PHP applying code from multiple classes that implement the same interface

Question

I'm currently implementing a project and have a question regarding multiple classes that implement the same interface.

So, right now I have multiple discount types that might be applied to an Order. So, I created a DiscountInterface that has the check() method, that checks if the discount is applicable and the apply() method that applies the discount.

So far, so good. Then I implemented the first discount type in a class that implements the DiscountInterface and has the logic for checking and applying this particular discount.

In my controller, I inject the DiscountInterface. When an Order is received I call both the check() and apply() methods and everything is running perfectly.

My question is the following. I to implement a second type of discount. Following my implementation, I will have to create a new class that implements DiscountInterface. But when it's time to call it on the controller how should it be done. Since I have to different classes, with the same methods.

The following code works if I have one class implementing these methods, but what happens if I have two classes implementing them?

public function discount(
    Request $request, 
    DiscountInterface $discount, 
    CustomerRepository $customer, 
    ProductRepository $product, 
    ValidatorInterface $validator,
    OrderServiceInterface $orderService
)
{
    $data = json_decode($request->getContent(), true);
    $order = $orderService->convertDataToOrder($data, $customer, $product, $validator);

    if($discount->check($order, $customer)){
        $order = $discount->apply($order);
    }

The code will continue to work, it'll take any class that satisfies the "DiscountInterface", so even when you have 2 or 300 classes, you can throw them into that method as they're completely interchangeable. — Royal Wares
– Royal Wares, Commented Oct 25, 2018 at 10:07
do you need to check against ALL discount types and figure out which ones apply? — delboy1978uk
– delboy1978uk, Commented Oct 25, 2018 at 10:08

delboy1978uk · Accepted Answer · 2018-10-25 10:42:26Z

2

If you need to check against all discount types, instead of passing the implementing class, why not pass a DiscountFactory instead?

<?php

use My\Discount\CrapDiscount;
use My\Discount\AwesomeDiscount;

class DiscountFactory
{
    /** @var DiscountInterface[] */
    private $discounts;

    public function __construct()
    {
        $this->discounts = [
            new CrapDiscount(),
            new AwesomeDiscount(),
        ];
    }

    public function getDiscounts(): array
    {
        return $this->discounts;
    }
}

Then your code could look something like this:

public function discount(
    Request $request, 
    DiscountFactory $discountFactory, 
    CustomerRepository $customer, 
    ProductRepository $product, 
    ValidatorInterface $validator,
    OrderServiceInterface $orderService
)
{
    $data = json_decode($request->getContent(), true);
    $order = $orderService->convertDataToOrder($data, $customer, $product, $validator);

    foreach ($discountFactory->getDiscounts() as $discount) {
        if($discount->check($order, $customer)){
            $order = $discount->apply($order);
        }
    }
    // etc

edited Oct 25, 2018 at 10:42

answered Oct 25, 2018 at 10:15

delboy1978uk

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4 Comments

Cláudio Ribeiro Over a year ago

Sounds like a plan! Thanks for the idea!

jeroen Over a year ago

@CláudioRibeiro Note that you might have to add checks to validate the order of the discounts; discounting an amount and then subtracting a percentage is not the same as doing it the other way around.

delboy1978uk Over a year ago

Cheers! If it works can you mark this correct please? :-D

delboy1978uk Over a year ago

@Claudio I moved it into the constructor

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