1.3. Factory Method

1.3.1. Purpose

The good point over the SimpleFactory is you can subclass it to implement different ways to create objects.

For simple cases, this abstract class could be just an interface.

This pattern is a “real” Design Pattern because it achieves the Dependency Inversion principle a.k.a the “D” in SOLID principles.

It means the FactoryMethod class depends on abstractions, not concrete classes. This is the real trick compared to SimpleFactory or StaticFactory.

1.3.2. UML Diagram

Alt FactoryMethod UML Diagram

1.3.3. Code

You can also find this code on GitHub

Logger.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

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

StdoutLogger.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

class StdoutLogger implements Logger
{
    public function log(string $message)
    {
        echo $message;
    }
}

FileLogger.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

class FileLogger implements Logger
{
    public function __construct(private string $filePath)
    {
    }

    public function log(string $message)
    {
        file_put_contents($this->filePath, $message . PHP_EOL, FILE_APPEND);
    }
}

LoggerFactory.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

interface LoggerFactory
{
    public function createLogger(): Logger;
}

StdoutLoggerFactory.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

class StdoutLoggerFactory implements LoggerFactory
{
    public function createLogger(): Logger
    {
        return new StdoutLogger();
    }
}

FileLoggerFactory.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod;

class FileLoggerFactory implements LoggerFactory
{
    public function __construct(private string $filePath)
    {
    }

    public function createLogger(): Logger
    {
        return new FileLogger($this->filePath);
    }
}

1.3.4. Test

Tests/FactoryMethodTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Creational\FactoryMethod\Tests;

use DesignPatterns\Creational\FactoryMethod\FileLogger;
use DesignPatterns\Creational\FactoryMethod\FileLoggerFactory;
use DesignPatterns\Creational\FactoryMethod\StdoutLogger;
use DesignPatterns\Creational\FactoryMethod\StdoutLoggerFactory;
use PHPUnit\Framework\TestCase;

class FactoryMethodTest extends TestCase
{
    public function testCanCreateStdoutLogging()
    {
        $loggerFactory = new StdoutLoggerFactory();
        $logger = $loggerFactory->createLogger();

        $this->assertInstanceOf(StdoutLogger::class, $logger);
    }

    public function testCanCreateFileLogging()
    {
        $loggerFactory = new FileLoggerFactory(sys_get_temp_dir());
        $logger = $loggerFactory->createLogger();

        $this->assertInstanceOf(FileLogger::class, $logger);
    }
}