How to declare async function as a variable in flutter(dart)? - flutter

Question

In flutter, we can declare a function as variable and call it like this
MyWidget((){print('HI');});
class MyWidget extends StatelessWidget{
final Function sayHi;
MyWidget(this.sayHi);
#override
Widget build(BuildContext context) {
sayHi();
return ...
}
}
But what if sayHi() is a async function? How to declare a async function as variable? There seems no class like AsyncFunction. So how to achive that?

Answer 1

Async functions are normal functions with some sugar on top. Here, the function variable type just needs to specify that it returns a Future:
class Example {
Future<void> Function() asyncFuncVar;
Future<void> asyncFunc() async => print('Do async stuff...');
Example() {
asyncFuncVar = asyncFunc;
asyncFuncVar().then((_) => print('Hello'));
}
}
void main() => Example();
Hope this helps.

Answer 2

Like this
class MyApp extends StatelessWidget {
void asyncFunc() async {
await Future.delayed(Duration(seconds: 5));
print('printed after 5 seconds');
}
#override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
debugShowCheckedModeBanner: false,
home: Scaffold(
body: Center(
child: MyWidget(asyncFunc),
),
),
);
}
}
class MyWidget extends StatelessWidget {
final void Function() asyncFunc;
const MyWidget(this.asyncFunc);
#override
Widget build(BuildContext context) {
return MaterialButton(
onPressed: asyncFunc,
);
}
}