Overview of the Single Responsibility Principle (SRP): each class should have only one reason to change. Example: a `UserService` handles user-related business logic, while a separate `UserRepository` deals with database operations.

Overview of the Open/Closed Principle (OCP): software entities should be open for extension but closed for modification. Example: extending payment methods by implementing a new `IPaymentProcessor` without changing existing processor code.

Overview of the Liskov Substitution Principle (LSP): derived classes must be substitutable for their base classes. Example: a `Rectangle` class and a `Square` subclass both implement `setWidth` and `setHeight` correctly so that client code using `Rectangle` works with `Square` as well.

Overview of the Interface Segregation Principle (ISP): clients should not be forced to depend on interfaces they do not use. Example: splitting a broad `IMachine` interface into `IPrinter` and `IScanner` so that a printer client isn’t forced to implement scan functionality.

Swift Lessons: Solid Principles

Principles