1. 模式定义

装饰器模式能够从一个对象的外部动态地给对象添加功能。
一般给一个对象加功能有如下三种方式：

直接修改这个对象 => 这是不可取的，会影响其他调用这个对象的类或对象
派生对于的子类来拓展 => 随着扩展越多，子类会越膨胀
对象组合 √

装饰器模式就是采用动态组合的方式，实现给一个对象加功能。
最常见的示例是 Web服务层——为REST 服务提供json 和xml装饰器

2. UML类图

image.png

3. 示例代码

RenderInterface.php

<?php


namespace DesignPattern\Structural\Decorator;

/**
 * 渲染器接口
 * Class RendererInterface
 * @package DesignPattern\Structural\Decorator
 */
interface RendererInterface
{
    //渲染数据
    public function renderData();
}

WebService.php

<?php

namespace DesignPattern\Structural\Decorator;

class WebService implements RendererInterface
{
    protected $data;

    public function __construct($data)
    {
        $this->data = $data;
    }


    public function renderData()
    {
        return $this->data;
    }
}

Decorator.php

<?php

namespace DesignPattern\Structural\Decorator;

/**
 * 装饰器： 必须实现 RendererInterface 接口, 这是装饰器模式的主要特点，
 * 否则的话就不是装饰器而只是个包裹类
 * Class Decorator
 * @package DesignPattern\Structural\Decorator
 */
abstract class Decorator implements RendererInterface
{
    /** @var $renderer RendererInterface */
    protected $renderer;

    public function __construct(RendererInterface $renderer)
    {
        $this->renderer = $renderer;
    }
}

RenderXml .php

<?php

namespace DesignPattern\Structural\Decorator;

/**
 * 渲染成XML
 * Class RenderXml
 * @package DesignPattern\Structural\Decorator
 */
class RenderXml extends Decorator
{
    public function renderData()
    {
        $data = $this->renderer->renderData();

        $doc = new \DOMDocument();
        foreach ($data as $key => $val) {
            $doc->appendChild($doc->createElement($key, $val));
        }

        return $doc->saveXML();

    }
}

RenderJson.php

<?php

namespace DesignPattern\Structural\Decorator;

/**
 * 渲染成 Json
 * Class RenderXml
 * @package DesignPattern\Structural\Decorator
 */
class RenderJson extends Decorator
{
    public function renderData()
    {
        $output = $this->renderer->renderData();
        return json_encode($output);
    }
}

DecoratorTest

<?php

namespace DesignPattern\Tests;

use DesignPattern\Structural\Decorator\RenderJson;
use DesignPattern\Structural\Decorator\RenderXml;
use DesignPattern\Structural\Decorator\WebService;
use PHPUnit\Framework\TestCase;

class DecoratorTest extends TestCase
{
    protected $service;

    protected function setUp(): void
    {
        $this->service = new WebService(array('foo' => 'bar'));

    }

    public function testJsonDecorator()
    {
        // Wrap service with a JSON decorator for renderers
        $service = new RenderJson($this->service);
        // Our Renderer will now output JSON instead of an array
        $this->assertEquals('{"foo":"bar"}', $service->renderData());
    }

    public function testXmlDecorator()
    {
        // Wrap service with a XML decorator for renderers
        $service = new RenderXml($this->service);
        // Our Renderer will now output XML instead of an array
        $xml = '<?xml version="1.0"?><foo>bar</foo>';
        $this->assertXmlStringEqualsXmlString($xml, $service->renderData());
    }

    /**
     * The first key-point of this pattern :
     */
    public function testDecoratorMustImplementsRenderer()
    {
        $className = 'DesignPattern\Structural\Decorator\Decorator';
        $interfaceName = 'DesignPattern\Structural\Decorator\RendererInterface';
        $this->assertTrue(is_subclass_of($className, $interfaceName));
    }

}