6.1 KiB
isChild, anchor
| isChild | anchor |
|---|---|
| true | complex_problem |
Complex Problem
If you have ever read about Dependency Injection then you have probably seen the terms "Inversion of Control" or "Dependency Inversion Principle". These are the complex problems that Dependency Injection solves.
Inversion of Control
Inversion of Control is as it says, "inverting the control" of a system by keeping organizational control entirely separate from our objects. In terms of Dependency Injection, this means loosening our dependencies by controlling and instantiating them elsewhere in the system.
For years, PHP frameworks have been achieving Inversion of Control, however, the question became, which part of control are we inverting, and where to? For example, MVC frameworks would generally provide a super object or base controller that other controllers must extend to gain access to its dependencies. This is Inversion of Control, however, instead of loosening dependencies, this method simply moved them.
Dependency Injection allows us to more elegantly solve this problem by only injecting the dependencies we need, when we need them, without the need for any hard coded dependencies at all.
S.O.L.I.D.
Single Responsibility Principle
The Single Responsibility Principle is about actors and high-level architecture. It states that “A class should have only one reason to change.” This means that every class should only have responsibility over a single part of the functionality provided by the software. The largest benefit of this approach is that it enables improved code reusability. By designing our class to do just one thing, we can use (or re-use) it in any other program without changing it.
Open/Closed Principle
The Open/Closed Principle is about class design and feature extensions. It states that “Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.” This means that we should design our modules, classes and functions in a way that when a new functionality is needed, we should not modify our existing code but rather write new code that will be used by existing code. Practically speaking, this means that we should write classes that implement and adhere to interfaces, then type-hint against those interfaces instead of specific classes.
The largest benefit of this approach is that we can very easily extend our code with support for something new without having to modify existing code, meaning that we can reduce QA time, and the risk for negative impact to the application is substantially reduced. We can deploy new code, faster, and with more confidence.
Liskov Substitution Principle
The Liskov Substitution Principle is about subtyping and inheritance. It states that “Child classes should never break the parent class’ type definitions.” Or, in Robert C. Martin’s words, “Subtypes must be substitutable for their base types.”
For example, if we have a FileInterface interface which defines an embed() method, and we have Audio and Video
classes which both implement the FileInterface interface, then we can expect that the usage of the embed() method will always
do the thing that we intend. If we later create a PDF class or a Gist class which implement the FileInterface
interface, we will already know and understand what the embed() method will do. The largest benefit of this approach
is that we have the ability to build flexible and easily-configurable programs, because when we change one object of a
type (e.g., FileInterface) to another we don't need to change anything else in our program.
Interface Segregation Principle
The Interface Segregation Principle (ISP) is about business-logic-to-clients communication. It states that “No client should be forced to depend on methods it does not use.” This means that instead of having a single monolithic interface that all conforming classes need to implement, we should instead provide a set of smaller, concept-specific interfaces that a conforming class implements one or more of.
For example, a Car or Bus class would be interested in a steeringWheel() method, but a Motorcycle or Tricycle
class would not. Conversely, a Motorcycle or Tricycle class would be interested in a handlebars() method, but a
Car or Bus class would not. There is no need to have all of these types of vehicles implement support for both
steeringWheel() as well as handlebars(), so we should break-apart the source interface.
Dependency Inversion Principle
The Dependency Inversion Principle is about removing hard-links between discrete classes so that new functionality can be leveraged by passing a different class. It states that one should "Depend on Abstractions. Do not depend on concretions.". Put simply, this means our dependencies should be interfaces/contracts or abstract classes rather than concrete implementations. We can easily refactor the above example to follow this principle.
{% highlight php %}
<?php namespace Database; class Database { public function __construct(protected AdapterInterface $adapter) { } } interface AdapterInterface {} class MysqlAdapter implements AdapterInterface {} {% endhighlight %} There are several benefits to the `Database` class now depending on an interface rather than a concretion. Consider that we are working in a team and the adapter is being worked on by a colleague. In our first example, we would have to wait for said colleague to finish the adapter before we could properly mock it for our unit tests. Now that the dependency is an interface/contract we can happily mock that interface knowing that our colleague will build the adapter based on that contract. An even bigger benefit to this method is that our code is now much more scalable. If a year down the line we decide that we want to migrate to a different type of database, we can write an adapter that implements the original interface and injects that instead, no more refactoring would be required as we can ensure that the adapter follows the contract set by the interface.