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空對(duì)象模式不是一個(gè)GoF設(shè)計(jì)模式,但是它出現(xiàn)非常頻繁足以被認(rèn)為是設(shè)計(jì)模式。它的好處如下:
返回一個(gè)對(duì)象或null的方法應(yīng)該返回一個(gè)對(duì)象或NullObject。NullObjects簡化了樣板代碼,如if (!is_null($obj)) {$obj->callSomething();}只需要$obj->callSomething();通過消除客戶端代碼中的條件簽入。
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 } }
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(); } }
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