案例介紹

一個(gè)有著基本功能的購物車程序長什么樣子:

//定義商品Item類
class Item {
  double price;
  String name;
  Item(name, price) {
    this.name = name;
    this.price = price;
  }
}

//定義購物車類
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 '購物車信息:' +
          '\n-----------------------------' +
          '\n用戶名: ' + name+ 
          '\n優(yōu)惠碼: ' + code + 
          '\n總價(jià): ' + price().toString() +
          '\n日期: ' + date.toString() +
          '\n-----------------------------';
  }
}

void main() {
  ShoppingCart sc = ShoppingCart('張三', '123456');
  sc.bookings = [Item('蘋果',10.0), Item('鴨梨',20.0)];
  print(sc.getInfo());
}

類抽象改造

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

class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;
  price() {...}
  //刪掉了構(gòu)造函數(shù)函數(shù)體
  ShoppingCart(this.name, this.code) : date = DateTime.now();
...
}

類抽象改造

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

class ShoppingCart {
  String name;
  DateTime date;
  String code;
  List<Item> bookings;
  price() {...}
  //刪掉了構(gòu)造函數(shù)函數(shù)體
  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

我們先看看 price 屬性的 get 方法：

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

使用 Dart 的箭頭函數(shù)來進(jìn)一步簡化實(shí)現(xiàn)函數(shù)：

class Item extends Meta{
  ...
  //重載了+運(yùn)算符，合并商品為套餐商品
  Item operator+(Item item) => Item(name + item.name, price + item.price); 
}

class ShoppingCart extends Meta{
  ...
  //把迭代求和改寫為歸納合并
  double get price => bookings.reduce((value, element) => value + element).price;
  ...
  getInfo() {...}
}

getInfo 方法如何優(yōu)化:

getInfo () => '''
購物車信息:
-----------------------------
  用戶名: $name
  優(yōu)惠碼: $code
  總價(jià): $price
  Date: $date
-----------------------------
''';

對(duì)象初始化方式的優(yōu)化

class ShoppingCart extends Meta{
  ...
  //默認(rèn)初始化方法，轉(zhuǎn)發(fā)到withCode里
  ShoppingCart({name}) : this.withCode(name:name, code:null);
  //withCode初始化方法，使用語法糖和初始化列表進(jìn)行賦值，并調(diào)用父類初始化方法
  ShoppingCart.withCode({name, this.code}) : date = DateTime.now(), super(name,0);

  //??運(yùn)算符表示為code不為null，則用原值，否則使用默認(rèn)值"沒有"
  getInfo () => '''
購物車信息:
-----------------------------
  用戶名: $name
  優(yōu)惠碼: ${code??"沒有"}
  總價(jià): $price
  Date: $date
-----------------------------
''';
}

void main() {
  ShoppingCart sc = ShoppingCart.withCode(name:'張三', code:'123456');
  sc.bookings = [Item('蘋果',10.0), Item('鴨梨',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 類，并調(diào)整 ShoppingCart 的聲明：

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

class ShoppingCart extends Meta with PrintHelper{
...
}
class Meta {
  double price;
  String name;
  //成員變量初始化語法糖
  Meta(this.name, this.price);
}

class Item extends Meta{
  Item(name, price) : super(name, price);
  //重載+運(yùn)算符，將商品對(duì)象合并為套餐商品
  Item operator+(Item item) => Item(name + item.name, price + item.price); 
}

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

//with表示以非繼承的方式復(fù)用了另一個(gè)類的成員變量及函數(shù)
class ShoppingCart extends Meta with PrintHelper{
  DateTime date;
  String code;
  List<Item> bookings;
  //以歸納合并方式求和
  double get price => bookings.reduce((value, element) => value + element).price;
  //默認(rèn)初始化函數(shù)，轉(zhuǎn)發(fā)至withCode函數(shù)
  ShoppingCart({name}) : this.withCode(name:name, code:null);
  //withCode初始化方法，使用語法糖和初始化列表進(jìn)行賦值，并調(diào)用父類初始化方法
  ShoppingCart.withCode({name, this.code}) : date = DateTime.now(), super(name,0);

  //??運(yùn)算符表示為code不為null，則用原值，否則使用默認(rèn)值"沒有"
  @override
  getInfo() => '''
購物車信息:
-----------------------------
  用戶名: $name
  優(yōu)惠碼: ${code??"沒有"}
  總價(jià): $price
  Date: $date
-----------------------------
''';
}

void main() {
  ShoppingCart.withCode(name:'張三', code:'123456')
  ..bookings = [Item('蘋果',10.0), Item('鴨梨',20.0)]
  ..printInfo();

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

公眾號(hào)：碼農(nóng)架構(gòu)