面向对象的洗礼：设计模式（八）之工厂方法

工厂方法：定义一个工厂接口，用来创建产品对象，将实际创建工作推迟到子类当中。
工厂方法来源于简单工厂模式，是简单工厂模式的一个衍生品。核心的工厂类不再进行类的实例化，核心工厂类不再负责产品的创建，核心的工厂类只负责子类的接口，使核心工厂类抽象化，成为一个抽象工厂。
工厂方法的优点：在简单工厂模式的基础上再次对核心工厂类进行抽象，在需要添加新的产品时，更好的依赖于开放-封闭原则，不需要修改具体的工厂角色即可扩展。
场景：简单计算器。以PHP为代码实现环境。
简单工厂模式：

<?php
class Calculator{
    public $numberA;
    public $numberB;
    public $result;
    public function returnResult(){

    }
}
class Add extends Calculator{
    public function __construct($a, $b){
        $this->numberA = $a;
        $this->numberB = $b;
    }
    public function returnResult(){
        $this->result = $this->numberA + $this->numberB;
        return $this->returnResult();
    }
}
class Sub extends Calculator{
    public function __construct($a, $b){
        $this->numberA = $a;
        $this->numberB = $b;
    }
    public function returnResult(){
        $this->result = $this->numberA - $this->numberB;
        return $this->returnResult();
    }
}
class Factory{
    public function calculatorFactory($operator, $numberA, $numberB){
        $obj = '';
        switch($operator){
            case '+':
                $obj = new Add($numberA, $numberB);
                break;
            case '-':
                $obj = new Sub($numberA, $numberB);
                break;
        }
        $result = $obj->returnResult();
        return $result;
    }
}
//客户端/接口
$operation = isset($_GET['operation']) ? $_GET['operation'] : '+';
$numberA = $_GET['numberA'];
$numberB = $_GET['numberB'];
$obj = new Factory();
echo $obj->calculatorFactory($operation, $numberA, $numberB);
?>

工厂方法：

<?php
class Calculator{
    public $numberA;
    public $numberB;
    public $result;
    public function returnResult(){}
}
class Add extends Calculator{
    public function __construct($a, $b){
        $this->numberA = $a;
        $this->numberB = $b;
    }
    public function returnResult(){
        $this->result = $this->numberA + $this->numberB;
        return $this->returnResult();
    }
}
class Sub extends Calculator{
    public function __construct($a, $b){
        $this->numberA = $a;
        $this->numberB = $b;
    }
    public function returnResult(){
        $this->result = $this->numberA - $this->numberB;
        return $this->returnResult();
    }
}
class Factory extends Calculator{
    public static function create($class, $numberA, $numberb){
        return new $class($numberA, $numberb);
    }
}
//客户端/接口
$operator = isset($_GET['operation']) ? $_GET['operation'] : '+';
$numberA = $_GET['numberA'];
$numberB = $_GET['numberB'];
switch($operator){
    case '+':
        $class = 'objAdd';
        break;
    case '-':
        $class = 'objSub';
        break;
}
$obj = Factory::create($class, $numberA, $numberb);
echo $obj->returnResult();
?>

简单工厂模式VS工厂方法模式：
简单工厂的选择在工厂类，