String+Emoji.swift

import UIKit

extension String {

    var glyphCount: Int {

        let richText = NSAttributedString(string: self)
        let line = CTLineCreateWithAttributedString(richText)
        return CTLineGetGlyphCount(line)
    }

    var isSingleEmoji: Bool {

        return glyphCount == 1 && containsEmoji
    }

    var containsEmoji: Bool {

        return unicodeScalars.contains { $0.isEmoji }
    }

    var containsOnlyEmoji: Bool {

        return !isEmpty
            && !unicodeScalars.contains(where: {
                !$0.isEmoji
                    && !$0.isZeroWidthJoiner
            })
    }

    // The next tricks are mostly to demonstrate how tricky it can be to determine emoji's
    // If anyone has suggestions how to improve this, please let me know
    var emojiString: String {

        return emojiScalars.map { String($0) }.reduce("", +)
    }

    var emojis: [String] {

        var scalars: [[UnicodeScalar]] = []
        var currentScalarSet: [UnicodeScalar] = []
        var previousScalar: UnicodeScalar?

        for scalar in emojiScalars {

            if let prev = previousScalar, !prev.isZeroWidthJoiner && !scalar.isZeroWidthJoiner {

                scalars.append(currentScalarSet)
                currentScalarSet = []
            }
            currentScalarSet.append(scalar)

            previousScalar = scalar
        }

        scalars.append(currentScalarSet)

        return scalars.map { $0.map{ String($0) } .reduce("", +) }
    }

    fileprivate var emojiScalars: [UnicodeScalar] {

        var chars: [UnicodeScalar] = []
        var previous: UnicodeScalar?
        for cur in unicodeScalars {

            if let previous = previous, previous.isZeroWidthJoiner && cur.isEmoji {
                chars.append(previous)
                chars.append(cur)

            } else if cur.isEmoji {
                chars.append(cur)
            }

            previous = cur
        }

        return chars
    }
}

extension UnicodeScalar {

    var isEmoji: Bool {

        switch value {
        case 0x1F600...0x1F64F, // Emoticons
            0x1F300...0x1F5FF, // Misc Symbols and Pictographs
            0x1F680...0x1F6FF, // Transport and Map
            0x1F1E6...0x1F1FF, // Regional country flags
            0x2600...0x26FF,   // Misc symbols
            0x2700...0x27BF,   // Dingbats
            0xFE00...0xFE0F,   // Variation Selectors
            0x1F900...0x1F9FF,  // Supplemental Symbols and Pictographs
            127000...127600, // Various asian characters
            // 65024...65039, // Variation selector, duplicate to 0xFE00...0xFE0F
            9100...9300, // Misc items
            8400...8447: // Combining Diacritical Marks for Symbols
            return true

        default: return false
        }
    }

    var isZeroWidthJoiner: Bool {

        return value == 8205
    }
}

Some emojis consist of two unicodeScalar value instead of one, e.g. ☺️ is the combination of 9786 & 65039.
Therefore the number of emojis within a string is not always equals to the number of unicodeScalars it contains.

If you want to know how many emojis a string contains, try look into Character, it always sees an emoji as one character.

extension String {
    var countWithEmoji: Int {
        var count = 0
        for substring in self {
            if substring.isEmoji {
                count += 2
            } else {
                count += 1
            }
        }
        return count
    }
}

extension Character {
    var isEmoji: Bool {
        for scalar in unicodeScalars {
            if scalar.isEmoji {
                return true
            }
        }
        return false
    }
}

extension UnicodeScalar {
    var isEmoji: Bool {
        switch value {
        case 0x1F600...0x1F64F, // Emoticons
        0x1F300...0x1F5FF, // Misc Symbols and Pictographs
        0x1F680...0x1F6FF, // Transport and Map
        0x1F1E6...0x1F1FF, // Regional country flags
        0x2600...0x26FF,   // Misc symbols
        0x2700...0x27BF,   // Dingbats
        0xFE00...0xFE0F,   // Variation Selectors
        0x1F900...0x1F9FF,  // Supplemental Symbols and Pictographs
        127000...127600, // Various asian characters
        65024...65039, // Variation selector
        9100...9300, // Misc items
        8400...8447: // Combining Diacritical Marks for Symbols
            return true
        default:
            return false
        }
    }
}