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Hitesh Meghwal
Hitesh Meghwal

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💡 Understanding SOLID Principles in Flutter – Part 3 (D)

#flutter #dart #solidprinciples #mobile

Welcome to the final part of our blog series on mastering the SOLID principles in Flutter.
If you’ve followed along, we’ve already covered:

Today, we complete the series with:

D - Dependency Inversion Principle (DIP)

📘 Definition:

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

Also: Abstractions should not depend on details, but details should depend on abstractions.

🔌 Real-Life Analogy:

You charge your phone using a charger interface — not hard-wired to the wall.
Tomorrow if the power source changes (solar, generator, battery), your charger still works.

In code:

  • High-level logic (like your app’s business rules) shouldn’t depend on low-level implementation details like storage, network APIs.
  • Instead, both rely on interfaces.

💡 Flutter-Specific Example:

❌ Wrong: Tight Coupling

class AuthService {
  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;

  Future<void> login(String email, String password) {
    return _firebaseAuth.signInWithEmailAndPassword(email: email, password: password);
  }
}
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Problem:

  • The app is tightly coupled to Firebase.
  • If you switch to Supabase or REST API, you’ll have to rewrite all AuthService logic.

✅ Right: Inversion via Abstraction

abstract class IAuthProvider {
  Future<void> login(String email, String password);
}

// Firebase implementation
class FirebaseAuthProvider implements IAuthProvider {
  final FirebaseAuth _auth = FirebaseAuth.instance;

  @override
  Future<void> login(String email, String password) {
    return _auth.signInWithEmailAndPassword(email: email, password: password);
  }
}

// Higher-level logic
class AuthService {
  final IAuthProvider provider;

  AuthService(this.provider);

  Future<void> login(String email, String password) {
    return provider.login(email, password);
  }
}
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Now:

  • AuthService depends only on IAuthProvider — not on Firebase.
  • You can swap with Supabase, REST, or a Mock class for testing.

📦 Real Flutter App Use Case: Dependency Injection

Using packages like get_it, riverpod, or provider:

final authService = AuthService(FirebaseAuthProvider());
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  • You inject concrete implementations only at runtime or app startup — not hard-coded inside services.

🔑 Key Takeaways:

  • High-level classes depend on abstractions, not concrete tools (like Firebase, SQLite, APIs).
  • Makes testing easier by mocking implementations.
  • Improves flexibility and maintainability.

🧠 Interview One-Liner:

“DIP helps me keep my business logic decoupled from frameworks like Firebase or Dio. I design with interfaces and inject actual implementations using Provider or GetIt in Flutter.”

✅ Summary of All 5 SOLID Principles (Quick Recall)

Principle Meaning Flutter Example
S One class = One job Separate UI, Service, and Navigation
O Open to extend, closed to modify Use interfaces for payment methods
L Child class must honor parent behavior Avoid misusing inheritance (e.g., Ostrich)
I Keep interfaces small & focused Split validators, user roles
D Depend on abstractions, not concrete Inject Firebase via interface

Popular Libraries Supporting SOLID:

  • get_it: Dependency injection
  • provider/riverpod: State management with DI
  • flutter_bloc: Separation of concerns
  • dio + retrofit: Abstract HTTP clients

✅ That’s a Wrap on Part 3 – And the Whole SOLID Series!
🎯 Final Thoughts
By applying the SOLID principles, you’re not just writing cleaner code —
you’re creating an architecture that can scale, adapt, and survive real-world complexity.

Whether you’re prepping for an interview or improving a legacy app, these principles will guide you toward better software design.

🙌 Thank You for Following the Series!
If this series helped you:
✅ Drop a like
💬 Share your thoughts in the comments
📢 Share with other devs

And let’s continue building SOLID Flutter apps together 🚀

🚀 A complete Flutter project applying all 5 SOLID principles is coming soon — stay tuned for the GitHub repo drop!

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