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Playing with dependency injection in PHP

Dependency Injection is a software design pattern that allows avoiding hard-coding dependencies and makes possible to change the dependencies both at runtime and compile time.

By using Dependency Injection we can write more maintainable, testable, and modular code. All projects have dependencies. The larger the project the more dependencies is it bound to have; now having a great number of dependencies is nothing bad by itself however how those dependencies are managed and maintained is.

Dependency Injection is not a new pattern and it has been commonly used on many languages like Java, but this pattern is somewhat new in the PHP world and it's gaining traction quickly thanks for frameworks like laravel

Let's exemplify these concepts by creating a pair of classes first without dependency injection and then rewriting the code to use the dependency injection pattern; since I'm primarily a Magento developer I'll be really original (wink, wink) and create a Product and a StockItem class.

At first glance the code looks pretty normal and it's what many PHP developers would call good code, however if we take a closer look at it using the S.O.L.I.D principle we can see that the code actually has many problems:

Let's Inject something!

In the other hand by using dependency injection we can correct most of these problems, let's take at the same code but using dependency injection:

Constructor Injection

By using dependency injection we have more maintainable code, in the previous example we are using a type of dependency injection normally referred as Constructor Injection. By doing a simple change we can reduce the level of complexity of our code and improve the overall quality; not to mention that now we can easily run unit tests.

Constructor injection is by far the most common method used and there are several advantages by using this particular type of injection:

These two points make Constructor Injection extremely useful, however there is also a few drawbacks that make it unsuitable for all scenarios:

Setter Injection

Another common type of dependency injection is called setter injection and the same code as above would look something like this:

As we can see, with Setter Injection the dependencies are provided to our class after it has been instantiated using setter methods. Setter Injection has a few advantages:

Setter Injection might be more suitable for situations where more flexibility is required.

So is Dependency Injection right for my application?

At the end of the day is up to each developer the one that has to make the decision about what design patterns are the right fit for his application be it Dependency Injection or something else; that being said from personal experience using patterns like this might be an overkill for smaller projects.

But if you are working on large and long running project, then there is good chance that dependency injection might the right solution for your project.

If you have any stories about using Dependency Injection in the real world and you would like to share please leave a comment down below.

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