Dependencies and Dependency Injection

Autoloading

(Almost) no code comes without dependencies on other PHP classes.

In order to use a class defined in another file, it needs to get imported at the beginning of your file using use, following a namespace.

The namespace, starting with Foodsharing gets interpreted by the composer autoloader. This is configured in /composer.json under the key "autoload", where Foodsharing gets mapped to /src. Other namespaces than Foodsharing link to external libraries, which have their source code somewhere inside the /vendor directory.

The composer autoloader loads files corresponding to conventions specified by the PSR-4 standard. That means that every subnamespace following the Foodsharing namespace gets interpreted as subdirectory under /src.

use is not necessary for using classes in the same namespace (roughly: same directory).

Loading a depencency via require is just used in files like index.php, which are not in the /src directory and therefore can't use autoloading. require basically executes the PHP file unter the given file name, so you can use the class declaration defined in that file. Don't use require if you can use autoloading.

Services

A service class is a class whose main purpose is not representing an object structure, but providing functionality. Service classes are used to structure operations using object oriented design patterns.

What exactly a service is and what not, is not well defined (see Blog Post Services in Domain-Driven Design), but there are some characteristics that are typical for services

• only one instance is created per requests and then shared by all classes
• services depend on other services and the non-service objects (like DTOs) they operate on
• non-service objects don't depend on services

As we don't have any entity classes, except for the DTO classes, nearly all of our classes are services.

Because service classes only need to be instantiated once, we don't use the new statement to create instances. Instead, we use the Dependency Injection pattern. This enables us to share service instances throughout the application.

Automatic service injection

Responsible for creating and injecting the instances is the Symfony Dependency Injection component.

If we want Symfony to inject a dependency into our service, all we need to do is mentioning the class in our constructor (__construct), e.g.

class ActivityXhr extends Control {
  private $mailboxGateway; /* attributes (member variables) */
  public function __construct(ActivityModel $model, MailboxGateway $mailboxGateway)
    { /* using arguments for setting attributes */ }
... }

Dependency injection (Symfony) then makes sure that every service we request is created and injected when our service is instanciated.

This works because we only need at most one object of every service class. This is because, as already mentioned, service classes are not about representing objects using instances, but about the functionality they provide.

Symfony config is in /config/services.yml. This configuration makes sure that services get created automatically. See the Symfony docs for further reference.