Dartlang 等待不止一个未来

我想做的东西后，很多未来的函数都做了，但我不知道如何写的代码在省道？ 代码是这样的:

for (var d in data) {
d.loadData().then()
}
// when all loaded
// do something here

但我不想一个一个的等:

for (var d in data) {
await d.loadData(); // NOT NEED THIS
}

怎么用 Dart 写这些代码？

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小开

You can use Future.wait to wait for a list of futures:

import 'dart:async';


Future main() async {
var data = [];
var futures = <Future>[];
for (var d in data) {
futures.add(d.loadData());
}
await Future.wait(futures);
}

DartPad example

小开

Existing answer gives enough information, but I want to add a note/warning. As stated in the docs:

The value of the returned future will be a list of all the values that were produced in the order that the futures are provided by iterating futures.

So, that means that the example below will return 4 as the first element (index 0), and 2 as the second element (index 1).

import 'dart:async';


Future main() async {
print('start');


List<int> li = await Future.wait<int>([
fetchLong(),  // longer (which gives 4) is first
fetchShort(), // shorter (which gives 2) is second
]);


print('results: ${li[0]} ${li[1]}'); // results: 4 2
}


Future<int> fetchShort() {
return Future.delayed(Duration(seconds: 3), () {
print('Short!');
return 2;
});
}


Future<int> fetchLong() {
return Future.delayed(Duration(seconds: 5), () {
print('Long!');
return 4;
});
}
小开

If you want to wait for multiple futures of different types and also support null-safety then you can add a helper function similar to the following.

import 'package:tuple/tuple.dart';


Future<Tuple2<T1, T2>> waitConcurrently<T1, T2>(
Future<T1> future1, Future<T2> future2) async {
late T1 result1;
late T2 result2;


await Future.wait([
future1.then((value) => result1 = value),
future2.then((value) => result2 = value)
]);


return Future.value(Tuple2(result1, result2));
}

In order for this to work you need tuples. At the moment Dart does not provide tuples natively, but there is a package from Google which does: https://pub.dev/packages/tuple

小开

In addition, I'd like to supplement Günter Zöchbauer's answer with FutureOr variant. You'll need to convert your FutureOr<T> variable to Future<T> first and then call wait:

Future.wait(list.map((x) async => x))

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