Dart: Convert a `String` to a `Uint8List` and `Uint8List` to `String` (UTF-16)

main.dart

import 'dart:typed_data';

void main() {
  // Source
  String source = 'Hello! Cześć! 你好! ご挨拶！Привет! ℌ𝔢𝔩𝔩𝔬! 🅗🅔🅛🅛🅞!';
  print(source.length.toString() + ': "' + source + '" (' + source.runes.length.toString() + ')');
  
  // String (Dart uses UTF-16) to bytes
  var list = new List<int>();
  source.runes.forEach((rune) {
    if (rune >= 0x10000) {
      rune -= 0x10000;
      int firstWord = (rune >> 10) + 0xD800;
      list.add(firstWord >> 8);
      list.add(firstWord & 0xFF);
      int secondWord = (rune & 0x3FF) + 0xDC00;
      list.add(secondWord >> 8);
      list.add(secondWord & 0xFF);
    }
    else {
    	list.add(rune >> 8);
    	list.add(rune & 0xFF);
    }
  });
  Uint8List bytes = Uint8List.fromList(list);
  
  // Here you have `Uint8List` available
  
  // Bytes to UTF-16 string
  StringBuffer buffer = new StringBuffer();
  for (int i = 0; i < bytes.length;) {
    int firstWord = (bytes[i] << 8) + bytes[i + 1];
    if (0xD800 <= firstWord && firstWord <= 0xDBFF) {
      int secondWord = (bytes[i + 2] << 8) + bytes[i + 3];
      buffer.writeCharCode(((firstWord - 0xD800) << 10) + (secondWord - 0xDC00) + 0x10000);
      i += 4;
    }
    else {
    	buffer.writeCharCode(firstWord);
      i += 2;
    }
  }
  
  // Outcome
  String outcome = buffer.toString();  
  print(outcome.length.toString() + ': "' + outcome + '" (' + outcome.runes.length.toString() + ')');
}