosition.GetPositionAtOffset(-function.CaretOffset);
rtbFormulaString.Focus();
rtbFormulaString.CaretPosition = safeActualCaretPositionInRichTextBox;

The insertion seems to work but I can't set the caret to the right position. It always starts at the beginning of the text. What do I do wrong or what am I missing?

EDIT: As I use MVVM in my project, I have a property in my ViewModel which needs to be updated after every text change (RichTextBox_TextChanged event is used):

vm.FormulaString = new TextRange(rtbFormulaString.Document.ContentStart, rtbFormulaString.Document.ContentEnd).Text;

When I delete this line, it works sometimes (I don't know why or how and I don't understand why it behaves like that).

Answer 1

To make the caret move to the end of the text you just added programatically you can use the following code :

TextRange newText = new TextRange(yourRichTextBox.CaretPosition, yourRickTextBox.CaretPosition);
newText.Text = yourString;
yourRichTextBox.CaretPosition = newText.End;

It's pretty clear to understand : pickup the current position of your caret, write some text at it and then move the caret to the end of it

EDIT :

To save the position before inserting and to get the offset added by your new text you can use a method like this :

public static uint GetDistanceUntil(this TextPointer start, TextPointer end)
{
    uint ret = 0;
    TextPointer currentPosition = start;
    while (currentPosition.CompareTo(end) != 0)
    {
        // if currentPosition becomes null, you passed through the whole document that contains start and didn't find end 
        // (most likely because start and end aren't in the same document or because start was after end)
        currentPosition = currentPosition.GetPositionAtOffset(1, LogicalDirection.Forward); 
        ret++;
    }
    return ret;
}

To obtain the current position of your carret as a number : yourRichTextBox.ContentStart.GetDistanceUntil(yourRichTextBox.CaretPosition);

And to get the "length" of your text : newText.Start.GetDistanceUntil(newText.End);

Once you're done adding the text you can obtain the TextPointer where your caret should be by using the method that does the "conversion" the other way :

public static TextPointer MoveTo(this TextPointer start, uint distance)
{
    TextPointer ret = start;
    while (distance > 0)
    {
        // if ret becomes null, you passed through the whole document that contains start and didn't move enough yet according to given distance
        ret = ret.GetPositionAtOffset(1, LogicalDirection.Forward); 
        distance--;
    }
    return ret;
}

And set the CaretPosition of your RichTextBox to yourRichTextBox.ContentStart.MoveTo(savedPosition + textLength);

I'm pretty sure you can adapt both methods if you don't always want to use LogicalDirection.Forward