Understanding Dependency Injection And Containers

Dependency injection and dependency injection containers are different things:

• dependency injection is a method for writing better code
• a container is a tool to help injecting dependencies

You don't need a container to do dependency injection. However a container can help you.

Classic PHP code

Here is how a code not using DI will roughly work:

• Application needs Foo (e.g. a controller), so:
• Application creates Foo
• Application calls Foo
  • Foo needs Bar (e.g. a service), so:
  • Foo creates Bar
  • Foo calls Bar
    • Bar needs Bim (a service, a repository, …), so:
    • Bar creates Bim
    • Bar does something

Using dependency injection

Here is how a code using DI will roughly work:

• Application needs Foo, which needs Bar, which needs Bim, so:
• Application creates Bim
• Application creates Bar and gives it Bim
• Application creates Foo and gives it Bar
• Application calls Foo
  • Foo calls Bar
    • Bar does something

This is the pattern of Inversion of Control. The control of the dependencies is inverted from one being called to the one calling.

The main advantage: the one at the top of the caller chain is always you. You can control all dependencies and have complete control over how your application works. You can replace a dependency by another (one you made for example).

For example what if Library X uses Logger Y and you want to make it use your logger Z? With dependency injection, you don't have to change the code of Library X.

Using a container

Now how does a code using a container works:

• Application needs Foo so:
• Application gets Foo from the Container, so:
  • Container creates Bim
  • Container creates Bar and gives it Bim
  • Container creates Foo and gives it Bar
• Application calls Foo
  • Foo calls Bar
    • Bar does something

In short, the container takes away all the work of creating and injecting dependencies.