3.1. Chain Of Responsibilities ¶

3.1.1. Purpose¶

To build a chain of objects to handle a call in sequential order. If one object cannot handle a call, it delegates the call to the next in the chain and so forth.

3.1.2. Examples¶

logging framework, where each chain element decides autonomously what to do with a log message
a Spam filter
Caching: first object is an instance of e.g. a Memcached Interface, if that “misses” it delegates the call to the database interface
Yii Framework: CFilterChain is a chain of controller action filters. the executing point is passed from one filter to the next along the chain, and only if all filters say “yes”, the action can be invoked at last.

3.1.3. UML Diagram¶

3.1.4. Code¶

You can also find this code on GitHub

Handler.php

<?php

namespace DesignPatterns\Behavioral\ChainOfResponsibilities;

use Psr\Http\Message\RequestInterface;
use Psr\Http\Message\ResponseInterface;

abstract class Handler
{
    /**
     * @var Handler|null
     */
    private $successor = null;

    public function __construct(Handler $handler = null)
    {
        $this->successor = $handler;
    }

    /**
     * This approach by using a template method pattern ensures you that
     * each subclass will not forget to call the successor
     *
     * @param RequestInterface $request
     *
     * @return string|null
     */
    final public function handle(RequestInterface $request)
    {
        $processed = $this->processing($request);

        if ($processed === null) {
            // the request has not been processed by this handler => see the next
            if ($this->successor !== null) {
                $processed = $this->successor->handle($request);
            }
        }

        return $processed;
    }

    abstract protected function processing(RequestInterface $request);
}

Responsible/FastStorage.php

<?php

namespace DesignPatterns\Behavioral\ChainOfResponsibilities\Responsible;

use DesignPatterns\Behavioral\ChainOfResponsibilities\Handler;
use Psr\Http\Message\RequestInterface;

class HttpInMemoryCacheHandler extends Handler
{
    /**
     * @var array
     */
    private $data;

    /**
     * @param array $data
     * @param Handler|null $successor
     */
    public function __construct(array $data, Handler $successor = null)
    {
        parent::__construct($successor);

        $this->data = $data;
    }

    /**
     * @param RequestInterface $request
     *
     * @return string|null
     */
    protected function processing(RequestInterface $request)
    {
        $key = sprintf(
            '%s?%s',
            $request->getUri()->getPath(),
            $request->getUri()->getQuery()
        );

        if ($request->getMethod() == 'GET' && isset($this->data[$key])) {
            return $this->data[$key];
        }

        return null;
    }
}

Responsible/SlowStorage.php

<?php

namespace DesignPatterns\Behavioral\ChainOfResponsibilities\Responsible;

use DesignPatterns\Behavioral\ChainOfResponsibilities\Handler;
use Psr\Http\Message\RequestInterface;

class SlowDatabaseHandler extends Handler
{
    /**
     * @param RequestInterface $request
     *
     * @return string|null
     */
    protected function processing(RequestInterface $request)
    {
        // this is a mockup, in production code you would ask a slow (compared to in-memory) DB for the results

        return 'Hello World!';
    }
}

3.1.5. Test¶

Tests/ChainTest.php

<?php

namespace DesignPatterns\Behavioral\ChainOfResponsibilities\Tests;

use DesignPatterns\Behavioral\ChainOfResponsibilities\Handler;
use DesignPatterns\Behavioral\ChainOfResponsibilities\Responsible\HttpInMemoryCacheHandler;
use DesignPatterns\Behavioral\ChainOfResponsibilities\Responsible\SlowDatabaseHandler;
use PHPUnit\Framework\TestCase;

class ChainTest extends TestCase
{
    /**
     * @var Handler
     */
    private $chain;

    protected function setUp()
    {
        $this->chain = new HttpInMemoryCacheHandler(
            ['/foo/bar?index=1' => 'Hello In Memory!'],
            new SlowDatabaseHandler()
        );
    }

    public function testCanRequestKeyInFastStorage()
    {
        $uri = $this->createMock('Psr\Http\Message\UriInterface');
        $uri->method('getPath')->willReturn('/foo/bar');
        $uri->method('getQuery')->willReturn('index=1');

        $request = $this->createMock('Psr\Http\Message\RequestInterface');
        $request->method('getMethod')
            ->willReturn('GET');
        $request->method('getUri')->willReturn($uri);

        $this->assertEquals('Hello In Memory!', $this->chain->handle($request));
    }

    public function testCanRequestKeyInSlowStorage()
    {
        $uri = $this->createMock('Psr\Http\Message\UriInterface');
        $uri->method('getPath')->willReturn('/foo/baz');
        $uri->method('getQuery')->willReturn('');

        $request = $this->createMock('Psr\Http\Message\RequestInterface');
        $request->method('getMethod')
            ->willReturn('GET');
        $request->method('getUri')->willReturn($uri);

        $this->assertEquals('Hello World!', $this->chain->handle($request));
    }
}

3.1. Chain Of Responsibilities¶