3.11. Template Method

3.11.1. Purpose

Template Method is a behavioral design pattern.

Perhaps you have encountered it many times already. The idea is to letsubclasses of this abstract template “finish” the behavior of analgorithm.

A.k.a the “Hollywood principle”: “Don’t call us, we call you.” Thisclass is not called by subclasses but the inverse. How? With abstractionof course.

In other words, this is a skeleton of algorithm, well-suited forframework libraries. The user has just to implement one method and thesuperclass do the job.

It is an easy way to decouple concrete classes and reduce copy-paste,that’s why you’ll find it everywhere.

3.11.2. UML Diagram

3.11.3. Code

You can also find this code on GitHub

Journey.php

<?php
 
namespace DesignPatterns\Behavioral\TemplateMethod;
 
abstract class Journey
{
    /**
     * @var string[]
     */
    private $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
     *
     * @return null|string
     */
    protected function buyGift()
    {
        return null;
    }
 
    private function buyAFlight(): string
    {
        return 'Buy a flight ticket';
    }
 
    private function takePlane(): string
    {
        return 'Taking the plane';
    }
 
    /**
     * @return string[]
     */
    public function getThingsToDo(): array
    {
        return $this->thingsToDo;
    }
}

BeachJourney.php

<?php
 
namespace DesignPatterns\Behavioral\TemplateMethod;
 
class BeachJourney extends Journey
{
    protected function enjoyVacation(): string
    {
        return "Swimming and sun-bathing";
    }
}

CityJourney.php

<?php
 
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";
    }
}

3.11.4. Test

Tests/JourneyTest.php

<?php
 
namespace DesignPatterns\Behavioral\TemplateMethod\Tests;
 
use DesignPatterns\Behavioral\TemplateMethod;
use PHPUnit\Framework\TestCase;
 
class JourneyTest extends TestCase
{
    public function testCanGetOnVacationOnTheBeach()
    {
        $beachJourney = new TemplateMethod\BeachJourney();
        $beachJourney->takeATrip();
 
        $this->assertEquals(
            ['Buy a flight ticket', 'Taking the plane', 'Swimming and sun-bathing', 'Taking the plane'],
            $beachJourney->getThingsToDo()
        );
    }
 
    public function testCanGetOnAJourneyToACity()
    {
        $beachJourney = new TemplateMethod\CityJourney();
        $beachJourney->takeATrip();
 
        $this->assertEquals(
            [
                'Buy a flight ticket',
                'Taking the plane',
                'Eat, drink, take photos and sleep',
                'Buy a gift',
                'Taking the plane'
            ],
            $beachJourney->getThingsToDo()
        );
    }
}