WinUI3 get TextBlock character bounding box.

TextBlockExtensions.cs

public static class TextBlockExtensions
{
    // https://github.com/microsoft/microsoft-ui-xaml/blob/main/src/dxaml/xcp/core/text/TextBlock/TextBlockView.cpp#L606
    public static TextPointer GetAdjustedPosition(this TextBlock textBlock, int charIndex)
    {
        const int PlaceHolderPositionsForInlines = 2;

        var contentLength = textBlock.Text.Length + 2 * PlaceHolderPositionsForInlines;

        int charCount = charIndex;
        int adjustedPosition = 0;

        // Correct out of bound indexes to valid ones
        if (charIndex < 0)
        {
            adjustedPosition = PlaceHolderPositionsForInlines;
        }
        else if (charIndex >= (contentLength - (int)PlaceHolderPositionsForInlines))
        {
            adjustedPosition = contentLength - (int)PlaceHolderPositionsForInlines;
        }
        else
        {
            adjustedPosition = charIndex + PlaceHolderPositionsForInlines;
            System.Diagnostics.Debug.Assert(adjustedPosition <= (int)contentLength - (int)PlaceHolderPositionsForInlines);
        }

        return textBlock.ContentStart.GetPositionAtOffset(adjustedPosition, LogicalDirection.Forward);
    }

    public static TextBlockCharacterBoundingBox GetElementBoundingBox(this TextBlock textBlock, int charIndex)
    {
        var text = textBlock.Text;

        ArgumentOutOfRangeException.ThrowIfLessThan(charIndex, 0);
        ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(charIndex, text.Length);

        var pointer = GetAdjustedPosition(textBlock, charIndex);
        var nextPointer = pointer.GetPositionAtOffset(1, LogicalDirection.Forward);
        var rect1 = pointer.GetCharacterRect(LogicalDirection.Forward);
        var rect2 = nextPointer.GetCharacterRect(LogicalDirection.Backward);
        var ch = text[charIndex];

        var p1x = Math.Min(rect1.X, rect2.X);
        var p1y = Math.Min(rect1.Y, rect2.Y);
        var p2x = Math.Max(rect1.X, rect2.X);
        var p2y = Math.Max(rect1.Y + rect1.Height, rect2.Y + rect2.Height);

        var rect = new Rect(new Point(p1x, p1y), new Point(p2x, p2y));

        return new TextBlockCharacterBoundingBox(charIndex, ch, rect);
    }

    public record struct TextBlockCharacterBoundingBox(int CharacterIndex, char Character, Rect BoundingBox);
}