Strategy Pattern en acción con PHP

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En el artículo anterior ya hemos visto la definición del Strategy Patern y como se aplica, ahora vamos a ver un ejemplo de la vida real, para eso vamos implementar los métodos de pago que podría tener nuestra tienda virtual.

Contexto

Nuestra pequeña tienda virtual permite que nuestros clientes paguen con tarjeta de crédito, pero como el negocio va creciendo tenemos que adaptarnos al entorno, así que tenemos que implementar otras formas adicionales de pagos como: deposito en cuenta, pago a través de teléfonos móviles, pago contra entrega, cheque, etc.

Cada método de pago tiene una forma distinta de implementarse, ya que cada uno requiere y maneja datos e información distinta, entonces debemos encapsular cada uno de ellos.

Para este ejemplo vamos a implementar el pago con tarjeta de crédito y con PayPal

UML

Implementación

En primer lugar vamos a crear la interfaz de nuestra estrategia, en nuestro caso, la cantidad que se va pagar lo vamos a pasar como argumento.

// PaymentInterface.php
namespace Behavioral\Strategy\Example\ShoppingCart;

interface PaymentInterface
{
    public function pay($amount);
}

Ahora tendremos que crear las estrategias concretas (algoritmos) para el pago mediante tarjeta de crédito y PayPal.

//CreditCardPayment.php
namespace Behavioral\Strategy\Example\ShoppingCart;

use Behavioral\Strategy\Example\ShoppingCart\PaymentInterface;

class CreditCardPayment implements PaymentInterface
{

    private $name;
    private $cardNumber;
    private $cvv;
    private $dateOfExpiry;

    public function __construct($name, $cardNumber, $cvv, $dateOfExpiry)
    {
        $this->name         = $name;
        $this->cardNumber   = $cardNumber;
        $this->cvv          = $cvv;
        $this->dateOfExpiry = $dateOfExpiry;
    }

    public function pay($amount)
    {
        echo $amount." paid with credit/debit card";
    }
}

//PaypalPayment.php
namespace Behavioral\Strategy\Example\ShoppingCart;

use Behavioral\Strategy\Example\ShoppingCart\PaymentInterface;

class PaypalPayment implements PaymentInterface
{

    private $emailId;
    private $password;

    public function __construct($email, $password)
    {
        $this->emailId              = $email;
        $this->password = $password;
    }

    public function pay($amount)
    {
        echo $amount." paid using Paypal";
    }
}

Ahora nuestras estrategias (algoritmos) están listos y podemos implementar el Carrito de Compras. Observa que el método de pago del Carrito de Compras, requiere el algoritmo de pago como argumento

//ShoppingCart.php
namespace Behavioral\Strategy\Example\ShoppingCart;

use Behavioral\Strategy\Example\ShoppingCart\Item;
use Behavioral\Strategy\Example\ShoppingCart\PaymentInterface;

class ShoppingCart
{

    private $items = [];

    public function addItem(Item $item)
    {
        $this->items[$item->getId()] = $item;
    }

    public function removeItem($itemId)
    {
        if (isset($this->items[$itemId])) {
            unset($this->items[$itemId]);
        }
    }

    public function getItems()
    {
        return $this->items;
    }

    public function calculateTotal()
    {
        $sum = 0;
        foreach ($this->items as $items) {
            $sum += $items->getPrice();
        }
        return $sum;
    }

    public function pay(PaymentInterface $paymentMethod)
    {
        $amount = $this->calculateTotal();
        $paymentMethod->pay($amount);
    }
}

Crearemos una clase Item para almacenar la información de los productos y pasárselo al carrito de compra

//Item.php
namespace Behavioral\Strategy\Example\ShoppingCart;

class Item
{

    private $id;
    private $upcCode;
    private $price;

    public function __construct($id, $upc, $cost)
    {
        $this->id      = $id;
        $this->upcCode = $upc;
        $this->price   = $cost;
    }

    public function getId()
    {
        return $this->id;
    }

    public function getUpcCode()
    {
        return $this->upcCode;
    }

    public function getPrice()
    {
        return $this->price;
    }
}

Vamos a probar como funciona nuestra programa

use Behavioral\Strategy\Example\ShoppingCart\Item;
use Behavioral\Strategy\Example\ShoppingCart\ShoppingCart;
use Behavioral\Strategy\Example\ShoppingCart\CreditCardPayment;
use Behavioral\Strategy\Example\ShoppingCart\PaypalPayment;

$cart = new ShoppingCart();

$item1 = new Item(1, '124', 10);
$item2 = new Item(2, '14', 40);

$cart->addItem($item1);
$cart->addItem($item2);

/** Si mi cliente va usar una tarjeta de crédito, uso esto* */
//Pago con Tarjeta de Crédito
$cart->pay(new CreditCardPayment("Jhon W.", "101012123456", "786", "12/15"));
//El resultado sera:
//50 paid with credit/debit card

/** Si mi cliente va usar su cuenta paypal uso esto* */
//Pago con Paypal
$cart->pay(new PaypalPayment("my_email@example.com", "my_password"));
//El resultado sera:
//50 paid using Paypal

De esa manera podemos agregar muchas mas formas de pago sin necesidad de tocar las demás clases haciendo mas flexible nuestro programa.

¿Sencillo no?

Los archivos lo puedes encontrar en en el siguiente enlace:
https://github.com/benjamingb/DesignPatternsPHP/tree/master/Behavioral/Strategy