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10 May 2017
How Does Inversion of Control Really Work?
IoC seems to have become the cornerstone concept of many frameworks and object-oriented designs since it was described by Martin Fowler, Robert Martin and others ten years ago. Despite its popularity IoC is misunderstood and overcomplicated all too often.
Look at this code:
It is very straight forward: we retrieve the title from the book and
simply give it to the
print() procedure, or whatever else it might be. We are in charge,
the control is in our hands.
In contrast to this, here is the inversion:
We give the entire book to the procedure
title() when it feels like it. That is, we delegate control.
This is pretty much everything you need to know about IoC.
Does it have anything to do with
dependency injection containers?
Well, of course, we could put the book into a container, inject the entire
print(), let it retrieve the book from the container and
title(). But that's not what IoC is really about—it's merely
one of its
perverted usage scenarios.
The main point of IoC is exactly the same as I was proposing
in my previous posts about naked data
and object friends:
we must not deal with data, but instead only with
object composition. In the
given example the design would be even better if we got rid of the
print() procedure altogether and replaced it with an object:
That would be pure object composition.
There is not much more to say on this subject; I hope I have cleared it up for you—it is just as simple as that.