TL;DR

public static Size CalculateTextSize(string myText, string myFontResourceKey, int myFontSize)
{
  if (Application.Current?.TryFindResource(myFontResourceKey, out object? fontObj) != true)
    throw new ArgumentException($"Cannot find a resource named {myFontResourceKey} in this application's resources.");

  if (!(fontObj is FontFamily myFont))
    throw new ArgumentException($"The resource {myFontResourceKey} is not a FontFamily.");

  var ts = TextShaper.Current;
  var typeface = new Typeface(myFont);
  ShapedBuffer shaped = ts.ShapeText(myText, new TextShaperOptions(typeface.GlyphTypeface, myFontSize));
  var run = new ShapedTextRun(shaped, new GenericTextRunProperties(typeface, myFontSize));
  return run.Size;
}

###########

The API reference at https://reference.avaloniaui.net/api/ seems to still target version 11.0 at the time i’m writing this. The Avalonia.Media.TextFormatting namespace contains a bunch of classes related to the measuring text, but they are slightly different than version 11.1.

After some trial-and-error, i finally got it working:

1. Get your font resource. In my case, it was defined in a ResourceDictionary xaml file at the application level, and the only function which worked to retrieve it was StyledElement.FindResource. It can be accessed from anywhere using Application.Current.FindResource("MyFontResourceKey", out object? fontObj);
2. Cast it to FontFamily
3. Create a TypeFace from your FontFamily
4. Get a TextShaper instance
5. Get a ShapedBuffer (named GlyphRun in avalonia 11.0, it seems) from the TextShaper
6. Transform it into a ShapedTextRun
7. Get the size of your rendered text from the ShapedTextRun.Size property.

Don’t hesitate to answer if i missed something simpler !