Five design principles to make your code more maintainable, easier to test, and ready to scale in your iOS/Swift projects.

SRP

OCP

LSP

ISP

DIP

Why SOLID Matters in iOS?

iOS projects tend to be complex: huge modules (Auth, Dashboard, Payment), long lifecycles (Banking Apps, Super Apps), and constant refactoring. Without SOLID, you end up with God Objects (ViewControllers with 2000+ lines), nightmare unit tests, and ripple effects where one small change breaks everything.

With SOLID, your code becomes modular, easy to mock, and flexible enough to swap implementations (e.g., URLSession to Alamofire) effortlessly.

1. S – Single Responsibility Principle (SRP)

“A class should have only one reason to change.”

A class or struct should have one main responsibility. If you find yourself changing a class for multiple different reasons (e.g., changing UI logic AND API logic), it violates SRP.

UserManager_Bad.swift

                    class UserManager {

                      // 1. Networking

                      func fetchUser(id: String, completion: @escaping (User?) -> Void) { ... }

                      // 2. Validation

                      func isValidEmail(_ email: String) -> Bool { ... }

                      // 3. Persistence

                      func saveUserToLocal(_ user: User) { ... }

                    }

The Fix: Separate responsibilities into distinct components.

UserRepo_Good.swift

                    struct UserValidator { /* Validation logic */ }

                    class UserAPIService: UserRemoteDataSource { /* API logic */ }

                    class UserLocalStorage: UserLocalDataSource { /* CoreData logic */ }

                    class UserRepository {

                      let remote: UserRemoteDataSource

                      let local: UserLocalDataSource

                      let validator: UserValidator

                      // Repository now only coordinates!

                    }

2. O – Open/Closed Principle (OCP)

“Software entities should be open for extension, but closed for modification.”

Avoid massive switch statements. When adding a new feature (like a new payment method), you should be adding a new class, not editing an existing stable file.

Payment_Good.swift

                    protocol PaymentMethod {

                      func pay(amount: Double)

                    }

                    class CreditCardPayment: PaymentMethod { ... }

                    class EWalletPayment: PaymentMethod { ... }

                    // To add "PayLater", just create a new class. No need to touch PaymentProcessor.

3. L – Liskov Substitution Principle (LSP)

“Objects of a superclass should be replaceable with objects of its subclasses without breaking the correctness of the program.”

Don't force a subclass to inherit methods it can't use. If you have a Penguin class inheriting from Bird, and Bird has a fly() method, you have a problem. Use protocols like Flyable instead.

4. I – Interface Segregation Principle (ISP)

“Clients should not be forced to depend upon interfaces they do not use.”

Don't create "Fat Protocols". If a View only needs to login, it shouldn't have access to registration or deletion methods.

ISP_Protocol.swift

                    protocol UserLoginService {

                      func login(...)

                    }

                    protocol UserRegistrationService {

                      func register(...)

                    }

                    // A class can conform to both, but the client only sees what it needs.

5. D – Dependency Inversion Principle (DIP)

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

Your ViewModel should not know about AuthService directly. It should know about AuthServicing protocol. This allows you to inject a MockAuthService during unit tests.

DIP_Injection.swift

                    class LoginViewModel {

                      private let service: AuthServicing // Protocol, not concrete class!

                      init(service: AuthServicing) {

                        self.service = service

                      }

                    }

Summary

SRP: One class, one job.
OCP: Extend, don't edit.
LSP: Subclasses must behave.
ISP: Small protocols > Big protocols.
DIP: Depend on protocols, use Injection.

Writing

Post Not Found