An iOS Engineer’s Perspective on SOLID Principles

How SOLID applies to the iOS and Swift worlds

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SOLID is made up of five design principles meant to make code more understandable, flexible, and maintainable. The principle first appeared in a 2000 paper called “Design Principles and Design Patterns” by Robert Martin, but the acronym SOLID was introduced by Michael Feathers, according to Wikipedia.

I hope when you land yourself on this article, you have the desire to actually want to become a better engineer or write better code. When engineers write code, we should think about the scalability of the code and if the code is readable by different engineers. If we don’t, we’ve got a big problem — which we need to address by following the SOLID principles.

Principle #1: Single Responsibility

Each class should have only one responsibility

By looking at the line of code, we could probably somehow measure how many responsibilities actually lie in the class. With that, I’m taking different examples from different sources to better help us measure how many lines of code we should have.

• According to Code Complete, a study from 1996 found that classes with more routines had more defects.
• According to Ben Nadel’s “Object Calisthenics,” classes over 50 lines usually do more than one thing, which makes them harder to understand and harder to reuse. Fifty-line classes have the added benefit of being visible on one screen without scrolling, which makes them easier to grasp quickly.
• According to one of the discussions on Stack Exchange, 200 lines is a good guideline.
• SwiftLint, a tool that help enforce the Swift style by using the default configuration, believes a class should have less than 500 lines of code. (I’m aware you may alter the configuration of such settings.)

As for how many lines of code you should have in your project, I’ll leave it to your own jurisdiction, as I believe we know best our own project. I trust we should know how long our code can be, but I definitely like the idea of keeping it at less than 200 lines.