3.7. 空对象模式
3.7.1. 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。
3.7.2. 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
3.7.3. UML 图

3.7.4. 代码
在 GitHub 上查看代码
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. 测试
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}