Future, async & await

It is possible for an external function to return a Future value.

To make things easy, we added a simple wrapper for Dart's Future object. You can use the 'then' method to do something when the Dart Future is completed.

Example:

import 'package:hetu_script/hetu_script.dart';

Future<void> fetch() {
  // Imagine that this function is fetching user info from another service or database.
  return Future.delayed(
      const Duration(seconds: 2), () => print('Hello world after 2 seconds!'));
}

void main() {
  final hetu = Hetu();
  hetu.init(externalFunctions: {'fetch': fetch});
  hetu.eval(r'''
      external fun fetch
      final future = fetch()
      future.then((value) {
        print('future completed!')
      })
  ''');
}

Create a Future

After 0.4.0, you can manually create a Future object by using the constructor of the Future class binding:

final a = Future( () => 42 )
a.then( (value) => print(value) )
print(41)

The above output should be 41 then 42.

And you could also create a async function by add async keyword before its definition block:

// for declaration
fun testAsync async {
  // some code
}

// for literal function
() async {
  // some code
} ().then((value) {
  // you can immediately call this future literal function to do things just like in Javascript.
})

Hetu will create a Future object by using the script function as its argument.

Wait for a list of future

After 0.4.0, you can use the static wait api on Future class to wait for a bunch of values to be returned.

async asyncFunc1 => 41
async asyncFunc2 => 42

Future.wait([asyncFunc1(), asyncFunc2()]).then((valueList) {
  print(valueList)
})

Above code will print:

[
  41,
  42
]

await

After Hetu version 0.5.0, it possible to use await keyword within async function to avoid complex then callbacks.

However, this is an experimental feature, and it's only partly supported.

You can now use await in expressions statement (like a function call statement), variable declaration initialization,

but you still cannot use await within For statement initialization or a function call's arguments in this version.

Example:

final result = await fetch() * await valueFuture() * await sumAll();