Extension Methods trong Dart

extension StringExtension on String {
  String capitalize() {
    if (isEmpty) return this;
    return '${this[0].toUpperCase()}${substring(1)}';
  }
}
 
void main() {
  print('hello'.capitalize());  // Hello
}

extension IntExtension on int {
  bool get isEven => this % 2 == 0;
  bool get isOdd => this % 2 != 0;
  
  int squared() => this * this;
  
  Duration get seconds => Duration(seconds: this);
  Duration get minutes => Duration(minutes: this);
  Duration get hours => Duration(hours: this);
}
 
void main() {
  print(5.squared());    // 25
  print(4.isEven);       // true
  print(5.seconds);      // 0:00:05.000000
}

extension ListExtension<T> on List<T> {
  T? get firstOrNull => isEmpty ? null : first;
  T? get lastOrNull => isEmpty ? null : last;
  
  List<T> unique() => toSet().toList();
  
  bool containsAll(List<T> items) {
    return items.every((item) => contains(item));
  }
}
 
void main() {
  var list = [1, 2, 2, 3, 3, 3];
  print(list.unique());     // [1, 2, 3]
  print([].firstOrNull);    // null
}

extension DateTimeExtension on DateTime {
  String get formatted => '$day/$month/$year';
  
  bool get isToday {
    var now = DateTime.now();
    return year == now.year && 
           month == now.month && 
           day == now.day;
  }
  
  bool get isWeekend => 
      weekday == DateTime.saturday || 
      weekday == DateTime.sunday;
  
  DateTime addDays(int days) => add(Duration(days: days));
}
 
void main() {
  var date = DateTime.now();
  print(date.formatted);   // 25/12/2024
  print(date.isToday);     // true
}

extension on String {
  String operator *(int times) {
    var result = '';
    for (var i = 0; i < times; i++) {
      result += this;
    }
    return result;
  }
}
 
void main() {
  print('Hi! ' * 3);  // Hi! Hi! Hi! 
}

extension NullableExtension<T> on T? {
  T orDefault(T defaultValue) => this ?? defaultValue;
  
  R? let<R>(R Function(T) transform) {
    if (this != null) {
      return transform(this as T);
    }
    return null;
  }
}
 
void main() {
  String? name;
  print(name.orDefault('Guest'));  // Guest
  
  int? value = 5;
  print(value.let((v) => v * 2));  // 10
}

extension StringUtils on String {
  String reverse() => split('').reversed.join();
}
 
extension AnotherString on String {
  String reverse() => 'Another reverse';
}
 
void main() {
  // Sử dụng tên extension khi conflict
  print(StringUtils('hello').reverse());  // olleh
}