使用 Timer 而不是 Future.delayed(...) 创建异步等待步骤

Question

是否可以使用Timer而不是执行异步步骤Future.delayed？主要原因是类cancel()中方法的可用性Timer，它允许我在 Flutter 中处理小部件时取消正在运行的计时器。

import 'dart:async';

const kFirstStepDuration = Duration(seconds: 1);
const kSecondStepDuration = Duration(seconds: 1);
const kThirdStepDuration = Duration(seconds: 1);

void main() async{
  await Future.delayed(kFirstStepDuration);
  print('first step');
  await Future.delayed(kSecondStepDuration);
  print('second step');
  await Future.delayed(kThirdStepDuration);
  print('end');
}

---

使用 Richard Heap 答案后的计时器，我想尽可能取消这些步骤，但下面的代码提示：

first timer to begin
cancel first
cancel second
cancel third

我的期望是提示：

first timer to begin
cancel first
second timer to begin
cancel second
third timer to begin
cancel third
executed in 0:00:00.06000

import 'dart:async';

const kFirstStepDuration = Duration(seconds: 1);
const kSecondStepDuration = Duration(seconds: 1);
const kThirdStepDuration = Duration(seconds: 1);

Timer? firstTimer;
Timer? secondTimer;
Timer? thirdTimer;

  final firstCompleter = Completer();
  final secondCompleter = Completer();
  final thirdCompleter = Completer();

void main() {
  threadA();
  threadB();
}

void threadA() async {
  print('first timer to begin');
  firstTimer = Timer(kFirstStepDuration, ()=> firstCompleter.complete());
  await firstCompleter.future;
  
  print('second timer to begin');
  secondTimer = Timer(kSecondStepDuration, ()=> secondCompleter.complete());
  await secondCompleter.future;
  
  print('third timer to begin');
  thirdTimer = Timer(kThirdStepDuration, ()=> thirdCompleter.complete());
  await thirdCompleter.future;
}

void threadB() async {
  await Future.delayed(Duration(milliseconds: 20));
  firstTimer?.cancel();
  print('cancel first');
  await Future.delayed(Duration(milliseconds: 20));
  secondTimer?.cancel();
  print('cancel second');
  await Future.delayed(Duration(milliseconds: 20));
  thirdTimer?.cancel();
  print('cancel third');
}

Answer 1

awaitcomplete您在计时器回调中的未来。

print('starting');
final completer = Completer();
final t = Timer(Duration(seconds: 3), () => completer.complete());
await completer.future;
print('done');

这样做的问题是，如果取消计时器，完成器将无法完成，因为永远不会调用回调。所以你必须取消计时器并完成完成器。

在之后检查小部件是否仍然安装不是更容易吗Future.delayed？

编辑

更新您的问题后，您似乎没有遇到上面的“问题”。您可以将所有功能封装到一个类中，例如：

class PointlessTimer {
  PointlessTimer(Duration d) {
    _timer = Timer(d, () => _completer.complete());
  }

  late final Timer _timer;
  final _completer = Completer();

  Future get future => _completer.future;

  void cancel() {
    _timer.cancel();
    _completer.complete();
  }
}

并像这样使用它：

void main() async {
  print('starting');
  final pointless = PointlessTimer(Duration(seconds: 3));
  Timer(Duration(seconds: 2), () => pointless.cancel());
  await pointless.future;
  print('done');
}