目的

空对象模式不是一个GoF设计模式，但是它出现非常频繁足以被认为是设计模式。它的好处如下：

• 简化客户端代码
• 减少空指针异常的次数
• 减少测试用例的复杂性

返回一个对象或null的方法应该返回一个对象或NullObject。NullObjects简化了样板代码，如if (!is_null($obj)) {$obj->callSomething();}只需要$obj->callSomething();通过消除客户端代码中的条件签入。

例子

• Null logger或Null输出以保持对象之间交互的标准方式，即使他们不做任何事情
• 责任链模式中的空处理程序
• 命令模式中的空命令

UML 图

代码

Service.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject;

class Service
{
    public function __construct(private Logger $logger)
    {
    }

    /**
     * do something ...
     */
    public function doSomething()
    {
        // notice here that you don't have to check if the logger is set with eg. is_null(), instead just use it
        $this->logger->log('We are in ' . __METHOD__);
    }
}

Logger.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject;

/**
 * Key feature: NullLogger must inherit from this interface like any other loggers
 */
interface Logger
{
    public function log(string $str);
}

PrintLogger.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject;

class PrintLogger implements Logger
{
    public function log(string $str)
    {
        echo $str;
    }
}

NullLogger.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject;

class NullLogger implements Logger
{
    public function log(string $str)
    {
        // do nothing
    }
}

测试

Tests/LoggerTest.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject\Tests;

use DesignPatterns\Behavioral\NullObject\NullLogger;
use DesignPatterns\Behavioral\NullObject\PrintLogger;
use DesignPatterns\Behavioral\NullObject\Service;
use PHPUnit\Framework\TestCase;

class LoggerTest extends TestCase
{
    public function testNullObject()
    {
        $service = new Service(new NullLogger());
        $this->expectOutputString('');
        $service->doSomething();
    }

    public function testStandardLogger()
    {
        $service = new Service(new PrintLogger());
        $this->expectOutputString('We are in DesignPatterns\Behavioral\NullObject\Service::doSomething');
        $service->doSomething();
    }
}