Contracts

Introduction

Much of the Streams Platform and it's addons utilize the interface pattern. This allows developers to implement their own implementation of the interface.

Contracts and implementations and resolving from the service container is used heavily within the Streams Platform in order to allow a very wide and low-level approach to extending core functionality.

/vendor/anomaly/streams-module/src/Entry/Contract/EntryInterface

Loose Coupling

First, let's review some code that is tightly coupled to a cache implementation. Consider the following:

<?php

namespace App\Orders;

class Repository
{
    /**
     * The cache instance.
     */
    protected $cache;

    /**
     * Create a new repository instance.
     *
     * @param  \SomePackage\Cache\Memcached  $cache
     * @return void
     */
    public function __construct(\SomePackage\Cache\Memcached $cache)
    {
        $this->cache = $cache;
    }

    /**
     * Retrieve an Order by ID.
     *
     * @param  int  $id
     * @return Order
     */
    public function find($id)
    {
        if ($this->cache->has($id))    {
            //
        }
    }
}

In this class, the code is tightly coupled to a given cache implementation. It is tightly coupled because we are depending on a concrete Cache class from a package vendor. If the API of that package changes our code must change as well.

Likewise, if we want to replace our underlying cache technology (Memcached) with another technology (Redis), we again will have to modify our repository. Our repository should not have so much knowledge regarding who is providing them data or how they are providing it.

Instead of this approach, we can improve our code by depending on a simple, vendor agnostic interface:

<?php

namespace App\Orders;

use Illuminate\Contracts\Cache\Repository as Cache;

class Repository
{
    /**
     * The cache instance.
     */
    protected $cache;

    /**
     * Create a new repository instance.
     *
     * @param  Cache  $cache
     * @return void
     */
    public function __construct(Cache $cache)
    {
        $this->cache = $cache;
    }
}

Now the code is not coupled to any specific vendor, or even Laravel. Since the contracts package contains no implementation and no dependencies, you may easily write an alternative implementation of any given contract, allowing you to replace your cache implementation without modifying any of your cache consuming code.

How To Use Contracts

So, how do you get an implementation of a contract? It's actually quite simple.

Many types of classes are resolved through the service container, including controllers, event listeners, middleware, queued jobs, and even route Closures. So, to get an implementation of a contract, you can just "type-hint" the interface in the constructor of the class being resolved.

For example, take a look at this event listener:

<?php

namespace App\Listeners;

use App\User;
use App\Events\OrderWasPlaced;
use Illuminate\Contracts\Redis\Factory;

class CacheOrderInformation
{
    /**
     * The Redis factory implementation.
     */
    protected $redis;

    /**
     * Create a new event handler instance.
     *
     * @param  Factory  $redis
     * @return void
     */
    public function __construct(Factory $redis)
    {
        $this->redis = $redis;
    }

    /**
     * Handle the event.
     *
     * @param  OrderWasPlaced  $event
     * @return void
     */
    public function handle(OrderWasPlaced $event)
    {
        //
    }
}

When the event listener is resolved, the service container will read the type-hints on the constructor of the class, and inject the appropriate value. To learn more about registering things in the service container, check out its documentation.