This is something I've made in Swift language, inspired by this question: https://stackoverflow.com/questions/21443625/core-text-calculate-letter-frame-in-ios about how to calculate the exact bounding box for every character in iOS Core Text.
The CoreTextSwift class wrapper was made by @krzyzanowskim at https://github.com/krzyzanowskim/CoreTextSwift
What I got:
My wish (Furigana in green)
