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DIP: Takeaways

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The two rules of the dependency inversion principle give us clear instructions on how two classes - like CommentSpamManager and RegexSpamWordHelper - should interact.

"Inversion"? What got Inverted?

But before we talk about the pros and cons of DIP... why is this called dependency inversion? What is the "inversion"?

This took me a long time to wrap my head around. I expected that dependency inversion somehow meant that the two classes literally started depending on each other in some... different way. Like suddenly we would inject the CommentSpamManager into RegexSpamWordHelper... instead of the other way around, actually "inverting" the dependency.

But, as you can see... that is not the case. On a high level, these two classes depend on each other in the exact same way as they always did: the low level, details class - RegexSpamWordHelper - is injected into the high-level class - CommentSpamManager.

The "inversion" part is... more of an abstract concept. Before we refactored our code to create and use the interface, I would have said:

CommentSpamManager depends on RegexSpamWordHelper. If we decide to modify RegexSpamWordHelper, we will then need to update CommentSpamManager to make it work with those changes. RegexSpamWordHelper is the boss.

But after the refactoring, specifically, after we created an interface based on the needs of CommentSpamManager, I would now say this:

CommentSpamManager depends on any class that implements CommentSpamCounterInterface. In reality, this is the RegexSpamWordHelper class. But if we decided to refactor how RegexSpamWordHelper works, it would still be responsible for implementing CommentSpamCounterInterface. In other words, when RegexSpamWordHelper changes, our high level CommentSpamManager class will not need to change.

That is the inversion: it's an inversion of control: a "reversal" of who is in charge. Thanks to the new interface, the high-level class - CommentSpamManager - has taken control over what its dependency needs to look like.

Pros and Cons of DIP

So now that we understand the dependency inversion principle, what are its benefits?

Simply put: DIP is all about decoupling. CommentSpamManager is now decoupled from RegexSpamWordHelper. We could even replace it with a different class that implements this interface without touching any code from the high-level class.

This is one of the core strategies to writing "framework agnostic" code. In this situation, developers create interfaces in their code and only depend on those interfaces, instead of on the interfaces or classes from whatever framework they're using.

However, in my code, I rarely follow the dependency inversion principle. Well, let me clarify. If I were working on an open source, reusable library, like Symfony itself, I would definitely create interfaces, like we just did. Why? Because I want to allow the users of my code to replace this service with some other class, like maybe someone wants to replace our simple RegexSpamWordHelper in their app with a class that uses an API to find these spam words.

But if I were writing this in my own application, I would skip creating the interface: I would make my code look like it originally did with CommentSpamManager relying directly on RegexSpamWordHelper with no interface.

Most Dependencies Don't Need Inverting

Why? As Dan North points out in his blog post: not all dependencies need to be inverted. If something you depend on will truly need to be swapped out for a different class or implementation later, then that dependency is almost more of an "option". If we had that situation, we probably would want to apply DIP. By creating and type-hinting an interface, we're saying:

Please pass me the "option" that you would like to use for counting spam words.

But, most of the time, to partially quote Dan:

Dependencies aren't options: they're just the way we are going to count spam words in this situation.

If you followed DIP perfectly, you end up with a code base with a lot of interfaces which are implemented by only one class each. That adds flexibility... which you likely won't need. The "cost" is misdirection: your code is harder to follow.

For example, in CommentSpamManager, it now takes a bit more work to figure out which class counts the spam words and how everything is working. And if you ever do try to change a dependency to use a different, concrete class, you might discover that, even though you followed DIP, it's not so easy change!

For example, changing from one database system to another is probably going to be an ugly job... even if you created an interface to abstract away the differences beforehand. It might still be worth doing... if you do think your database will change, but it's not a silver bullet that will make that an easy task.

So my advice is this: unless you're writing code that will be shared across projects, do not create an interface until you have more than one class that would implement it... which we actually saw earlier with our scoring factors. This is a perfectly nice use of interfaces.

But! I fully admit that not everyone agrees with my opinion on this! And if you do disagree, awesome! Do what you think is best. There are plenty of smart people out there that do create extra interfaces in their code to decouple from whatever frameworks or libraries they're using. I'm just not one of them.

SOLID in Review

Ok friends, that's it! We are done with the SOLID principles! Let's do a quick recap... using our simplified definitions.

One: the single responsibility principle says:

Write classes so that your code "fits in your head".

Two: the open-closed principle says:

Design your classes so that you can change their behavior without changing their code.

This is never entirely possible... and in my app code, I rarely follow this.

Three: the Liskov substitution principle says:

If a class extends a base class or implements an interface, make your class behave like it is supposed to.

PHP protects against most violations of this principle by throwing syntax errors.

Four: the interface segregation principle says:

If a class has a large interface - so a lot of methods - and you often inject the class and only use some of these methods - consider splitting your class into smaller pieces.

And five: the dependency inversion principle says:

Prefer type-hinting interfaces and allow each interface to be designed for the "high level" class that will use it, instead of for the low-level class that will implement it.

In my app, I do type-hint interfaces whenever they exist, usually because services from Symfony or other libraries provide an interface. But I don't create my own interfaces until I have multiple classes that need to implement them.

My opinions are, of course, just that: opinions! And I tend to be much more pragmatic than dogmatic... for better or worse. People will definitely disagree... and that's great! SOLID forces us to think critically.

Also the SOLID principles aren't the only "game" in town when it comes to writing clean code. There are design patterns, composition over inheritance, the law of demeter and other principles to guide your path.

If you have any questions or ideas, as always, we would love to hear from you down in the comments.

Alright, friends, seeya next time!