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Refactoring your legacy code using SOLID principles

Muhammad Sumon Molla Selim�@SumonMSelim

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Working with legacy code

  • Lots of reading and exploring
  • Time consuming
  • Hard to understand

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Why SOLID?

  • Maintainable codebase
  • Easy to extend and modify
  • Easy to read and understand

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Single Responsibility Principle

Gather together the things that change for the same reasons. Separate those things that change for different reasons.

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Single Responsibility Principle

  • A class should be responsible for only one thing
  • Have only one reason to change

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Open/Closed Principle

You should be able to extend the behavior of a system without having to modify that system.

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Open/Closed Principle

  • Classes, modules, functions, etc. should be open for extension, but closed for modification
  • Extend functionality by adding new code instead of modifying existing code

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Liskov Substitution Principle

Let φ(x) be a property provable about objects x of type T. Then φ(y) should also be true for objects y of type S where S is a subtype of T.

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Liskov Substitution Principle

  • Any derived class should be able to substitute its parent class
  • Every class that implements an interface must be able to substitute any reference throughout the code that implements the same interface

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Interface Segregation Principle

No client should be forced to depend on methods it does not use.

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Interface Segregation Principle

  • Tailor interfaces to individual client interface
  • 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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Dependency Inversion Principle

High-level modules should not depend on low-level modules. Both should depend on abstractions.

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Dependency Inversion Principle

  • Never depend on anything concrete, only depend on abstractions
  • Able to change an implementation easily without altering the high level code

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REMEMBER

  • These are only principles, not rules
  • There is no hard rules that you will need to implement all of these
  • Use your common sense to utilize these principles in your codebase properly

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Resources

Video: https://youtu.be/TWY9Bin9TY0

Code: https://github.com/SumonMSelim/solid-principles-example-laravel

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Muhammad Sumon Molla Selim

Senior Software Engineer�AWS Community Builder�@SumonMSelim

https://www.sumonselim.com

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