Dart面向对象理解

/// 传统的面向对象写法 单继承
class Item {
  double price;
  String name;
  Item(name, price) {
    this.name = name;
    this.price = price;
  }
}
class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;
  ShoppingCart(name, code) {
    this.name = name;
    this.code = code;
    this.date = DateTime.now();
  }
  price() {
    double sum = 0.0;
    for (var i in bookings) {
      sum += i.price;
    }
    return sum;
  }
  getInfo() {
    return '购物车信息：' + 
      '\n----------------------' + 
      '\n 用户名：' + name +
      '\n 优惠码：' + code +
      '\n 总价：' + price().toString() +
      '\n 日期：' + date.toString() + 
      '\n---------------------';
  }
}
main() {
    ShoppingCart sc = ShoppingCart("张三", '123456');
    sc.bookings = [Item("aa", 10.0), Item("bb", 20.0)];
    print(sc.getInfo());
}

通过重构修改了上面的代码，用到了许多dart才有的特性
1、简化构造过程，构造函数简写
2、命名构造函数，一个类可以有多个构造函数
3、构造时初始化额外参数，super前初始化
4、简单成员函数用get或者set关键字改造
5、a??b运算符替代三元运算符
6、重载类的+（加号）运算符
7、连级运算符

abstract class PrintHelper {
  printInfo() => print(getInfo());
  getInfo();
}
class Meta {
  double price;
  String name;
  Meta(this.name, this.price);
}
class Item extends Meta {
  Item(name, price) : super(name, price);
  Item operator +(Item item) => Item(name + item.name, price + item.price);
}
class ShoppingCart extends Meta with PrintHelper {
  DateTime date;
  String code;
  List<Item> bookings;
  ShoppingCart({name}) : this.withCode(name: name, code: null);
  ShoppingCart.withCode({name, this.code})
      : date = DateTime.now(),
        super(name, 0);
  double get price => bookings.reduce((value, element) => value + element).price;
  getInfo() => '''
        购物车信息： 
        ---------------------
          用户名：$name         
          优惠码：${code ?? '无'}         
          总价：$price        
          日期：$date
        ---------------------
        ''';
}
main() {
  ShoppingCart.withCode(name: "张三", code: '123456')
    ..bookings = [Item("aa", 10.0), Item("bb", 20.0)]
    ..printInfo();
}