what is Typedefs and what is the use of Typedefs ?

Overview

Typedef (type alias) in Dart allows you to create a named alias for a function type or any type, making complex function signatures readable and reusable.

Typedef for Function Types

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// Without typedef -- hard to read
final Map<String, void Function(String, int)> handlers = {};

// With typedef -- much cleaner
typedef EventHandler = void Function(String event, int timestamp);

final Map<String, EventHandler> handlers = {};

void handleClick(String event, int timestamp) {
  print('$event at $timestamp');
}

handlers['click'] = handleClick; // Clean!

Common Uses

Callback Types

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// Define reusable callback signatures
typedef OnSuccess<T> = void Function(T data);
typedef OnError = void Function(String message, Exception? error);
typedef Predicate<T> = bool Function(T value);
typedef JsonMapper<T> = T Function(Map<String, dynamic> json);

// Use in function signatures
void fetchData({
  required OnSuccess<List<User>> onSuccess,
  required OnError onError,
}) async {
  try {
    final users = await api.getUsers();
    onSuccess(users);
  } catch (e) {
    onError('Fetch failed', e as Exception);
  }
}

// Use
fetchData(
  onSuccess: (users) => setState(() => _users = users),
  onError: (msg, e) => showSnackBar(msg),
);

Generic Typedef (Dart 2.13+)

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// Generic type alias
typedef Transformer<T, R> = R Function(T input);

Transformer<String, int> strLength = (s) => s.length;
Transformer<int, String> intToStr = (n) => n.toString();

print(strLength('hello')); // 5
print(intToStr(42));       // '42'

Non-function Typedef (Dart 2.13+)

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// Type alias for any type -- not just functions
typedef IntList = List<int>;
typedef UserMap = Map<String, User>;
typedef Coordinate = (double, double); // Record type alias

IntList numbers = [1, 2, 3];  // Same as List<int>
UserMap cache = {};            // Same as Map<String, User>
Coordinate location = (12.97, 77.59);

In Flutter -- Practical Examples

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// Widget builder typedef
typedef WidgetBuilder = Widget Function(BuildContext context);
typedef IndexedWidgetBuilder = Widget Function(BuildContext context, int index);

// Builder pattern
void showCustomDialog({
  required WidgetBuilder builder,
}) {
  showDialog(context: context, builder: builder);
}

// State management
typedef StateUpdater<T> = T Function(T currentState);

class Store<T> {
  T _state;
  Store(this._state);
  void update(StateUpdater<T> updater) => _state = updater(_state);
}

Typedef vs inline Function Types

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// Inline -- valid but verbose in complex cases
void process(void Function(String, int) callback) { ... }

// Typedef -- reusable and named
typedef Callback = void Function(String, int);
void process(Callback callback) { ... } // Cleaner

Best Use: Use typedef to name complex function signatures, make callback types reusable across files, and improve code readability. Dart 2.13+ also supports non-function type aliases.