3.12. 模板方法模式

3.12.1. 目的

模板方法是一种行为设计模式。

你可能已经遇到过很多次了。它的思想是让这个抽象模板的子类【完成】算法的行为策略。

也就是【好莱坞原则】：【别打给我们，我们打给你】。这个类不是由子类调用的，而是由相反的子类调用的。怎么样？当然是抽象的。

换句话说，这是一个算法框架，非常适合框架库。用户只需要实现一个方法，超类就可以完成这项工作。

它是解耦具体类和减少复制黏贴的一种简单方法，这就是为什么你会发现它无处不在。

3.12.2. UML 图

3.12.3. 代码

在 GitHub 上查看代码

Journey.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\TemplateMethod;

abstract class Journey
{
    /**
     * @var string[]
     */
    private array $thingsToDo = [];

    /**
     * This is the public service provided by this class and its subclasses.
     * Notice it is final to "freeze" the global behavior of algorithm.
     * If you want to override this contract, make an interface with only takeATrip()
     * and subclass it.
     */
    final public function takeATrip()
    {
        $this->thingsToDo[] = $this->buyAFlight();
        $this->thingsToDo[] = $this->takePlane();
        $this->thingsToDo[] = $this->enjoyVacation();
        $buyGift = $this->buyGift();

        if ($buyGift !== null) {
            $this->thingsToDo[] = $buyGift;
        }

        $this->thingsToDo[] = $this->takePlane();
    }

    /**
     * This method must be implemented, this is the key-feature of this pattern.
     */
    abstract protected function enjoyVacation(): string;

    /**
     * This method is also part of the algorithm but it is optional.
     * You can override it only if you need to
     */
    protected function buyGift(): ?string
    {
        return null;
    }

    private function buyAFlight(): string
    {
        return 'Buy a flight ticket';
    }

    private function takePlane(): string
    {
        return 'Taking the plane';
    }

    /**
     * @return string[]
     */
    final public function getThingsToDo(): array
    {
        return $this->thingsToDo;
    }
}

BeachJourney.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\TemplateMethod;

class BeachJourney extends Journey
{
    protected function enjoyVacation(): string
    {
        return "Swimming and sun-bathing";
    }
}

CityJourney.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\TemplateMethod;

class CityJourney extends Journey
{
    protected function enjoyVacation(): string
    {
        return "Eat, drink, take photos and sleep";
    }

    protected function buyGift(): ?string
    {
        return "Buy a gift";
    }
}

3.12.4. 测试

Tests/JourneyTest.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\TemplateMethod\Tests;

use DesignPatterns\Behavioral\TemplateMethod\BeachJourney;
use DesignPatterns\Behavioral\TemplateMethod\CityJourney;
use PHPUnit\Framework\TestCase;

class JourneyTest extends TestCase
{
    public function testCanGetOnVacationOnTheBeach()
    {
        $beachJourney = new BeachJourney();
        $beachJourney->takeATrip();

        $this->assertSame(
            ['Buy a flight ticket', 'Taking the plane', 'Swimming and sun-bathing', 'Taking the plane'],
            $beachJourney->getThingsToDo()
        );
    }

    public function testCanGetOnAJourneyToACity()
    {
        $cityJourney = new CityJourney();
        $cityJourney->takeATrip();

        $this->assertSame(
            [
                'Buy a flight ticket',
                'Taking the plane',
                'Eat, drink, take photos and sleep',
                'Buy a gift',
                'Taking the plane'
            ],
            $cityJourney->getThingsToDo()
        );
    }
}