fix: Close barcode scanner and free it's resources

[M123-dev]

• Possible fix for The second attempt to scan results in a black screen on iPhone #817 & MissingPluginException: MissingPluginException(No implementation found for method vision#startBarcodeScanner on channel g... #1155

I wondered why it would fail but only on the second attempt, maybe this fixes it, even if not its good for freeing unused resources

cc: @cli1005

[monsieurtanuki]

You close the barcodeScanner but you never reopen it...
I guess the open/close behavior should be somehow similar to the controller's.

[M123-dev]

Added documentation @monsieurtanuki

[monsieurtanuki]

@M123-dev You're missing the point: how do you expect the barcodeScanner to work after being closed? This stop method is called on an onPause callback, with a corresponding onResume callback. If onResume is called after onPause, the barcodeScanner will be closed.

[M123-dev]

No I understand what you mean @monsieurtanuki, its just that there is no way to manually start the scanner. ML Kit just has three public methods: https://developers.google.com/android/reference/com/google/mlkit/vision/barcode/BarcodeScanner?hl=en

The scanner is "started" when giving it it's first image

[monsieurtanuki]

there is no way to manually start the scanner

Then your solution cannot work. After you close the barcodeScanner with onPause, will it be automagically restarted with the next onResume or the next scan? Maybe onPause and onResume are not correct names, they should be onCreate and onDestroy in order to leave no hope of resuming.

[M123-dev]

I think we are talking past one another here @monsieurtanuki

The lifecycle looks like it:

idle...

while(running){
      image => ML Kit
      ML Kit starts
      ML Kit => barcode
      ML Kit goes back to idle
}



dispose()
{
  stopImageStream()
  stopMLKit() // Just frees remaining recources
}

There isn't even a public start method from ml kit, it's all done internally

On android it worked fine without closing the scanner but maybe this differs from android to iOS maybe they have a check that the scanner can't handle a new pic if the last one is still in there without responsing or so, honestly idk but it's worth trying

[monsieurtanuki]

@M123-dev If you want to destroy the barcode scanner, I guess the correct syntax would be:

await barcodeScanner?.close();
barcodeScanner = null;

Then come my previous remarks about the difference between onPause/onResume (e.g. we temporarily sleep while a dialog is displayed but will go back again) and onCreate/onDestroy (e.g. when the app is killed because another app needs tons of memory: if we go back again, it will be from scratch). Here the LifeCycleManager is about onPause/onResume.

In this current PR, assuming that the mere presence of the barcode scanner is a nuisance, I suggest that you just create a new BarcodeScanner in _startLiveFeed and close/nullify it in _stopImageStream.

[monsieurtanuki]

It would be cleaner with no default value at construction time:

BarcodeScanner? barcodeScanner;

instead of

BarcodeScanner? barcodeScanner = GoogleMlKit.vision.barcodeScanner();

But that's ok too I guess.