NSString+ComposedCharacterLength.h

@interface NSString (ComposedCharacter)

/**
 * @description @b NSString use UTF-16 encode to store characters internally, in
 * most cast, the @c length of a @b NSString is the number visible characters.
 * But not for some special Characters like Emoji.
 * @"😃🤡⛹🏿‍♀️⚫️🤥🤕💪🏾" this @b NSString only has 7 Emojis, but its @c length is
 * 20. Sometimes we only want the human visible charater count.
 *
 * @code
 * @"😃🤡⛹🏿‍♀️⚫️🤥🤕💪🏾".length == 20
 * @"😃🤡⛹🏿‍♀️⚫️🤥🤕💪🏾".rt_composedCharacterLength == 7
 *
 * ++ ++ ++++++ ++ ++ ++ ++++
 * 😃 🤡 ⛹🏿‍♀️    ⚫️ 🤥 🤕 💪🏾
 * @endcode
 *
 *
 * @return number of visible charaters
 * @warning Emoji is System sensitive, and its behavior differ on different
 * systems, so dose this property.
 */
@property (nonatomic, assign, readonly) NSUInteger rt_composedCharacterLength;
@end

NSString+ComposedCharacterLength.m

@implementation NSString (ComposedCharacter)

- (NSUInteger)rt_composedCharacterLength __attribute((const))
{
    NSUInteger numberOfComposedCharaters = 0;

    NSUInteger length = self.length;
    while (length > 0) {
        NSRange range = [self rangeOfComposedCharacterSequenceAtIndex:length - 1];
        length -= range.length;
        ++ numberOfComposedCharaters;
    }
    return numberOfComposedCharaters;
}

@end