Clean Architecture

Clean Architecture is a software design philosophy that aims to create systems that are:

• Maintainable: Easy to understand, change, and extend.
• Testable: Easy to write and run tests.
• Independent of frameworks, UI, databases, and external agencies: Ensures that changes in one layer do not affect others.

Key Concepts:

• Layers: The architecture is divided into concentric layers, with the core logic at the center.
  • Entities: Represent business rules and data structures.
  • Use Cases (Interactors): Implement specific application business rules. They orchestrate the flow of data to and from entities and manage the execution of actions.
  • Interface Adapters: Convert data from the format most convenient for use cases and entities to the format most convenient for external agencies (UI, database, web, etc.).
  • Frameworks and Drivers: Contain details about frameworks, databases, UI, and external systems. They depend on the core logic but do not impact it directly.

Dependency Rule:

• Inner layers should not depend on outer layers. The dependencies point inwards, ensuring that changes in external layers (like frameworks) do not affect the business rules.

SOLID Principles

SOLID is an acronym for five design principles intended to make software designs more understandable, flexible, and maintainable.

1. Single Responsibility Principle (SRP):
   • Definition: A class should have one, and only one, reason to change.
   • Example: A class handling user authentication should not be responsible for logging.
2. Open/Closed Principle (OCP):
   • Definition: Software entities should be open for extension but closed for modification.
   • Example: Adding new functionality through inheritance or interfaces rather than altering existing code.
3. Liskov Substitution Principle (LSP):
   • Definition: Objects of a superclass should be replaceable with objects of a subclass without affecting the functionality.
   • Example: If class Bird has a method fly, any subclass (like Eagle or Penguin) should also implement fly in a way that it doesn’t break the application’s functionality.
4. Interface Segregation Principle (ISP):
   • Definition: Clients should not be forced to depend on interfaces they do not use.
   • Example: Instead of one large interface for multiple functionalities, create smaller, specific interfaces.
5. Dependency Inversion Principle (DIP):
   • Definition: High-level modules should not depend on low-level modules. Both should depend on abstractions (interfaces). Abstractions should not depend on details. Details should depend on abstractions.
   • Example: A PaymentService should depend on an IPaymentProcessor interface rather than a concrete PayPalProcessor.

Test-Driven Development (TDD)

Test-Driven Development (TDD) is a software development approach where tests are written before the code that makes the tests pass.

Process:

1. Red: Write a test for a new function or improvement. Initially, the test will fail since the function doesn’t exist yet.