如何在 Dart 中实现即发即忘功能

Question

我有以下 Dart 程序：

import 'package:pedantic/pedantic.dart';

Future someFunc() async {
  for (var i = 0; i < 100; i++) {
    print('Value of i in someFunc(): ' + i.toString());
  }
}

Future someOtherFunc() async {
  for (var i = 0; i < 100; i++) {
    print('Value of i in someOtherFunc(): ' + i.toString());
  }
}

Future<int> main() async {
  unawaited(someFunc());
  unawaited(someOtherFunc());
  print('End of program...');

  return 0;
}

当我执行这个时，输出是：

Value of i in someFunc(): 0
Value of i in someFunc(): 1
Value of i in someFunc(): 2 
...
Value of i in someFunc(): 99
Value of i in someOtherFunc(): 0
Value of i in someOtherFunc(): 1
Value of i in someOtherFunc(): 2 
...
Value of i in someOtherFunc(): 99
End of program...

看起来一切都是同步执行的。我期待类似的事情：

Value of i in someFunc(): 0
Value of i in someFunc(): 1
Value of i in someOtherFunc(): 0
Value of i in someFunc(): 3
End of program...
Value of i in someOtherFunc(): 1
(etc...)

如何在 Dart 中创建异步“即发即忘”函数/方法？

Answer 1

您看到此代码同步执行的原因是因为 Dart 中的异步方法执行起来就好像它们是同步的，直到第一个await，此时该方法的执行将暂停，直到await完成。如果您在方法中执行实际的异步工作，您会看到如您所期望的交错执行。

例如，对您的代码进行一些细微的修改，大致会产生您期望的行为：

import 'package:pedantic/pedantic.dart';

Future someFunc() async {
  for (var i = 0; i < 100; i++) {
    // Future.microtask is used to schedule work on the microtask queue,
    // which forces this method to suspend execution and continue on to someOtherFunc.
    // If you were doing some actual asynchronous work, you wouldn't need this.
    await Future.microtask(() => print('Value of i in someFunc(): ' + i.toString()));
  }
}

Future someOtherFunc() async {
  for (var i = 0; i < 100; i++) {
    await Future.microtask(() => print('Value of i in someOtherFunc(): ' + i.toString()));
  }
}

Future<int> main() async {
  unawaited(someFunc());
  unawaited(someOtherFunc());
  print('End of program...');

  return 0;
}

我强烈建议您阅读Dart 事件循环和微任务队列，这将使您更好地了解异步在 Dart 中的工作原理