laravel4cookbook

Packages 143
43
44 $provider = new EntryProvider($database);
45 $archiver = new Archiver($provider);
46 $archiver->archive();
Woah Nelly! That’s a lot of code when compared to the first snippet; but it’s worth it. Firstly; we’re
exposing so few details to the Archiver class that the entire entry dependency could be replaced in
a heartbeat. This is possible because we’ve moved the database dependency out of the Archiver and
into the EntryProvider.
We’re also type-hinting an interface instead of a concrete class; which lets us swap the concrete
class out with anything else that implements EntryProviderInterface. The concrete class can fetch
the entries from the database, or the filesystem or whatever.
We can test the EntryProvider class by swapping in a fake Database instance. We can test the
Archiver class by swapping in a fake EntryProvider instance.
So, to recap the requirements for dependency injection:
1. Don’t create class instances in other classes (if they are dependencies) — pass them into the
class from outside.
2. Don’t type-hint concrete classes — create interfaces.
“When you go and get things out of the refrigerator for yourself, you can cause problems.
You might leave the door open, you might get something Mommy or Daddy doesn’t
want you to have. You might even be looking for something we don’t even have or
which has expired.
What you should be doing is stating a need, ‘I need something to drink with lunch,’ and
then we will make sure you have something when you sit down to eat.”
— John Munsch
You can learn more about Dependency Injection, from Taylor’s book, at: https://leanpub.com/laravel.
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Inversion Of Control
Inversion of Control (IoC) is the name given to the process that involves assembling class instances
(and the resolution of class instances in general) within a container or registry. Where the registry
pattern involves defining class instances and then storing them in some global container; IoC