Correct use of a decorator

Question

I have a Products class. Now I want to add some kind of discount module to my site which should interact with the Products class.

Currently the only solution I can think of is using some kind of decorator pattern to wrap around the product class so it can alter the price of the product.

Like this:

class Product {
    function price() {
        return 10;
    }
}

class ProductDiscountDecorator {
    private $product;

    function __construct($product) {
        $this->product = $product;
    }

    function price() {
        return $this->product->price()*0.8;
    }
}

$product = new ProductDiscountDecorator(new Product());
echo $product->price();

This being the discount, the prices should be adjusted on every page on the website. So every page using the Product class should also add the decorator. The only way I could think of solving this is using a factory that automatically adds this decorator.

$product = $factory->get('product'); // returns new ProductDiscountDecorator(new Product());

It would probably work, but I feel like I'm misusing the decorator pattern here.

Do you guys have any thoughts on this? How would you implement something like this?

Answer 1

A Decorator is one option. Another option would be to use a Strategy Pattern for calculating the discount

Example

class Product
{
    protected $discountStrategy;
    protected $price;

    public function __construct(DiscountStrategy $discountStrategy)
    {
        $this->discountStrategy = $discountStrategy;
    }

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

and then capsule the various possible discounts into classes of their own:

interface DiscountStrategy 
{
    public function applyDiscount($cent);
}

class NoDiscount implements DiscountStrategy
{
    public function applyDiscount($cent)
    {
        return $cent;
    }
}

class PremiumDiscount implements DiscountStrategy
{
    public function applyDiscount($cent)
    {
        return $cent - $cent * 0.8;
    }
}

You'd still use a Factory to assemble the products but Strategies are more fine grained and you can reuse them in other situations as well, for instance, some loyal customers could get a Discount as well, even when the product itself doesnt have a discount attached.