Getters và Setters trong Dart

class Rectangle {
  double width, height;
  
  Rectangle(this.width, this.height);
  
  // Getter
  double get area => width * height;
  
  // Setter
  set area(double value) {
    width = sqrt(value);
    height = sqrt(value);
  }
}
 
var rect = Rectangle(10, 20);
print(rect.area);  // 200.0
rect.area = 100;   // width = height = 10

class User {
  int _age = 0;
  
  int get age => _age;
  
  set age(int value) {
    if (value >= 0 && value <= 150) {
      _age = value;
    }
  }
}

class CounterState {
  int _count = 0;
  
  int get count => _count;
  
  void increment() => _count++;
  void decrement() => _count--;
}