username static method

Implementation

static Validator username() {
  return (String? value) {
    if ((value?.isEmpty ?? true)) {
      return "Field cannot be empty.";
    } else if (value!.contains(" ")) {
      return "Username cannot contain spaces";
    } else if (value.containsUsernameSpecial()) {
      return "Username can only contain letters, numbers and underscores";
    }
    return null;
  };
}