dart - async Future StreamSubscription Error -

could please explain what's wrong following code. i'm making 2 calls function finputdata. first works ok, second results in error : "unhandled exception" "bad state: stream has subscriber"

i need write test console program inputs multiple parameters.

import "dart:async" async; import "dart:io";   void main() {   finputdata ("enter nr of iterations : ")   .then((string sresult){     int iiters;     try {       iiters = int.parse(sresult);       if (iiters < 0) throw new exception("invalid");     } catch (oerror) {       print ("invalid entry");       exit(1);     }      print ("in main : iterations selected = ${iiters}");      finputdata("continue processing? (y/n) : ")   // call bombs     .then((string sinput){       if (sinput != "y" && sinput != "y")         exit(1);        fprocessdata(iiters);        print ("main completed");     });   }); }  async.future<string> finputdata(string sprompt) {   async.completer<string> ocompleter = new async.completer();     stdout.write(sprompt);   async.stream<string> ostream = stdin.transform(new stringdecoder());   async.streamsubscription osub;   osub = ostream.listen((string sinput) {     ocompleter.complete(sinput);     osub.cancel();   });   return ocompleter.future; }  void fprocessdata(int iiters) {   print ("in fprocessdata");   print ("iiters = ${iiters}");    (int ipos = 1; ipos <= iiters; ipos++ ) {     if (ipos%100 == 0) print ("processed = ${ipos}");   }   print ("in fprocessdata - completed ${iiters}"); }  

some background reading:

streams comes in 2 flavours: single or multiple (also known broadcast) subscriber. default, our stream single-subscriber stream. means if try listen stream more once, exception, , using of callback functions or future properties counts listening.

you can convert single-subscriber stream broadcast stream using asbroadcaststream() method.

so you've got 2 options - either re-use single subscription object. i.e. call listen once, , keep subscription object alive.

or use broadcast stream - note there number of differences between broadcast streams , single-subscriber streams, you'll need read , make sure suit use-case.

here's example of reusing subscriber ask multiple questions:

import 'dart:async'; import 'dart:io';  main() {     var console = new console();     var loop;     loop = () => ask(console).then((_) => loop());     loop(); }  future ask(console console) {     print('1 + 1 = ...');     return console.readline().then((line) {         print(line.trim() == '2' ? 'yup!' : 'nope :(');     }); }  class console {     streamsubscription<string> _subs;      console() {         var input = stdin             .transform(new stringdecoder())             .transform(new linetransformer());          _subs = input.listen(null);     }      future<string> readline() {         var completer = new completer<string>();         _subs.ondata(completer.complete);         return completer.future;     } }