language-dart

Dart

Null safety

• Sound null safety is on by default (Dart ≥2.12) — no unsound nulls sneak through.
• Non-nullable is the default; append ? only when a value can genuinely be absent (String? name).
• late defers initialization for values assigned before first read (DI fields, initState fields) — a late field read before assignment throws.
• ! (null-assertion) asserts non-null; prefer narrowing (if (x != null)) or ?./?? over !, since ! throws at runtime.
• ??= for lazy default assignment; ?? for fallback expressions.

String greet(String? name) => 'Hello, ${name ?? 'stranger'}';

Async — Futures and Streams

• Future<T> wraps a value that arrives later; async/await reads like sync code.
• Unhandled Future errors surface as unhandled exceptions — always try/catch around await, or attach .catchError.
• Stream<T> models a sequence over time; await for consumes it, StreamController produces it.
• Completer<T> bridges callback-based APIs into a Future.
• Future.wait([...]) for concurrent futures; don't await sequentially in a loop when the work is independent.

Future<User> fetchUser(String id) async {
  try {
    final res = await api.get('/users/$id');
    return User.fromJson(res.data);
  } on DioException catch (e) {
    throw UserFetchException(id, e);
  }
}

Widget composition

• StatelessWidget is the default; reach for StatefulWidget only when the widget owns mutable state across rebuilds.
• build() must be pure and fast — no side effects, no I/O; it can run many times per frame.
• Compose small widgets instead of deep inheritance — extract a widget class, not a private _buildX() method, when it needs its own const or rebuild boundary.
• const constructors on leaf widgets let Flutter skip rebuilding subtrees whose inputs didn't change.
• Pass a Key (ValueKey, ObjectKey) when reordering or diffing a list of like-typed widgets, or state attaches to the wrong element.