Getting a glyph boundingRect in draw#rect in UILabel - ios

Question

Using Swift, I want to get the boundingRect of a glyph, in draw#rect in a UILabel.
The UILabel already has a size (say 300x300 in the example) and qualities such as the text being centered.
class RNDLabel: UILabel {
override func draw(_ rect: CGRect) {
let manager = NSLayoutManager()
let store = NSTextStorage(attributedString: NSAttributedString(
string: text!,
attributes: [NSAttributedString.Key.font: font]))
store.addLayoutManager(manager)
let textContainer = NSTextContainer(size: rect.size)
// note, intrinsicContentSize is identical there, no difference
manager.addTextContainer(textContainer)
let glyphRange = manager.glyphRange(
forCharacterRange: NSRange(location: 0, length: 1),
actualCharacterRange: nil)
let glyphRect = manager.boundingRect(
forGlyphRange: glyphRange, in: textContainer)
print("glyphRect \(glyphRect)")
...context?.addRect(glyphRect), context?.drawPath(using: .stroke)
super.draw(rect)
}
The green square is not correct - it should be more like these red squares!
There seems to be a number of problems:
surely the layoutManager I make, should get the qualities of the UILabel, eg "centered text"? (I believe you can't actually directly access the layoutManager of a UILabel, though?)
should we be using something like CTLineGetOffsetForStringIndex? Is that even possible in draw#rect
notice as well as not having the correct offset, the green box seems the wrong height anyway. (It looks more like a plain old intrinsicContentSize rather than a glyph bounding box.)
How to?
For the record, my overall aim is to move the glyph around, based on the actual glyph box (which of course is different for "y", "X", etc). But in general there are many useful reasons to know the box of a glyph.

Answer 1

Your original question is sort of an unquestion, because it says two completely opposite things.
On the one hand, you say UILabel.
On the other hand, you use terms like NSLayoutManager and NSTextStorage and NSTextContainer — the TextKit stack.
Those are opposites because UILabel is not drawn with the TextKit stack. So what you're asking to do is like trying to lift a piece of paper with a magnet; magnets do lift things, but what they lift are magnetic materials, and paper is not one of those. To be sure, UILabel must draw in some deterministic way, but what that way is, is completely unknown and opaque and has nothing to do with TextKit.
On the other hand, UITextView, or a view that your draw yourself using TextKit, does use TextKit, and now all the TextKit tools apply. So what you're asking to do would be a lot more straightforward if this were a UITextView (or a view that you draw yourself using TextKit).
This need not change anything very much about the appearance of the interface. A UITextView can be made to act a lot like a UILabel (make it noneditable and nonscrollable) but with the important difference that the whole text kit stack is directly exposed. So that's where I'd start. I'm one of those people who prefers to use the framework rather than to fight it.

Answer 2

It turns out the answer to this question seems to be:
In fact, surprisingly or not, you basically have no access to glyph information in UILabel specifically.
So realistically you can't get those glyph "actual shapes" in UILabel.
In particular RobN. has pointed out that an investigation showed that in UILabel, _drawTextInRect:baselineCalculationOnly does the work and that is a big pile of ad hoc code.
A summary of the situation would seem to be that UILabel simply predates NSLayoutManager / Core Text and (as yet) just plain does not use those modern systems.
Only those modern systems give you this ...
... sort of access to glyph-by-glyph shapes.