Typedef in Dart

What is typedef?

A typedef creates an alias for a type — most commonly function types. It improves code readability by giving complex function signatures a meaningful name.

Function Type Aliases

// Without typedef:
void Function(String, int) callback;
List<Map<String, dynamic>> Function(String) queryFn;

// With typedef:
typedef Callback = void Function(String message, int code);
typedef QueryFn = List<Map<String, dynamic>> Function(String sql);
typedef Predicate<T> = bool Function(T value);
typedef Transformer<T, R> = R Function(T input);

// Usage:
void runWithCallback(Callback callback) {
  callback('Success', 200);
}

void main() {
  Callback myCallback = (msg, code) => print('[$code] $msg');
  runWithCallback(myCallback);  // [200] Success
}

Generic Typedef

typedef JSON = Map<String, dynamic>;
typedef JsonList = List<Map<String, dynamic>>;
typedef Mapper<T, R> = R Function(T);
typedef Comparator<T> = int Function(T a, T b);

// Use in a sortable list:
List<T> sortBy<T>(List<T> items, Comparator<T> compare) {
  var sorted = [...items];
  sorted.sort(compare);
  return sorted;
}

void main() {
  var numbers = [3, 1, 4, 1, 5, 9, 2, 6];
  var sorted = sortBy(numbers, (a, b) => a.compareTo(b));
  print(sorted);  // [1, 1, 2, 3, 4, 5, 6, 9]

  JSON user = {'name': 'Alice', 'age': 30};
  print(user['name']);  // Alice
}

Practical Example: Event System

typedef EventHandler<T> = void Function(T event);
typedef AsyncEventHandler<T> = Future<void> Function(T event);

class EventBus<T> {
  final List<EventHandler<T>> _handlers = [];

  void on(EventHandler<T> handler) => _handlers.add(handler);
  void emit(T event) { for (var h in _handlers) h(event); }
}

void main() {
  var bus = EventBus<String>();
  bus.on((msg) => print('Handler 1: $msg'));
  bus.on((msg) => print('Handler 2: $${msg.toUpperCase()}'));
  bus.emit('hello');
}

Handler 1: hello
Handler 2: HELLO

🧠 Quiz

Summary

typedef creates named type aliases, primarily for function signatures. Use typedef Name = ReturnType Function(ParamTypes). Generics work with typedef — typedef Mapper<T, R> = R Function(T). Typedef improves readability and is great for callback patterns and event systems.