CTRun opaque type represents a run delegate, which is assigned to a run (attribute range) to control typographic traits such glyph ascent, glyph descent, and glyph width.
- Core Text
The callbacks defined for
CTRun objects are provided by the owner of a run delegate and are used to modify glyph metrics during layout. The values returned by the delegate are applied to each glyph in the run or runs corresponding to the attribute with that delegate.