Code for Stack Overflow question https://stackoverflow.com/q/46408487/116862

character-boundary.swift

let str = "👨🏾‍🚒"

print(str.unicodeScalars.map { "0x\(String($0.value, radix: 16))" })
// → ["0x1f468", "0x1f3fe", "0x200d", "0x1f692"]
print(str.utf16.map { "0x\(String($0, radix: 16))" })
// → ["0xd83d", "0xdc68", "0xd83c", "0xdffe", "0x200d", "0xd83d", "0xde92"]
print(str.utf16.count)
// → 7

let utf16Offset = 2
let utf16Index = String.Index(encodedOffset: utf16Offset)

let char = str[utf16Index]
print(char)
// → 🏾‍🚒
print(char.unicodeScalars.map { "0x\(String($0.value, radix: 16))" })
// → ["0x1f3fe", "0x200d", "0x1f692"]

let trappingIndex = String.Index(encodedOffset: 1)
//str[trappingIndex]
// fatal error: Can't form a Character from a String containing more than one extended grapheme cluster

extension String.Index {
    func isOnCharacterBoundary(in str: String) -> Bool {
        return String.Index(self, within: str) != nil
    }
}

trappingIndex.isOnCharacterBoundary(in: str)
// → false (as expected)
utf16Index.isOnCharacterBoundary(in: str)
// → true (WTF!)