综合案例：掌握Dart核心特性

案例介绍

一个有着基本功能的购物车程序长什么样子:

//定义商品Item类
class Item {
  double price;
  String name;
  Item(name, price) {
    this.name = name;
    this.price = price;
  }
}

//定义购物车类
class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;

  price() {
    double sum = 0.0;
    for(var i in bookings) {
      sum += i.price;
    }
    return sum;
  }

  ShoppingCart(name, code) {
    this.name = name;
    this.code = code;
    this.date = DateTime.now();
  }

  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-----------------------------';
  }
}

void main() {
  ShoppingCart sc = ShoppingCart('张三', '123456');
  sc.bookings = [Item('苹果',10.0), Item('鸭梨',20.0)];
  print(sc.getInfo());
}

类抽象改造

class Item {
  double price;
  String name;
  Item(this.name, this.price);
}

class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;
  price() {...}
  //删掉了构造函数函数体
  ShoppingCart(this.name, this.code) : date = DateTime.now();
...
}

类抽象改造

class Item {
  double price;
  String name;
  Item(this.name, this.price);
}

class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;
  price() {...}
  //删掉了构造函数函数体
  ShoppingCart(this.name, this.code) : date = DateTime.now();
...
}

抽象 name, price:

class Meta {
  double price;
  String name;
  Meta(this.name, this.price);
}
class Item extends Meta{
  Item(name, price) : super(name, price);
}

class ShoppingCart extends Meta{
  DateTime date;
  String code;
  List<Item> bookings;
  
  double get price {...}
  ShoppingCart(name, this.code) : date = DateTime.now(),super(name,0);
  getInfo() {...}
}

方法改造

我们先看看 price 属性的 get 方法：

double get price {
  double sum = 0.0;
  for(var i in bookings) {
    sum += i.price;
  }
  return sum;
} 

使用 Dart 的箭头函数来进一步简化实现函数：

class Item extends Meta{
  ...
  //重载了+运算符，合并商品为套餐商品
  Item operator+(Item item) => Item(name + item.name, price + item.price); 
}

class ShoppingCart extends Meta{
  ...
  //把迭代求和改写为归纳合并
  double get price => bookings.reduce((value, element) => value + element).price;
  ...
  getInfo() {...}
}

getInfo 方法如何优化:

getInfo () => '''
购物车信息:
-----------------------------
  用户名: $name
  优惠码: $code
  总价: $price
  Date: $date
-----------------------------
''';

对象初始化方式的优化

class ShoppingCart extends Meta{
  ...
  //默认初始化方法，转发到withCode里
  ShoppingCart({name}) : this.withCode(name:name, code:null);
  //withCode初始化方法，使用语法糖和初始化列表进行赋值，并调用父类初始化方法
  ShoppingCart.withCode({name, this.code}) : date = DateTime.now(), super(name,0);

  //??运算符表示为code不为null，则用原值，否则使用默认值"没有"
  getInfo () => '''
购物车信息:
-----------------------------
  用户名: $name
  优惠码: ${code??"没有"}
  总价: $price
  Date: $date
-----------------------------
''';
}

void main() {
  ShoppingCart sc = ShoppingCart.withCode(name:'张三', code:'123456');
  sc.bookings = [Item('苹果',10.0), Item('鸭梨',20.0)];
  print(sc.getInfo());

  ShoppingCart sc2 = ShoppingCart(name:'李四');
  sc2.bookings = [Item('香蕉',15.0), Item('西瓜',40.0)];
  print(sc2.getInfo());
}

增加 PrintHelper 类，并调整 ShoppingCart 的声明：

abstract class PrintHelper {
  printInfo() => print(getInfo());
  getInfo();
}

class ShoppingCart extends Meta with PrintHelper{
...
}
class Meta {
  double price;
  String name;
  //成员变量初始化语法糖
  Meta(this.name, this.price);
}

class Item extends Meta{
  Item(name, price) : super(name, price);
  //重载+运算符，将商品对象合并为套餐商品
  Item operator+(Item item) => Item(name + item.name, price + item.price); 
}

abstract class PrintHelper {
  printInfo() => print(getInfo());
  getInfo();
}

//with表示以非继承的方式复用了另一个类的成员变量及函数
class ShoppingCart extends Meta with PrintHelper{
  DateTime date;
  String code;
  List<Item> bookings;
  //以归纳合并方式求和
  double get price => bookings.reduce((value, element) => value + element).price;
  //默认初始化函数，转发至withCode函数
  ShoppingCart({name}) : this.withCode(name:name, code:null);
  //withCode初始化方法，使用语法糖和初始化列表进行赋值，并调用父类初始化方法
  ShoppingCart.withCode({name, this.code}) : date = DateTime.now(), super(name,0);

  //??运算符表示为code不为null，则用原值，否则使用默认值"没有"
  @override
  getInfo() => '''
购物车信息:
-----------------------------
  用户名: $name
  优惠码: ${code??"没有"}
  总价: $price
  Date: $date
-----------------------------
''';
}

void main() {
  ShoppingCart.withCode(name:'张三', code:'123456')
  ..bookings = [Item('苹果',10.0), Item('鸭梨',20.0)]
  ..printInfo();

  ShoppingCart(name:'李四')
  ..bookings = [Item('香蕉',15.0), Item('西瓜',40.0)]
  ..printInfo();
}

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