🚀 What is SOLID?
SOLID is a set of five design principles that help developers write clean, maintainable, and scalable code. Originally introduced by Robert C. Martin (Uncle Bob), these principles are especially powerful in object-oriented programming — and yes, they apply beautifully in Flutter and Dart too!
The SOLID acronym stands for:
- S – Single Responsibility Principle
- O – Open/Closed Principle
- L – Liskov Substitution Principle
- I – Interface Segregation Principle
- D – Dependency Inversion Principle
🔍 Why Should Flutter Devs Care?
As your app grows, your codebase gets more complex. Without structure, you’ll end up with:
- Widgets doing everything (UI, logic, API)
- Spaghetti navigation code
- Difficult-to-test classes
- Painful refactoring when features change
SOLID helps prevent all that by encouraging:
✅ Separation of concerns
✅ Loose coupling
✅ Easy unit testing
✅ Flexible architecture
✅ Long-term maintainability
📚 Let's Break It Down – In 3 Parts
To keep things simple and digestible, we’ll explore SOLID in a 3-part blog series, with real-world analogies and Flutter code examples
🧩 Part 1: S & O – Writing Focused, Extendable Widgets
Avoiding God-widgets
Keeping UI, logic, and services separate
Making your classes open to grow, but stable in design
🧩 Part 2: L & I – Designing Safe and Focused Abstractions
Subclassing wisely
Creating lean interfaces
Avoiding broken contracts in inheritance
🧩 Part 3: D – Mastering Dependencies in Flutter Apps
Injecting services the clean way (Provider, GetIt)
Designing for flexibility and testability
Swapping Firebase or REST APIs without chaos
Part 1: S & O
S - Single Responsibility Principle (SRP)
📘 Definition:
A class should have only one reason to change.
🍕 Real-Life Analogy:
Imagine a pizza delivery guy who is also expected to:
- Cook the pizza 🍕
- Take customer calls 📞
- Drive the delivery vehicle 🚗
If anything goes wrong in any one of these, his role needs to change. That's too many responsibilities!
Instead, we split the roles:
- Chef 👨🍳 cooks
- Receptionist 📞 takes orders
- Delivery person 🚗 delivers
This is exactly what SRP encourages in your code.
💡 Flutter-Specific Example:
❌ Wrong:
class UserProfileWidget extends StatelessWidget {
final String userId;
UserProfileWidget(this.userId);
Future<User> fetchUserData(String id) async {
// Networking logic
}
void navigateToEdit(BuildContext context) {
// Navigation logic
}
@override
Widget build(BuildContext context) {
// UI code + network call + navigation
}
}
Problem:
This widget is doing too much:
- UI rendering
- Fetching user data
- Handling navigation
It has multiple reasons to change:
- UI design changes
- API structure changes
- Route logic changes
✅ Right:
Split into smaller classes:
// 1. Data Layer
class UserService {
Future<User> fetchUserData(String id) async {
// Only responsible for fetching user data
}
}
// 2. Navigation Helper
class NavigationHelper {
void goToEditProfile(BuildContext context) {
Navigator.pushNamed(context, '/editProfile');
}
}
// 3. UI Layer
class UserProfileWidget extends StatelessWidget {
final String userId;
final UserService userService;
final NavigationHelper navigator;
UserProfileWidget({
required this.userId,
required this.userService,
required this.navigator,
});
@override
Widget build(BuildContext context) {
// Only handles UI logic
}
}
🔑 Key Takeaways:
- One class = one purpose.
- Makes code easier to read, test, and maintain.
- Helps avoid spaghetti code in widgets.
- Widgets ≠ services ≠ business logic. Separate them!
🧠 Interview One-Liner:
“In Flutter, I follow SRP by ensuring that Widgets only build UI, Services handle data, and Helpers take care of navigation or logic.”
O - Open/Closed Principle (OCP)
📘 Definition:
Software entities should be open for extension, but closed for modification.
🧁 Real-Life Analogy:
Think of a custom cake shop 🍰.
The base cake recipe is fixed.
But customers can add new toppings (fruits, choco chips, nuts) without changing the original cake.
Similarly, in code:
You shouldn’t rewrite the original class every time a new requirement comes.
Instead, you should extend it — via inheritance, composition, or abstractions.
💡 Flutter-Specific Example:
❌ Wrong:
class PaymentProcessor {
void process(String type) {
if (type == 'credit') {
// Credit card logic
} else if (type == 'paypal') {
// PayPal logic
} else if (type == 'upi') {
// UPI logic
}
}
}
Problem:
Each time a new payment method is added, we have to modify the PaymentProcessor.
This violates OCP — it's not closed for modification
✅ Right:
Use abstraction with an interface:
abstract class PaymentMethod {
void pay(double amount);
}
class CreditCardPayment implements PaymentMethod {
@override
void pay(double amount) {
// Credit card logic
}
}
class PaypalPayment implements PaymentMethod {
@override
void pay(double amount) {
// PayPal logic
}
}
class UpiPayment implements PaymentMethod {
@override
void pay(double amount) {
// UPI logic
}
}
class PaymentProcessor {
final PaymentMethod method;
PaymentProcessor(this.method);
void process(double amount) {
method.pay(amount);
}
}
Now, if you want to add a new payment method (CryptoPayment), you just implement the interface.
No change to PaymentProcessor. ✔️
🔑 Key Takeaways:
- Design your classes so that you can extend behavior without editing existing code.
- Favor composition or interfaces over if-else/switch ladders.
- Makes your code future-proof and scalable.
🧠 Interview One-Liner:
I apply OCP in Flutter by using interfaces or abstract classes, so new features can be added without modifying existing logic — reducing bugs and regressions.
❤️ That’s a Wrap for Part 1!
We’ve just scratched the surface of the SOLID journey with the first two principles — Single Responsibility and Open/Closed — and how they apply beautifully in Flutter.
If you found this helpful, drop a like, share it with your dev circle, and don’t forget to leave your feedback or questions in the comments.
This is just the beginning!
👉 Stay tuned for Part 2, where we dive into:
The power of safe inheritance (Liskov)
Why small interfaces matter (Interface Segregation)
Let’s master clean code, one principle at a time. 🚀
Be part of the series — and let’s build better Flutter apps together!
Top comments (3)
I would like to suggest to develop a basic feature first approach flutter project with SOLID principle and state the git link in your next post. 💙 Happy fluttering...
Thanks for the suggestion! 🙌
Actually, I already build production-level Flutter projects following SOLID principles and clean architecture. In most cases, I focus heavily on:
S – Single Responsibility Principle
I – Interface Segregation Principle
D – Dependency Inversion Principle
These help keep the codebase modular, testable, and easy to maintain — especially as the project scales. Looking forward to sharing a sample project soon for others to explore and learn from! 💙
Super ✨