The Adapter Design Pattern is a structural pattern that allows objects with incompatible interfaces to work together. It acts as an intermediary (or adapter) between two objects, converting the interface of one object to the interface expected by the other. This allows classes that would otherwise be incompatible because they have different interfaces to cooperate without modifications to their original code.

Adapter Structure

The Adapter pattern is generally composed of three main elements:

• Client: The class that expects to work with objects of a specific interface.
• Adaptee: The class that has an interface that is incompatible with the client, but whose functionalities are necessary.
• Adapter: The class that implements the interface expected by the client and converts calls to the Adaptee interface.

Types of Adapters

1. Object Adapter: Composition-based. The Adapter contains an instance of the class it is adapting.
2. Class Adapter: Inheritance-based (usually in languages ​​that support multiple inheritance).

When to use the Adapter?

• When you want to use an existing class, but its interface does not match what the client expects.
• To integrate new functionality into a legacy system, without having to modify the old code.

This pattern is useful in systems that need to work with external libraries or APIs, allowing you to adapt their functionality without changing the code of these libraries.

Example Using PHPMailer

Here is an example of how to use the Adapter Design Pattern to integrate PHPMailer with a custom interface.

Situation:

Let's assume that your system expects any email sending class to implement an interface called IMailer, but PHPMailer does not follow this interface directly. The Adapter will be used to adapt PHPMailer to the interface expected by the system.

Install PHPMailer via Composer

composer require phpmailer/phpmailer

Directory System

?Adapter
 ┣ ?src
 ┃ ┣ ?Interfaces
 ┃ ┃ ┗ ?IMailer.php
 ┃ ┣ ?Adapters
 ┃ ┃ ┗ ?PHPMailerAdapter.php
 ┃ ┗ ?Services
 ┃   ┗ ?ServicoDeEmail.php
 ┣ ?vendor
 ┣ ?composer.json
 ┗ ?index.php

Autoload

In the composer.json file (located at the root of the project), add the App namespace to automatically load the classes:

{
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    },
    "require": {
        "phpmailer/phpmailer": "^6.5"
    }
}

Interface IMailer

namespace App\Interfaces;

interface IMailer {
    public function send($to, $subject, $message);
}

Class PHPMailerAdapter

namespace App\Adapters;

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;
use App\Interfaces\IMailer;

class PHPMailerAdapter implements IMailer {
    private $phpMailer;

    public function __construct() {
        $this->phpMailer = new PHPMailer(true);

        // Basic PHPMailer configuration
        $this->phpMailer->isSMTP();
        $this->phpMailer->Host = 'smtp.example.com';
        $this->phpMailer->SMTPAuth = true;
        $this->phpMailer->Username = '[email protected]';
        $this->phpMailer->Password = 'password';
        $this->phpMailer->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
        $this->phpMailer->Port = 587;
        $this->phpMailer->setFrom('[email protected]', 'Your Name');
    }
}

• Method send

public function send($to, $subject, $message) {
    try {
        $this->phpMailer->addAddress($to);
        $this->phpMailer->Subject = $subject;
        $this->phpMailer->Body = $message;
        $this->phpMailer->send();
        echo 'Email sent successfully!';
    } catch (Exception $e) {
        echo "Failed to send email: {$this->phpMailer->ErrorInfo}";
    }
}

Class EmailService

namespace App\Services;

use App\Interfaces\IMailer;

class EmailService {
    private $mailer;

    public function __construct(IMailer $mailer) {
        $this->mailer = $mailer;
    }
}

• Method sendEmailToClient

public function sendEmailToClient($to, $subject, $message) {
    $this->mailer->send($to, $subject, $message);
}

File index.php

require 'vendor/autoload.php';

use App\Adapters\PHPMailerAdapter;
use App\Services\EmailService;

// Creating the PHPMailer Adapter
$mailer = new PHPMailerAdapter();

// Using the email service with the IMailer interface
$emailService = new EmailService($mailer);

// Sending an email
$emailService->sendEmailToClient('[email protected]', 'Email Subject', 'Message content.');

Explanation of the Structure

• IMailer.php: Defines the IMailer interface that any email system should implement.
• PHPMailerAdapter.php: Adapts PHPMailer to the IMailer interface.
• EmailService.php: Email service that uses the IMailer interface to send emails.
• index.php: Main file that uses the email service to send a message.