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Introduction

Dependency Injection

Dependency Injection (here-in called DI) refers to the act of providing dependencies for an object during instantiation or via a method call. A basic example looks like this:

$b = new MovieLister(new MovieFinder());

Above, MovieFinder is a dependency of MovieLister, and MovieFinder was injected into MovieLister.

There are several forms of dependency injection:

  • Constructor injection
  • Setter injection
  • Property (or field) injection

While previous versions of laminas-di attempted to handle all three types, starting in version 3, the component focuses only on the first, constructor injection. This decision was made for several reasons:

  • It simplified the implementation dramatically.
  • The simplified implementation had performance benefits.
  • It enforces the order of object initialization, which both helps to prevent circular dependencies, and ensures the completeness of the instantiated object.

Further reading

If you are not familiar with the concept of DI, here are several great reads:

laminas-servicemanager

Since laminas-di purely provides automatic DI (aka auto wiring), it does not provide code-driven Inversion of Control (IoC).

However, Laminas does ship with another IoC component as well: laminas-servicemanager. Unlike laminas-di, laminas-servicemanager is code-driven, meaning that you tell it what class to instantiate, or provide a factory for the given class. This allows you more fine-grained control on how your objects will be instantiated.

In fact laminas-di is designed to play nicely with other IoC containers that implement PSR-11, especially with laminas-servicemanager. You can even use factories generated by laminas-di for laminas-servicemanager.

Dependency Injection Containers

When your code is written in such a way that all your dependencies are injected into consuming objects, you might find that the simple act of wiring an object has gotten more complex. When this becomes the case, and you find that this wiring is creating more boilerplate code, this makes for an excellent opportunity to utilize a Dependency Injection Container. These containers are also often referred to as IoC Containers.

In its simplest form, a Dependency Injection Container (here-in called a DiC for brevity) is an object that is capable of creating objects on request and managing the "wiring", or the injection of required dependencies, for those requested objects. Since the patterns that developers employ in writing DI capable code vary, DiC's are generally either in the form of smallish objects that suit a very specific pattern, or larger DiC frameworks.

The PHP FIG defined a standard interface for such DiCs via PSR-11 (ContainerInterface).

laminas-di is a DiC framework which provides an injector performing the wiring, and a simple imlementation of a DiC. The injector is able to consume any PSR-11 container, such as laminas-servicemanager, to obtain the instances of the dependencies.

While for the simplest use cases no configuration is needed, laminas-di allows developers to configure how to resolve dependencies for more complex use cases.