Lesson 38 of 59

5 mins read

Class Modifiers: sealed, final, base & interface

In Plain English

Dart 3 introduced fine-grained **Class Modifiers** to control how classes can be used across library boundaries. **`sealed`** creates closed algebraic data type hierarchies with exhaustive pattern matching; **`final`** disallows external subtyping; **`base`** enforces inheritance without implementation; and **`interface`** allows implementation without extension.

The Mental Model

"Class modifiers are like property zoning laws: "Residential Only" (interface) means you can build a house that looks like the blueprint, while "Historical Landmark" (final) means nobody is allowed to alter or extend the structure."

Deep Dive: How It Works

`sealed`: All subclasses must be declared in the same library file. Enables compiler exhaustiveness in switch expressions.

`final`: Outside the defining library, the class cannot be extended, implemented, or mixed in.

`base`: Outside the defining library, classes can only `extend` (preserving base constructor invariants), disallowing `implements`.

`interface`: Outside the defining library, classes can only `implements`, disallowing `extends`.

Syntax Blueprint

// Sealed hierarchy for state
sealed class ViewState {}
class Loading extends ViewState {}
class Success extends ViewState { final String data; Success(this.data); }
class Failure extends ViewState { final String err; Failure(this.err); }

Sealed class establishing an exhaustive state hierarchy.

Core Rules to Remember

Exhaustive Sealed Classes: Switches on sealed classes require no default case; missing subtypes cause compile errors.

API Boundary Control: Prevent consumers from breaking internal class assumptions.

Algebraic Data Types (ADT): The standard architectural pattern for modern state management in Flutter.

Common Beginner Traps & How to Fix Them

Mistake:Trying to extend a `sealed` class in a different `.dart` file.

Why it happens: Sealed classes require all subtypes to be defined in the same file to guarantee compile-time exhaustiveness.

How to fix: Define all subtypes of the sealed class in the same file.

Pro Tip: Use `sealed class` for UI states (Loading, Data, Error) to guarantee all states are handled by your widget rendering logic.

Live Interactive Example

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Sealed Class Hierarchy and Exhaustive Switch

Python 3.12

Output Console

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How it works: The sealed class enabled clean object destructuring and exhaustive compiler validation in switch expression.

Your Turn: Micro Challenge

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Micro Exercise

Define Sealed Auth State

Define `sealed class AuthState {}`.
Define `class AuthLoggedIn extends AuthState {}` and `class AuthLoggedOut extends AuthState {}`.
Write function `String getAuthText(AuthState s) => switch (s) { AuthLoggedIn() => "IN", AuthLoggedOut() => "OUT" };`.
In `main()`, print `Auth: ${getAuthText(AuthLoggedIn())}`.

Sandbox Output

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