使用 Timer 而不是 Future.delayed(...) 創建異步等待步驟

Question

是否可以使用Timer而不是Future.delayed執行異步步驟？ 主要原因是Timer class 中的cancel()方法的可用性，它允許我在 Flutter 中放置小部件時取消正在運行的計時器。

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 回答后的計時器，我想盡可能取消這些步驟，但下面的代碼會提示：

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

await您在計時器回調中complete的未來。

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

問題是如果定時器被取消，完成者將無法完成，因為回調永遠不會被調用。 所以你必須取消計時器並完成完成者。

在Future.delayed之后檢查小部件是否仍然安裝會更容易嗎？

編輯

在更新您的問題之后，您似乎沒有遇到上述“問題”。 您可以將所有功能封裝到 class 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();
  }
}

並像這樣使用它：

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

Answer 2

遵循 Richard Heap 的回答：

/// works as a Future.delayed. Returns a timer to be canceled.
/// useful when disposing widgets before a timer completion.
/// necessary when testing widgets that disposes before the timer completion.
  Future<Timer> _delayed(Duration duration) async {
    final completer = Completer();
    final timer = Timer(duration, () {
      completer.complete();
    });
    await completer.future;
    return timer;
  }