3.7. Null Object

3.7.1. Amaç

Boş Nesne, bir Dörtlü Takım (Gang of Four) tasarım deseni değildir. Ancak, bir şemadır ve görünen o ki sıklıkla bir desen olarak ele alınmaktadır.Aşağıdaki yararlıklara sahiptir:

  • Client code is simplified

  • Reduces the chance of null pointer exceptions

  • Fewer conditionals require less test cases

Bir nesne veya boş (null) döndüren nesneler, bunun yerine yine bir nesne veya NullObject döndürür. NullObject, istemci kodundaki koşullu kontrolü (if) atarak, yani if (!is_null($object)) $object->callSomething() yerine yalnızca $object->callSomething() kısmının kullanılabilmesini sağlayarak, ortak kodu (boilerplate) basitleştirmiştir.

3.7.2. Örnekler

  • Null logger or null output to preserve a standard way of interaction between objects, even if the shouldn’t do anything

  • null handler in a Chain of Responsibilities pattern

  • null command in a Command pattern

3.7.3. UML Diyagramı

Alt NullObject UML Diyagramı

3.7.4. Kod

Bu kodu Github üzerinde de bulabilirsiniz.

Service.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class Service
 8{
 9    public function __construct(private Logger $logger)
10    {
11    }
12
13    /**
14     * do something ...
15     */
16    public function doSomething()
17    {
18        // notice here that you don't have to check if the logger is set with eg. is_null(), instead just use it
19        $this->logger->log('We are in ' . __METHOD__);
20    }
21}

Logger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7/**
 8 * Key feature: NullLogger must inherit from this interface like any other loggers
 9 */
10interface Logger
11{
12    public function log(string $str);
13}

PrintLogger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class PrintLogger implements Logger
 8{
 9    public function log(string $str)
10    {
11        echo $str;
12    }
13}

NullLogger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class NullLogger implements Logger
 8{
 9    public function log(string $str)
10    {
11        // do nothing
12    }
13}

3.7.5. Test

Tests/LoggerTest.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject\Tests;
 6
 7use DesignPatterns\Behavioral\NullObject\NullLogger;
 8use DesignPatterns\Behavioral\NullObject\PrintLogger;
 9use DesignPatterns\Behavioral\NullObject\Service;
10use PHPUnit\Framework\TestCase;
11
12class LoggerTest extends TestCase
13{
14    public function testNullObject()
15    {
16        $service = new Service(new NullLogger());
17        $this->expectOutputString('');
18        $service->doSomething();
19    }
20
21    public function testStandardLogger()
22    {
23        $service = new Service(new PrintLogger());
24        $this->expectOutputString('We are in DesignPatterns\Behavioral\NullObject\Service::doSomething');
25        $service->doSomething();
26    }
27}