编程中的设计模式之状态模式

定义

状态模式（State Pattern）当一个对象的内部状态改变时允许改变其行为，这个对象看起来像是改变了其他类，属于行为型模式。

代码示例

在商城系统中，一个订单完成一个流程处理需求精力多个状态修改，这里只列举几种常见状态：未支付、已支付、已发货、已完成，抽象状态对象，通过这四个状态的具体对象实现，由公司类初始化时使用未支付的对象，传入不同的状态可以产生不同的行为。

<?php
abstract class State
{
    abstract public function action($company);
}

class Pending extends State
{
    public function action($company)
    {
        if($company->getOrderStatus() == 'pending'){
            echo '当前订单未支付';
        }else{
            $company->setState(new PaymentSuccess());
            $company->processOrder();
        }
    }
}

class PaymentSuccess extends State
{
    public function action($company)
    {
        if($company->getOrderStatus() == 'payment_success'){
            echo '订单已支付';
        }else{
            $company->setState(new Shipped());
            $company->processOrder();
        }
    }
}

class Shipped extends State
{
    public function action($company)
    {
        if($company->getOrderStatus() == 'shipped'){
            echo '订单已发货';
        }else{
            $company->setState(new Completed());
            $company->processOrder();
        }
    }
}

class Completed extends State
{
    public function action($company)
    {
        if($company->getOrderStatus() == 'completed'){
            echo '订单已完成';
        }else{
            echo '未知状态';
            exit;
        }
    }
}

class Company
{
    private $state;
    public $orderStatus;

    public function __construct()
    {
        $this->state = new Pending();
        $this->orderStatus = 'shipped';
    }

    public function setState($state)
    {
        $this->state = $state;
    }

    public function getState()
    {
        return $this->state;
    }

    public function getOrderStatus()
    {
        return $this->orderStatus;
    }

    public function processOrder()
    {
        $this->state->action($this);
    }
}

class DemoPattern
{
    public function handle()
    {
        $company = new Company();
        $company->processOrder();
        //修改状态再次判断
        $company->orderStatus = 'completed';
        $company->processOrder();
    }
}

$demoPattern = new DemoPattern();
$demoPattern->handle();

总结

如果一个对象中存在很多的状态判断，可以使用该模式把每个状态定义为一个新的子类，减少单个判断语句对整体的影响。