如何在 Dart/Flutter 中同时使用 Timer 和 await？

Question

I have a code like below:

Timer(Duration(seconds: 5),(){
   print("This is printed after 5 seconds.");
});
print("This is printed when Timer ends");

How can I use "await" in this case? I want when Timer ends then run next code below Timer. I used Future.delayed(), it can do it but I can't skip the delay time in Future.delayed() like in Timer(). Because i want to skip the delay time if the condition is true. Because I want to skip the delay time if the condition is true. If I use Future.delayed(), it doesn't have cancel() method like Timer(). Please tell me the solution. Thanks

Answer 1

Try Future.delayed instead of Timer

await Future.delayed(Duration(seconds: 5),(){

                print("This is printed after 5 seconds.");
              });

              print('This is printed when Timer ends');

Answer 2

Timer(Duration(seconds: 5),(){
   print("This is printed after 5 seconds.");
   printWhenTimerEnds();
});

void printWhenTimerEnds(){
 print("This is printed when Timer ends");
}

When you wish to skip the timer just call timer cancel and the printWhenTimerEnds() method

Answer 3

There are several ways to achieve what you need. You can useCompleter and Future.any like this:

import 'dart:async';

Completer<void> cancelable = Completer<void>();

// auxiliary function for convenience and better code reading
void cancel() => cancelable.complete();

Future.any(<Future<dynamic>>[
  cancelable.future,
  Future.delayed(Duration(seconds: 5)),
]).then((_) {
  if (cancelable.isCompleted) {
    print('This is print when timer has been canceled.');
  } else {
    print('This is printed after 5 seconds.');
  }
});

// line to test cancel, comment to test timer completion
Future.delayed(Duration(seconds: 1)).then((_) => cancel());

Basically we are creating two futures one is a delayed and another one is cancelable future. And we are waiting for the first one that completes using Future.any .

The other option is to use CancelableOperation or CancelableCompleter .

For example:

import 'dart:async';
import 'package:async/async.dart';

Future.delayed(Duration(seconds: 1)).then((_) => cancel());

var cancelableDelay = CancelableOperation.fromFuture(
  Future.delayed(Duration(seconds: 5)),
  onCancel: () => print('This is print when timer has been canceled.'),
);

// line to test cancel, comment to test timer completion
Future.delayed(Duration(seconds: 1)).then((_) => cancelableDelay.cancel());

cancelableDelay.value.whenComplete(() {
  print('This is printed after 5 seconds.');
});

Here we do practically the same thing as above but with already available classes. We are wrapping Future.delayed into CancelableOperation so we are able now to cancel that operation ( Future.delayed in our case).

Another way you can wrap Timer into future with Completer and so on.