| isChild | anchor |
|---|---|
true |
complex_problem |
If you have ever read about Dependency Injection then you have probably seen the terms "Inversion of Control" or "Dependency Inversion Principle". These are the complex problems that Dependency Injection solves.
Inversion of Control is as it says, "inverting the control" of a system by keeping organisational control entirely separate from our objects. In terms of Dependency Injection, this means loosening our dependencies by controlling and instantiating them elsewhere in the system.
For years, PHP frameworks have been achieving Inversion of Control, however, the question became, which part of control are you inverting, and where to? For example, MVC frameworks would generally provide a super object or base controller that other controllers must extend to gain access to its dependencies. This is Inversion of Control, however, instead of loosening dependencies, this method simply moved them.
Dependency Injection allows us to more elegantly solve this problem by only injecting the dependencies we need, when we need them, without the need for any hard coded dependencies at all.
Dependency Inversion Principle is the "D" in the S.O.L.I.D set of object oriented design principles that states one should "Depend on Abstractions. Do not depend on concretions.". Put simply, this means our dependencies should be interfaces/contracts or abstract classes rather than concrete implementations. We can easily refactor the above example to follow this principle.
{% highlight php %}
adapter = $adapter; } } interface AdapterInterface {} class MysqlAdapter implements AdapterInterface {} {% endhighlight %} There are several benefits to the `Database` class now depending on an interface rather than a concretion. Consider that you are working in a team and the adapter is being worked on by a colleague. In our first example, we would have to wait for said colleague to finish the adapter before we could properly mock it for our unit tests. Now that the dependency is an interface/contract we can happily mock that interface knowing that our colleague will build the adapter based on that contract. An even bigger benefit to this method is that our code is now much more scalable. If a year down the line we decide that we want to migrate to a different type of database, we can write an adapter that implements the original interface and inject that instead, no more refactoring would be required as we can ensure that the adapter follows the contract set by the interface.