Kerning issue when rendering string using the CoreText - ios

Question

I am trying to render string using the CoreText api. Each character is rendered in the CAShapeLayer and i am getting the layer offset (x coordinate) for each character like this:
let offset = CTLineGetOffsetForStringIndex(line, glyphIndex, nil)
But the resulting offset doesn't seem to respect kerning between letters. Here is a an image of what i mean - the top label is a UILabel, the bottom one is my custom rendering:
As you can see the "A" letter in my custom label is placed further from the "W" letter.
I would really appreciate any help.
Thanks in advance.

Answer 1

In case someone has the same problem, i used an approach from this place: https://github.com/MichMich/XMCircleType/blob/master/XMCircleType/Views/XMCircleTypeView.m, the kerningForCharacter function. Here is the function itself:
- (float)kerningForCharacter:(NSString *)currentCharacter afterCharacter:(NSString *)previousCharacter
{
//Create a unique cache key
NSString *kerningCacheKey = [NSString stringWithFormat:#"%#%#", previousCharacter, currentCharacter];
//Look for kerning in the cache dictionary
NSNumber *cachedKerning = [self.kerningCacheDictionary objectForKey:kerningCacheKey];
//If kerning is found: return.
if (cachedKerning) {
return [cachedKerning floatValue];
}
//Otherwise, calculate.
float totalSize = [[NSString stringWithFormat:#"%#%#", previousCharacter, currentCharacter] sizeWithAttributes:self.textAttributes].width;
float currentCharacterSize = [currentCharacter sizeWithAttributes:self.textAttributes].width;
float previousCharacterSize = [previousCharacter sizeWithAttributes:self.textAttributes].width;
float kerning = (currentCharacterSize + previousCharacterSize) - totalSize;
//Store kerning in cache.
[self.kerningCacheDictionary setValue:#(kerning) forKey:kerningCacheKey];
//Return kerning.
return kerning;
}