Typedef In Flutter

Typedef In Flutter :

Hello everybody friends, in this post, learn to typedef and how to use in dart.

typedef :

  • We declare a function with a name (alias) which we want, i.e. join two string is return String.
  • Typedefs are alias for functions which are object. So, Typedefs are object with an alias =).
  • VoidCallback, a synchronous version of this signature.
  • pageBuilder RoutePageBuilder typedef – just a function that match with the RoutePageBuilder typedef signature.
  • transitionsBuilder RouteTransitionsBuilder typedef – exactly as pageBuilder but matching the RouteTransitionsBuilder signature.
Typedef In Flutter
typedef bool FunctionName(String userName, String password);

bool functionName(String value1, String value2) {
  if (value1 == 'kamlesh' && value2 == 'kamlesh') {
    return true;
  return false;

main() {
  var x = functionName;
  var result = getValue(functionName, 'kamlesh', 'kamlesh');

bool getValue(FunctionName function, String userName, String password) {
  return function(userName, password);

typedef join :

typedef String Join(String first, String second);

main() {
  Join dotJoin = (String first, String second) => dot(first, second);
  print("${joinWith(dotJoin, "Hello", "Typedef!")}");
  print("${joinWith(snakeCase, "Hello", "Typedef!")}");

String dot(String first, String second) => "$first.$second";
String snakeCase(String first, String second) => "${first}_$second";
String joinWith(Join join, String first, String second) {
  return join(first, second);

Flutter ListView use typedef :

  • items is length. Check the length > 0 if display _buildList() else PlaceholderContent()
  • itemBuilder i.e. ListView one of property itemBuilder.
typedef Widget ItemWidgetBuilder<T>(BuildContext context, T item);
class ListItemsBuilder<T> extends StatelessWidget {
  ListItemsBuilder({this.items, this.itemBuilder});
  final List<T> items;
  final ItemWidgetBuilder<T> itemBuilder;

  Widget build(BuildContext context) {
    if (items != null) {
      if (items.length > 0) {
        return _buildList();
      } else {
        return PlaceholderContent();
    } else {
      return Center(child: CircularProgressIndicator());

  Widget _buildList() {
    return ListView.builder(
        itemCount: items.length,
        itemBuilder: (context, index) {
          return itemBuilder(context, items[index]);

 Widget _buildContent() {
    return ListItemsBuilder<Counter>(
        items: _counters,
        itemBuilder: (context, counter) {
          return Text(counter);

RestAPI call :

typedef bool LoginRequest(String email, String password);

class Account {
  static LoginRequest login(ApiState apiState) =>
      (username, password) => _login(apiState.send, apiState.token, email, password);

class ApiState {
  final String token;
  final HttpSend send;
  const ApiState(this.send, this.token);

typedef Response HttpSend(Request request);

Response send(Request request) {
  return Response("error", 404);

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