Dart创建工厂模式

定义Shape形状类，并输出其面积

Dart 支持抽象类，可以在一个文本中定义多个类
dart.math 是一个 Dart 核心库，其余的还有诸如 dart:core，dart:async，dart:convert 和 dart:collection 这样的核心库

import 'dart:math';

abstract class Shape {
  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => pi * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

main() {
  final circle = Circle(2);
  final square = Square(2);
  print(circle.area);
  print(square.area);
}

创建一个顶层的方法

import 'dart:math';

abstract class Shape {
  num get area;
}

Shape shapeFactory(String type) {
  if (type == 'circle') return Circle(2);
  if (type == 'square') return Square(2);
  throw 'Can\'t create $type.';
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => pi * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

main() {
  try {
    print(shapeFactory('circle').area);
    print(shapeFactory('square').area);
  } catch (err) {
    print(err);
  }
}

创建一个工厂模式的构造方法

使用 Dart 的 factory 关键字来创建一个工厂模式的构造方法

import 'dart:math';

abstract class Shape {
  factory Shape(String type) {
    if (type == 'circle') return Circle(2);
    if (type == 'square') return Square(2);
    throw 'Can\'t create $type.';
  }
  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => pi * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

main() {
  final circle = Shape('circle');
  final square = Shape('square');
  print(circle.area);
  print(square.area);
}