Tuesday, March 2, 2021

Dependency Inversion Review

Earlier this week, I decided to dig out a copy of Robert Martin's 1996 article on the Dependency Inversion Principle.

I don't find his example particularly satisfactory, in particular the way that he works the example confuses, I believe, a number of different concerns.  So I thought to try the exercise of a "purer" approach, as I would do it today.

To begin, let's consider the original starting implementation of Copy()

Now,my first priority is that I not break any existing clients. So my intention is to refactor this code without changing the behavior or the signature.

That means I'm going to work my way up to an "extract function" refactoring, where the new function has the re-usable design that we are looking for.  

To begin, we need to think about replacing our dependencies on the I/O functions with abstractions.  Martin dives quickly into "objects" to address this in his examples, but that seems an imprecise hammer to use, given that functions already permit a perfectly satisfactory abstraction - the function pointer.

With an couple of variables to capture the functions we are invoking in our default implementation, it's not trivial to extract our improved method.

And done.

There's no particular magic to the fact that I use function pointers here.  In the kingdom of nouns,  these would be abstraction class instances or interfaces.  In a language like python, it would be a "callable".

Note that we have changed the code by adding a third leaf dependency to the copy function.  Copy() is otherwise a lot simpler, but mostly because we've stashed the complexity under another card.  If CopyV2 is part of the published interface, then we have introduced a new capability that allows consumers to provide substitutes for ReadKayboard and WritePrinter; CopyV2 is likely easier to test than its predecessor.

On the other hand, we're introducing the liability of more code now, in the hopes of accruing some benefit later.  And this isn't a particularly difficult refactoring to introduce "later".

Is the ease with which this refactoring can be introduced representative of the code that we encounter in the wild?  I believe so - but spaghetti code is certainly a thing, and this example doesn't obviously demonstrate that handling entanglement is trivial.


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