SOLID principles with Dart examples: a quick guide for Flutter developers

As software engineers, we love clean, maintainable, and scalable code. But sometimes, our code turns into a tangled mess that’s hard to debug and even harder to maintain. That’s where the SOLID principles come in! These five principles are like a roadmap for writing better code. In this article, I’ll break down each principle with a simple Dart example, an easy-to-understand explanation, and common errors it helps to avoid.

Single Responsibility Principle (SRP)

Definition: A class should have one, and only one, reason to change.

Error avoided: Changes in one responsibility won’t unintentionally affect another. For instance, if you modify the notification logic, it won’t risk breaking order processing.

Explanation:

Each class should do one thing and one thing only. In the example above, OrderService is responsible for processing orders, while NotificationService handles sending notifications. By keeping these responsibilities separate, each class remains focused, making the code easier to read, test, and maintain.

Open/Closed Principle (OCP)

Definition: A class should be open for extension but closed for modification.

Error avoided: Frequent changes to existing code can introduce bugs. Here, adding a new payment method won’t disrupt the already-tested payment classes.

Explanation:

Your code should be designed to add new functionality without changing existing code. In this example, we can introduce new payment methods (e.g., ApplePayPayment) by creating a new class that implements Payment. The existing CreditCardPayment and PaypalPayment remain untouched. This reduces the risk of breaking old functionality when adding new features.

Liskov Substitution Principle (LSP)

Definition: Subclasses should be replaceable for their base class without altering the program's behavior.

Error avoided: Replacing a base class with a subclass should never break the program. Without LSP, using a Penguin in place of a Bird could cause exceptions or unexpected behavior.

Explanation:

Subclasses must adhere to the behavior expected from their parent class. In the example, both Sparrow and Penguin extend Bird and implement the move() method. This ensures the program behaves correctly, regardless of which Bird subclass is used.

Interface Segregation Principle (ISP)

Definition: Classes should not be forced to implement methods they don’t use.

Error avoided: Bloated interfaces force classes to implement methods they don’t need, leading to fragile, hard-to-maintain code. ISP keeps interfaces lean and focused.

Explanation:

Instead of creating one giant interface with many unrelated methods, break it into smaller, more specific interfaces. In the example, we split the functionality into Printer and Scanner. If a device only needs to print, it can implement Printer without being forced to handle scanning.

Dependency Inversion Principle (DIP)

Definition: High-level modules should depend on abstractions, not concrete implementations.

Error avoided: Tight coupling between high-level and low-level modules makes the code hard to adapt or test. DIP ensures loose coupling, allowing easy changes and better testing

Explanation:

High-level modules (like DataManager) should not depend on low-level modules (like FirebaseDatabase). Instead, both should rely on abstractions (like Database). This makes the code more flexible. For example, you could switch from Firebase to another database by providing a new implementation of Database.

By applying SOLID principles in your Flutter projects, you’ll write code that’s cleaner, more modular, and easier to maintain. Whether it’s preventing unintended side effects, reducing bugs, or preparing your code for future changes, these principles are the key to building robust apps. Start small, pick one principle, refactor your code, and feel the difference. Happy coding!