Getters và Setters trong Dart

class Circle {
  double radius;
  
  Circle(this.radius);
  
  // Getter - computed property
  double get area => 3.14159 * radius * radius;
  double get circumference => 2 * 3.14159 * radius;
  double get diameter => radius * 2;
}
 
void main() {
  var circle = Circle(5);
  print(circle.area);          // 78.53975
  print(circle.circumference); // 31.4159
}

class Person {
  String _name = '';
  
  // Getter
  String get name => _name;
  
  // Setter với validation
  set name(String value) {
    if (value.isNotEmpty) {
      _name = value;
    }
  }
}
 
void main() {
  var person = Person();
  person.name = 'Alice';
  print(person.name);  // Alice
  
  person.name = '';    // Không thay đổi vì rỗng
  print(person.name);  // Alice
}

class BankAccount {
  double _balance = 0;
  
  double get balance => _balance;
  
  set balance(double value) {
    if (value >= 0) {
      _balance = value;
    } else {
      throw ArgumentError('Balance cannot be negative');
    }
  }
  
  void deposit(double amount) {
    if (amount > 0) _balance += amount;
  }
}

class User {
  final String _id;
  String name;
  
  User(this._id, this.name);
  
  // Chỉ có getter, không có setter
  String get id => _id;
}
 
void main() {
  var user = User('123', 'Alice');
  print(user.id);  // 123
  // user.id = '456';  // Error! Không có setter
}

class Rectangle {
  double width, height;
  
  Rectangle(this.width, this.height);
  
  double get area => width * height;
  double get perimeter => 2 * (width + height);
  bool get isSquare => width == height;
}

class ExpensiveResource {
  List<int>? _data;
  
  List<int> get data {
    _data ??= _loadData();
    return _data!;
  }
  
  List<int> _loadData() {
    print('Loading data...');
    return [1, 2, 3, 4, 5];
  }
}

class Temperature {
  double _celsius = 0;
  
  double get celsius => _celsius;
  
  set celsius(double value) {
    if (value < -273.15) {
      throw ArgumentError('Below absolute zero!');
    }
    _celsius = value;
  }
  
  double get fahrenheit => _celsius * 9/5 + 32;
  
  set fahrenheit(double value) {
    celsius = (value - 32) * 5/9;
  }
}