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

Inversion of Control, (or IoC), is about getting freedom (You get married, you lost freedom and you are being controlled. You divorced, you have just implemented Inversion of Control. That's what we called, "decoupled". Good computer system discourages some very close relationship.) more flexibility (The kitchen in your office only serves clean tap water, that is your only choice when you want to drink. Your boss implemented Inversion of Control by setting up a new coffee machine. Now you get the flexibility of choosing either tap water or coffee.) and less dependency (Your partner has a job, you don't have a job, you financially depend on your partner, so you are controlled. You find a job, you have implemented Inversion of Control. Good computer system encourages in-dependency.)

When you use a desktop computer, you have slaved (or say, controlled). You have to sit before a screen and look at it. Using the keyboard to type and using the mouse to navigate. And a badly written software can slave you even more. If you replace your desktop with a laptop, then you somewhat inverted control. You can easily take it and move around. So now you can control where you are with your computer, instead of your computer controlling it.

By implementing Inversion of Control, a software/object consumer gets more controls/options over the software/objects, instead of being controlled or having fewer options.

With the above ideas in mind. We still miss a key part of IoC. In the scenario of IoC, the software/object consumer is a sophisticated framework. That means the code you created is not called by yourself. Now let's explain why this way works better for a web application.

Suppose your code is a group of workers. They need to build a car. These workers need a place and tools (a software framework) to build the car. A traditional software framework will be like a garage with many tools. So the workers need to make a plan themselves and use the tools to build the car. Building a car is not an easy business, it will be really hard for the workers to plan and cooperate properly. A modern software framework will be like a modern car factory with all the facilities and managers in place. The workers do not have to make any plan, the managers (part of the framework, they are the smartest people and made the most sophisticated plan) will help coordinate so that the workers know when to do their job (framework calls your code). The workers just need to be flexible enough to use any tools the managers give to them (by using Dependency Injection).

Although the workers give the control of managing the project on the top level to the managers (the framework). But it is good to have some professionals help out. This is the concept of IoC truly come from.

Modern Web applications with an MVC architecture depends on the framework to do URL Routing and put Controllers in place for the framework to call.

Dependency Injection and Inversion of Control are related. Dependency Injection is at the micro level and Inversion of Control is at the macro level. You have to eat every bite (implement DI) in order to finish a meal (implement IoC).