[web] Bad state: stream has already been listened too

[sarbogast]

On my web app, I have 2 different screens where I want to use MobileScanner. Both are stateful widgets where I initialize the controller in the state, and dispose it in the state's dispose() function.

When I use the scanner in the first screen, everything works fine. Then I switch to the other screen and I get this error:

======== Exception caught by flutter web plugins ===================================================
The following StateError was thrown during a framework-to-plugin message:
Bad state: Stream has already been listened to.

When the exception was thrown, this was the stack: 
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49  throw_
dart-sdk/lib/async/stream_controller.dart 676:7                               [_subscribe]
dart-sdk/lib/async/stream_controller.dart 827:19                              [_createSubscription]
dart-sdk/lib/async/stream_impl.dart 473:9                                     listen
packages/flutter_web_plugins/src/plugin_event_channel.dart 136:38             _listen
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50            <fn>
dart-sdk/lib/async/zone.dart 1685:54                                          runUnary
dart-sdk/lib/async/future_impl.dart 159:18                                    handleValue
dart-sdk/lib/async/future_impl.dart 766:44                                    handleValueCallback
dart-sdk/lib/async/future_impl.dart 795:13                                    _propagateToListeners
dart-sdk/lib/async/future_impl.dart 465:9                                     callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                              _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                               _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15           <fn>
====================================================================================================

======== Exception caught by services library ======================================================
The following MissingPluginException was thrown while activating platform stream on channel dev.steenbakker.mobile_scanner/scanner/event:
MissingPluginException(No implementation found for method listen on channel dev.steenbakker.mobile_scanner/scanner/event)

When the exception was thrown, this was the stack: 
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49  throw_
packages/flutter/src/services/platform_channel.dart 175:7                     _invokeMethod
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50            <fn>
dart-sdk/lib/async/zone.dart 1685:54                                          runUnary
dart-sdk/lib/async/future_impl.dart 159:18                                    handleValue
dart-sdk/lib/async/future_impl.dart 766:44                                    handleValueCallback
dart-sdk/lib/async/future_impl.dart 795:13                                    _propagateToListeners
dart-sdk/lib/async/future_impl.dart 601:5                                     [_completeWithValue]
dart-sdk/lib/async/future_impl.dart 639:7                                     callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                              _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                               _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15           <fn>
====================================================================================================

[Adeel-Sultan]

On my web app, I have 2 different screens where I want to use MobileScanner. Both are stateful widgets where I initialize the controller in the state, and dispose it in the state's dispose() function.

When I use the scanner in the first screen, everything works fine. Then I switch to the other screen and I get this error:

======== Exception caught by flutter web plugins ===================================================
The following StateError was thrown during a framework-to-plugin message:
Bad state: Stream has already been listened to.

When the exception was thrown, this was the stack: 
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49  throw_
dart-sdk/lib/async/stream_controller.dart 676:7                               [_subscribe]
dart-sdk/lib/async/stream_controller.dart 827:19                              [_createSubscription]
dart-sdk/lib/async/stream_impl.dart 473:9                                     listen
packages/flutter_web_plugins/src/plugin_event_channel.dart 136:38             _listen
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50            <fn>
dart-sdk/lib/async/zone.dart 1685:54                                          runUnary
dart-sdk/lib/async/future_impl.dart 159:18                                    handleValue
dart-sdk/lib/async/future_impl.dart 766:44                                    handleValueCallback
dart-sdk/lib/async/future_impl.dart 795:13                                    _propagateToListeners
dart-sdk/lib/async/future_impl.dart 465:9                                     callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                              _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                               _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15           <fn>
====================================================================================================

======== Exception caught by services library ======================================================
The following MissingPluginException was thrown while activating platform stream on channel dev.steenbakker.mobile_scanner/scanner/event:
MissingPluginException(No implementation found for method listen on channel dev.steenbakker.mobile_scanner/scanner/event)

When the exception was thrown, this was the stack: 
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49  throw_
packages/flutter/src/services/platform_channel.dart 175:7                     _invokeMethod
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50            <fn>
dart-sdk/lib/async/zone.dart 1685:54                                          runUnary
dart-sdk/lib/async/future_impl.dart 159:18                                    handleValue
dart-sdk/lib/async/future_impl.dart 766:44                                    handleValueCallback
dart-sdk/lib/async/future_impl.dart 795:13                                    _propagateToListeners
dart-sdk/lib/async/future_impl.dart 601:5                                     [_completeWithValue]
dart-sdk/lib/async/future_impl.dart 639:7                                     callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                              _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                               _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15           <fn>
====================================================================================================

Yes I'm facing same issue.
If anyone have solution , please share asap.

[EricRode]

Same problem here

[Antoni-Czaplicki]

Fixed in #76, waiting for merge

[Adeel-Sultan]

@juliansteenbakker please merge new update and also update this package as well.

[Adeel-Sultan]

@Antoni-Czaplicki owner not merging your request, we have used this package and still facing this issue.
can you please help ?

[juliansteenbakker]

Thanks for reporting. The suggested answer is depending on an external library which i rather not have. I am looking for a solution without the need of an external library. We just need to check wether the stream is already listened too.

[EricRode]

I found a solution to this problem. If you change
StreamController controller = StreamController(); to
StreamController controller = StreamController.broadcast();
there is no bad state error anymore. But I have the problem that I am just able to scan one time when I restarted the scanner (on the first time I can scan as many as I want but on the next times I can just scan one time, don't know what the problem is.) No error is displayed.

[EricRode]

I had the problem because allowDuplicates was set to false but I was scanning the same code. So My solution above solves the issue.

[dzero1]

Still I'm getting this problem. I'm using the latest version.

Try on: https://flutlab.io/sandbox/cd165080-8a01-4925-b1f8-6e74cf288f39
To recreate, Press Scan button. Go back page without scaning. Go to scan page again.
(This online IDE doesn't allow to read camera. But you can see the console to check the error)

[Adeel-Sultan]

Still I'm getting this problem. I'm using the latest version.

Try on: https://flutlab.io/sandbox/cd165080-8a01-4925-b1f8-6e74cf288f39 To recreate, Press Scan button. Go back page without scaning. Go to scan page again. (This online IDE doesn't allow to read camera. But you can see the console to check the error)

Please update your pubspace.yaml file
mobile_scanner:
git:
url: https://github.com/juliansteenbakker/mobile_scanner

[dzero1]

Thanks. That helps me.