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

Programming speaking

IoC in easy terms: It's the use of Interface as a way of specific something (such a field or a parameter) as a wildcard that can be used by some classes. It allows the re-usability of the code.

For example, let's say that we have two classes : Dog and Cat. Both shares the same qualities/states: age, size, weight. So instead of creating a class of service called DogService and CatService, I can create a single one called AnimalService that allows to use Dog and Cat only if they use the interface IAnimal.

However, pragmatically speaking, it has some backwards.

a) Most of the developers don't know how to use it. For example, I can create a class called Customer and I can create automatically (using the tools of the IDE) an interface called ICustomer. So, it's not rare to find a folder filled with classes and interfaces, no matter if the interfaces will be reused or not. It's called BLOATED. Some people could argue that "may be in the future we could use it". :-|

b) It has some limitings. For example, let's talk about the case of Dog and Cat and I want to add a new service (functionality) only for dogs. Let's say that I want to calculate the number of days that I need to train a dog (trainDays()), for cat it's useless, cats can't be trained (I'm joking).

b.1) If I add trainDays() to the Service AnimalService then it also works with cats and it's not valid at all.

b.2) I can add a condition in trainDays() where it evaluates which class is used. But it will break completely the IoC.

b.3) I can create a new class of service called DogService just for the new functionality. But, it will increase the maintainability of the code because we will have two classes of service (with similar functionality) for Dog and it's bad.