Apply skin tone modifiers to emojis per the Unicode spec.

Playground.swift

import Foundation

import UIKit
import CoreText

// You can run this by copying this file into a Swift playground.

extension String {
    // Returns the number of glyphs used to render the string. If this string
    // should behave like a single emoji character (regardless of whether it is,
    // internally, a sequence of characters), this property will return `1`.
    var visibleCount: Int {
        // https://crunchybagel.com/using-emoji-skin-tone-modifiers-in-swift/
        // suggests that we could enumerate composed character sequences
        // instead, but that implementation reports `1` even when the sequence
        // won't actually be rendered as a single character.
        let typesetString = CTLineCreateWithAttributedString(NSAttributedString(string: self))
        return CTLineGetGlyphCount(typesetString)
    }
    
    // If this string contains multiple modifier bases, this function will
    // apply the same modifier to all of them: `"👭".modifiedBy("🏻")` -> `"👭🏻"`.
    // The spec https://unicode.org/reports/tr51/#multiperson_skintones and
    // implementation (at least of macOS 10.15.4) allow for each base to take a
    // different modifier e.g. 👩🏻‍🤝‍👩🏿 but this function does not yet support that.
    // When we add support for that, we should enforce that modifying any base
    // requires modifying all bases (in some way or another), per the spec.
    //
    // This function doesn't attempt to assess RGI status; some modified
    // multi-person groupings, in particular, may not be rendered as of Emoji
    // 13.0 https://unicode.org/reports/tr51/#multiperson_skintones .
    // You can check `visibleCount` to guard against that, although the platform
    // may ignore unsupported modifiers anyway, e.g.
    // "\u{0001F46F}\u{0001F3FB}\u{200D}\u{2640}" and
    // "\u{0001F46F}\u{200D}\u{2640}" will both render as 👯‍♀ on macOS 10.15.4.
    func modifiedBy(_ modifier: Unicode.Scalar) -> String {
        assert(modifier.properties.isEmojiModifier, "Scalar must be emoji modifier.")
        
        return String(unicodeScalars.reduce(into: UnicodeScalarView(), { (modifiedScalars, scalar) in
            // Strip variation selectors following modifiers because a) they'll
            // break ZWJ sequences, at least on macOS 10.15.4 b) the spec says
            // that "the emoji modifier automatically implies the emoji
            // presentation" style https://unicode.org/reports/tr51/#Diversity
            // and so recommends against including emoji presentation
            // selectors. It's silent on whether text presentation selectors
            // should be included or no but insofar as that would break the
            // emoji modifier, seems like we should remove those too.
            //
            // Strip modifiers following modifiers because those are now
            // invalid.
            if scalar.properties.isVariationSelector || scalar.properties.isEmojiModifier,
                let previousScalar = modifiedScalars.last,
                previousScalar.properties.isEmojiModifier {
                return
            }
            
            // If the current scalar is an emoji presentation selector and an
            // emoji earlier in the sequence (but not immediately preceding)
            // has been modified, it may not be strictly necessary to preserve
            // this selector, depending on the preceding emoji's presentation
            // style. However, we preserve such selectors to support modifying
            // sequences generated by input devices, which the spec recommends
            // https://unicode.org/reports/tr51/#Emoji_Variation_Selector_Notes
            // only generate fully-qualified sequences.
            //
            // Also, if this scalar is a *text* presentation selector, it
            // *must* be preserved, as it will break a ZWJ sequence as
            // described in
            // https://unicode.org/reports/tr51/#Emoji_Variation_Selector_Notes.
            
            modifiedScalars.append(scalar)
            
            // Emoji modifiers immediately follow base characters per
            // https://unicode.org/reports/tr51/#Diversity , for single emojis
            // as well as in ZWJ sequences
            // https://unicode.org/reports/tr51/#multiperson_skintones .
            if scalar.properties.isEmojiModifierBase {
                modifiedScalars.append(modifier)
            }
        }))
    }
}

let modifier = Unicode.Scalar("\u{0001F3FB}")
let modifier2 = Unicode.Scalar("\u{0001F3FF}")

// Check that we're correctly applying the spec https://unicode.org/reports/tr51/#Diversity.
assert("🎉".modifiedBy(modifier) == "🎉", "Emojis that do not accept modifiers should not be modified.")
assert("🖐".modifiedBy(modifier) == "🖐🏻", "Emojis should be modified.")
assert("💆‍♂".modifiedBy(modifier) == "💆🏻‍♂", "Emoji sequences should be modified.")
assert("👭".modifiedBy(modifier) == "👭🏻", "Multi-person groupings should be modified.")
assert("🖐🏻".modifiedBy(modifier2) == "🖐🏿", "Previous modifiers should be overridden.")
assert("\u{0001F590}\u{0001F3FB}".modifiedBy(modifier2) == "\u{0001F590}\u{0001F3FF}", "Previous modifiers should be stripped.")

// We write the emojis below as escape sequences so that we can more easily see,
// and guarantee, when they do / don't contain variation selectors. Comments
// describe what they actually look like.

// ♂️ -> ♂️
assert("\u{2642}\u{FE0F}".modifiedBy(modifier) == "\u{2642}\u{FE0F}", "Variation selectors should be preserved when emojis are not modified.")
// ♂︎ -> ♂︎
assert("\u{2642}\u{FE0E}".modifiedBy(modifier) == "\u{2642}\u{FE0E}", "Variation selectors should be preserved when emojis are not modified.")
// 🖐️ -> 🖐🏻
assert("\u{0001F590}\u{FE0F}".modifiedBy(modifier) == "\u{0001F590}\u{0001F3FB}", "Variation selectors should be stripped when an emoji is modified.")
// 🖐︎ (should be text presentation, though macOS forces emoji it seems) -> 🖐🏻
assert("\u{0001F590}\u{FE0E}".modifiedBy(modifier) == "\u{0001F590}\u{0001F3FB}", "Variation selectors should be stripped when an emoji is modified.")
// 🕵️‍♂️ -> 🕵🏻‍♂️
// Note that this preserves the final selector i.e. outputs a fully-qualified
// sequence; see the note inside `modifiedBy(_:)`.
assert("\u{0001F575}\u{FE0F}\u{200D}\u{2642}\u{FE0F}".modifiedBy(modifier) == "\u{0001F575}\u{0001F3FB}\u{200D}\u{2642}\u{FE0F}", "Variation selectors should be stripped when an emoji sequence is modified.")


// Check that modifications never decompose into multiple emojis.
assert("\u{0001F590}".modifiedBy(modifier).visibleCount == 1);
assert("\u{0001F486}\u{200D}\u{2642}".modifiedBy(modifier).visibleCount == 1);
assert("\u{2642}\u{FE0F}".modifiedBy(modifier).visibleCount == 1);
assert("\u{2642}\u{FE0E}".modifiedBy(modifier).visibleCount == 1);
assert("\u{0001F590}\u{FE0F}".modifiedBy(modifier).visibleCount == 1);
assert("\u{0001F590}\u{FE0E}".modifiedBy(modifier).visibleCount == 1);
assert("\u{0001F575}\u{FE0F}\u{200D}\u{2642}\u{FE0F}".modifiedBy(modifier).visibleCount == 1)