flutter - 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 Future.delayed 执行异步步骤？主要原因是 Timer 类中提供了 cancel() 方法，该方法允许我在 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');
}

---

使用理查德·希普回答后的计时器，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

等待您在计时器回调中完成的 future 。

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，完成程序将无法完成，因为回调函数永远不会被调用。所以你必须取消计时器并完成完成器。

在 Future.delayed 之后检查小部件是否仍然安装不是更容易吗？

编辑

更新您的问题后，您似乎没有遇到上述“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');
}