What is Inversion of Control?

Question

Inversion of Control (IoC) can be quite confusing when it is first encountered.

1. What is it?
2. Which problem does it solve?
3. When is it appropria

Answer 1

I understand that the answer has already been given here. But I still think, some basics about the inversion of control have to be discussed here in length for future readers.

Inversion of Control (IoC) has been built on a very simple principle called Hollywood Principle. And it says that,

Don't call us, we'll call you

What it means is that don't go to the Hollywood to fulfill your dream rather if you are worthy then Hollywood will find you and make your dream comes true. Pretty much inverted, huh?

Now when we discuss about the principle of IoC, we use to forget about the Hollywood. For IoC, there has to be three element, a Hollywood, you and a task like to fulfill your dream.

In our programming world, Hollywood represent a generic framework (may be written by you or someone else), you represent the user code you wrote and the task represent the thing you want to accomplish with your code. Now you don't ever go to trigger your task by yourself, not in IoC! Rather you have designed everything in such that your framework will trigger your task for you. Thus you have built a reusable framework which can make someone a hero or another one a villain. But that framework is always in charge, it knows when to pick someone and that someone only knows what it wants to be.

A real life example would be given here. Suppose, you want to develop a web application. So, you create a framework which will handle all the common things a web application should handle like handling http request, creating application menu, serving pages, managing cookies, triggering events etc.

And then you leave some hooks in your framework where you can put further codes to generate custom menu, pages, cookies or logging some user events etc. On every browser request, your framework will run and executes your custom codes if hooked then serve it back to the browser.

So, the idea is pretty much simple. Rather than creating a user application which will control everything, first you create a reusable framework which will control everything then write your custom codes and hook it to the framework to execute those in time.

Laravel and EJB are examples of such a frameworks.

Reference:

https://martinfowler.com/bliki/InversionOfControl.html

https://en.wikipedia.org/wiki/Inversion_of_control