Dart: Note collections

extension_on_enum.dart

// Example about using extension on Enum.

enum ValidationError { empty }

// Magical happen in here but it's on this enum only. Not in prototype.
extension ValidationErrorMessage on ValidationError {
  String text(String label) {
    switch (this) {
      case ValidationError.empty:
        return 'Please enter a password';
    }
  }
}

// Example validate function
ValidationError? validate(value) {
  if (value == null) return ValidationError.empty;
  return null;
}

// Validate then return error message
var errorMessage = validate(null)?.text();

print('Error message: $errorMessage');

// FYI. If someone found issue as method doesn't exists on other library.
// Run `flutter clean`, reopen IDE and run `flutter pub get` again.