Dart essentials

Sound null safety

Also called null safety

What

A type like String can never hold null; only String? can. The compiler forces you to handle the null case with ?., ??, a null check or late before you use the value.

Why

Null crashes are the most common runtime error in apps. Sound null safety moves them to compile time, so a missing value is handled on purpose instead of crashing a screen.

String? nickname;
final shown = nickname ?? 'Guest';
final length = nickname?.length;

if (nickname != null) {
  print(nickname.toUpperCase());
}

late final String token;

var, final and const

What

var can be reassigned, final is set once at runtime, and const is a compile-time constant that is created once and shared.

Why

Prefer final for most variables. const widgets are built once and Flutter can skip rebuilding them, which is a cheap performance win.

var count = 0;
count++;
final startedAt = DateTime.now();
const padding = EdgeInsets.all(16);
const title = Text('Settings');

Classes and immutable models

Also called immutable model, mixin

What

Dart classes have fields, constructors with named and required parameters, and factory constructors. App models are usually immutable, with a copyWith method to create changed copies.

Why

Immutable models make state changes explicit, which is what Flutter state management libraries expect and what makes bugs easy to trace.

class User {
  const User({required this.id, required this.name});
  final String id;
  final String name;

  User copyWith({String? name}) =>
      User(id: id, name: name ?? this.name);

  factory User.fromJson(Map<String, dynamic> j) =>
      User(id: j['id'] as String, name: j['name'] as String);
}

Sealed classes, records and patterns

Also called switch expression, pattern matching, sealed class

What

Dart 3 adds sealed class families, records such as (int, String) and patterns that destructure values. A switch over a sealed type must cover every subtype.

Why

Modelling states like loading, success and error as a sealed family means the compiler tells you when a screen forgets to handle one.

sealed class Shape {}
class Circle extends Shape { Circle(this.r); final double r; }
class Square extends Shape { Square(this.s); final double s; }

double area(Shape shape) => switch (shape) {
  Circle(:final r) => pi * r * r,
  Square(:final s) => s * s,
};

final (name, age) = ('Ana', 21);

List, Map and Set

What

The core collections: ordered List, key-value Map and unique Set. Collection if, collection for and the spread operator ... build them inline.

Why

Widget trees are built from lists, so collection if and for let you show or repeat children without temporary variables.

final scores = <String, int>{'ana': 90, 'li': 72};
final passed = [
  for (final e in scores.entries)
    if (e.value >= 75) e.key,
];
final everyone = {...passed, 'guest'};
final top = scores.values.reduce(max);

Futures, async and await

Also called event loop

What

A Future is a value that arrives later, such as an HTTP response. async marks a function that returns a Future, and await pauses it until the value is ready without blocking the UI.

Why

Every network call, file read and database query in Flutter is a Future, so you need to await them and handle their errors with try and catch.

Future<String> fetchName() async {
  await Future.delayed(const Duration(seconds: 1));
  return 'Ana';
}

Future<void> main() async {
  try {
    final (a, b) = await (fetchName(), fetchName()).wait;
    print('$a and $b');
  } catch (e) { print('Failed: $e'); }
}

Streams

What

A Stream delivers many values over time, like location updates or database changes. You read it with listen, await for or a StreamBuilder widget, and create one with an async* function.

Why

Anything that changes repeatedly, from a chat feed to a Firestore query, is modelled as a Stream so the UI can update as values arrive.

Stream<int> countdown(int from) async* {
  for (var i = from; i >= 0; i--) {
    await Future.delayed(const Duration(seconds: 1));
    yield i;
  }
}

final sub = countdown(3).listen(print);
await sub.cancel();

Isolates

What

Dart runs your code on one thread per isolate, and isolates share no memory. Isolate.run executes a function on a separate isolate and returns its result.

Why

Heavy work on the main isolate, like parsing a large JSON file or resizing images, blocks frames and makes the app stutter. Moving it to another isolate keeps scrolling smooth.

final items = await Isolate.run(() {
  final data = jsonDecode(hugeJson) as List<dynamic>;
  return data.map((e) => Item.fromJson(e)).toList();
});

Packages and pubspec.yaml

Also called pub.dev, pubspec

What

pubspec.yaml lists your app's dependencies, assets and SDK constraints. Packages come from pub.dev and are added with flutter pub add.

Why

Most features, such as HTTP, storage and notifications, come from packages, so you need to pick well-maintained ones and keep them up to date.

flutter pub add http
flutter pub add dev:build_runner
flutter pub get
flutter pub outdated