Abstract Classes and Interfaces

Beyond Classes: Interfaces, Mixins, Generics, and Async Work

This chapter tours the rest of Dart's toolkit in brief. Prerequisites: you already know basic Dart classes and methods from earlier chapters.

Abstract Classes and Interfaces

An abstract class is a blueprint. It lists what its children must do but does not do it itself. You cannot create one directly. Any class in Dart automatically acts as an interface too — a promise of certain behaviors — using the implements keyword.

abstract class Shape {
  double area();
}

class Circle implements Shape {
  final double radius;
  Circle(this.radius);
  @override
  double area() => 3.14159 * radius * radius;
}

void main() {
  final Shape s = Circle(2);
  print(s.area()); // 12.56636
}

Mixins

A mixin shares behavior across unrelated classes. Use mixin to define one and with to add it.

mixin Logger {
  void log(String msg) => print('[log] $msg');
}

class Bank with Logger {}

Generics

Generics let you write one function or class that works with any type, safely. List<String> means "a list that only holds text."

T firstOf<T>(List<T> items) => items[0];

Exceptions and Error Handling

An exception is a problem that stops your code mid-run. Wrap risky work in try, and respond in catch.

try {
  int x = int.parse('oops');
} catch (e) {
  print('failed: $e');
}

Futures, async, and await

A Future is a value that arrives later, like a meal ordered at a restaurant. Mark a function async, then use await to pause until the value arrives — without freezing your program.

Future<String> fetch() async {
  await Future.delayed(Duration(seconds: 1));
  return 'done';
}

Streams

A Stream is like a Future, but it delivers many values over time — like a radio broadcast.

await for (final i in Stream.periodic(Duration(milliseconds: 100), (i) => i).take(3)) {
  print('tick $i');
}

Isolates

Dart runs your code in isolates: separate workers with their own memory that talk by passing messages. This avoids the locking headaches of shared-memory threading.

Packages, pub, and Tooling

Pub is Dart's package manager — a storehouse of free code others wrote. Run dart pub add <package> to use one, and dart run to execute your program. The dart command also formats and analyzes your code for you.