Becoming a better developer by using the SOLID design principles

Becoming a better developer by using the SOLID design principles

Talk from Laracon EU 2018

The focus of this presentation is on what distinguishes a good PHP developer from a strong one and how to stand out from the crowd. It explains how to incorporate the SOLID principles in your daily work and design your code for extendibility. You'd learn how to write code in a way that will make it easy to go back to a feature you developed a year ago and extend it with additional functionality in minutes, not hours.


Katerina Trajchevska

August 31, 2018


  1. Becoming a better developer by using the SOLID design principles

    How to write code that’s easy to maintain, extend and understand
  2. Katerina Trajchevska • Senior Software Engineer & co-founder of Adeva

    • Remote Work Advocate • Community Volunteer • Consultant with startups and Fortune 500 companies Software Engineer and co-founder of Adeva
  3. @ktrajchevska Working on legacy code • Re-reading code multiple times

    to get to the part you need to change • Hard to understand what a method does • Spending a lot of time to fix a minor bug • You spend more time reading than writing code.
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  5. @ktrajchevska Working on a startup product • In charge of

    the development process • Constantly adding new features • No formal process • Very dynamic environment, no time to worry about code structure • What is it like to go back to your code after 2 years?
  6. @ktrajchevska What will we talk about today? • What is

    SOLID and how it can make our lives easier • What’s the purpose of each of the SOLID design principles • How to not use SOLID
  7. @ktrajchevska The purpose of SOLID design principles • To make

    the code more maintainable. • To make it easier to quickly extend the system with new functionality without breaking the existing ones. • To make the code easier to read and understand, thus spend less time figuring out what it does and more time actually developing the solution. • Introduced by Robert Martin (Uncle Bob), named by Michael Feathers.
  8. Single Responsibility Principle A class should have one, and only

    one, reason to change.
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  10. @ktrajchevska Single Responsibility Principle • A class should only be

    responsible for one thing. • There’s a place for everything and everything is in its place. • Find one reason to change and take everything else out of the class. • Very precise names for many small classes > generic names for large classes.
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  15. Open/Closed Principle An entity should be open for extension, but

    closed for modification.
  16. @ktrajchevska Open/Closed Principle • An entity should be open for

    extension but closed for modification. • Extend functionality by adding new code instead of changing existing code. • Separate the behaviors, so the system can easily be extended, but never broken. • Goal: get to a point where you can never break the core of your system.
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  24. Liskov Substitution Principle Let φ(x) be a property provable about

    objects x of type T. Then φ(y) should be true for objects y of type S where S is a subtype of T.
  25. @ktrajchevska Liskov Substitution Principle • Any derived class should be

    able to substitute its parent class without the consumer knowing it. • Every class that implements an interface, must be able to substitute any reference throughout the code that implements that same interface. • Every part of the code should get the expected result no matter what instance of a class you send to it, given it implements the same interface.
  26. Image credits: Derick Bailey

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  30. Interface Segregation Principle No client should be forced to depend

    on methods it does not use.
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  32. @ktrajchevska Interface Segregation Principle • A client should never be

    forced to depend on methods it doesn't use. • Or, a client should never depend on anything more than the method it’s calling. • Changing one method in a class shouldn’t affect classes that don’t depend on it. • Replace fat interfaces with many small, specific interfaces.
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  36. Dependency Inversion Principle High-level modules should not depend on low-level

    modules. Both should depend on abstractions.
  37. Image credits: Derick Bailey

  38. @ktrajchevska Dependency Inversion Principle • Never depend on anything concrete,

    only depend on abstractions. • High level modules should not depend on low level modules. They should depend on abstractions. • Able to change an implementation easily without altering the high level code.
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  43. @ktrajchevska ⚠ Don’t get trapped by SOLID • SOLID design

    principles are principles, not rules. • Always use common sense when applying SOLID. • Avoid over-fragmenting your code for the sake of SRP or SOLID. • Don’t try to achieve SOLID, use SOLID to achieve maintainability.
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  46. @ktrajchevska Final Thoughts • The purpose of SOLID principles is

    to make your code more maintainable, easy to extend and reason about. • It requires spending more time writing code, so you can spend less reading it later. • SOLID principles are principles, not rules. • Always know your trade-offs and use common sense. • SOLID is your tool, not your goal.
  47. Thank you @ktrajchevska Connecting tech companies with world-class developers