Some explanations about completer

Question

Hello everyone

I have one class 'Example' who needs to do some computation. I call start() which call _next(). During the computation _next() calls itself couple of time but in my example I simulate that with a Timer. here is my code

import "dart:async";


main() {

    Example ex = new Example();

    for (var i = 0 ; i < 3 ; i++) {
        ex.start().then((nbr) {
            print(nbr);
        });
    }

}

class Example {

    /// for _next
    Completer _insideCompleter;

    /// from start() to outside
    Completer _outsideCompleter;

    Example();

    /// start is just a better public api than next when we start the exercise
    Future<int> start() {

        _insideCompleter = new Completer();
        _outsideCompleter = new Completer();


        _next().then((int value) {
            print("value: $value");
            _outsideCompleter.complete(value);

        }).catchError((message) {
            print("information: $message");
        });

        return _outsideCompleter.future;
    }


    /// _next handle the flow with the status
    Future<int> _next() {

        new Timer(new Duration(seconds: 6), () {
            _insideCompleter.complete(15);
        });

        return _insideCompleter.future;
    }
}

it finishes with : Bad state: Future already completed. but as you can see in the start(). the Completer are re-created with new. So I don't understand why it is already complete.

If anybody can explain why it's not correct to code like that and maybe give me some interesting links it would be great

Cheers!

Answer 1

I'm not entirely sure what your intention with the code is but I think you should either

Example ex = new Example();

for (var i = 0 ; i < 3 ; i++) { // create a new Example() for each iteration here
    ex.start().then((nbr) {     // or ensure that the next iteration is not executed 
        print(nbr);             // before the previous is completed.
    });
}

with this code ex.start() is called 3 times before the first call is completed.

Answer 2

Here the main issue is about completers called in the callback function

_next().then((int value) {
            print("value: $value");
            _outsideCompleter.complete(value); // this line

        })

and

 new Timer(new Duration(seconds: 6), () {
            _insideCompleter.complete(15); // this line
        });

Because this 2 function are called after your loop and your completer are attribute, all the callback will use the latest _outsideCompleter and _insideCompleter created.

So after than one of the callback have "consume" your completer, the others will create exception of 'Bad state: Future already completed'

here a version that works

import "dart:async";


main() {

  Example ex = new Example();

  for (var i = 0 ; i < 3 ; i++) {
    ex.start().then((nbr) {
      print(nbr);
    });
  }

}

class Example {

  Example();

  /// start is just a better public api than next when we start the exercise
  Future<int> start() {

    var outsideCompleter = new Completer(); // create localy each times


    _next().then((int value) {
      print("value: $value");
      outsideCompleter.complete(value);

    }).catchError((message) {
      print("information: $message");
    });

    return outsideCompleter.future;
  }


  /// _next handle the flow with the status
  Future<int> _next() {
    var insideCompleter = new Completer(); // create localy each times

    new Timer(new Duration(seconds: 6), () {
      insideCompleter.complete(15);
    });

    return insideCompleter.future;
  }
}