Design Patterns - Factory Method Pattern

Purpose

The purpose of the design pattern

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

UML

UML design pattern diagram

Code

Code snippets

Logger

Logger Interface - factories must adhere to this interface.


namespace DesignPatterns\Creational\FactoryMethod;

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

LoggerFactory

LoggerFactory Interface - Concrete subclasses must adhere to this interface.


namespace DesignPatterns\Creational\FactoryMethod;

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

FileLogger

The FileLogger class is a concrete subclass which implements the Logger Interface, which forces it to have a log method.


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);
    }
}

StdOutLogger

The StdOutLogger class is a concrete subclass which implements the Logger Interface, which forces it to have a log method.


namespace DesignPatterns\Creational\FactoryMethod;

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

FileLoggerFactory

The FileLoggerFactory must implement the LoggerFactory interface, which forces it to have a createLogger method. The createLogger method returns an instance of Logger.


namespace DesignPatterns\Creational\FactoryMethod;

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

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

StdOutLoggerFactory

The StdOutLoggerFactory must implement the LoggerFactory interface, which forces it to have a createLogger method. The createLogger method returns an instance of Logger.


namespace DesignPatterns\Creational\FactoryMethod;

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

Tests

Examples of creating two class instances via the Factory Method.


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);
}