空对象模式

<?php

// php 技术群：781742505
// 返回一个对象或 null 应该用返回对象或者 NullObject 代替。
// NullObject 简化了死板的代码，消除了客户端代码中的条件检查，
// 例如 if (!is_null($obj)) { $obj->callSomething(); }
// 只需 $obj->callSomething(); 就行。

declare(strict_types=1);

namespace DesignPatterns\Behavioral\NullObject;

class Service
{
    /**
     * @var Logger
     */
    private $logger;

    /**
     * @param Logger $logger
     */
    public function __construct(Logger $logger)
    {
        $this->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__);
    }
}

interface Logger
{
    public function log(string $str);
}

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

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

$service = new Service(new NullLogger());
$service->doSomething();

$service = new Service(new PrintLogger());
$service->doSomething();