Flutter: Lifecycle of a Widget and Navigation

Question

I have written a flutter plugin, that displays a camera preview and scans for barcodes. I have a Widget called ScanPage that displays the CameraPreview and navigates to a new Route when a barcode is detected.

Problem: When I push a new Route (SearchProductPage) to the navigation stack, the CameraController continues to detect barcodes. I need to call stop() on my CameraController when the ScanPage is removed from the screen. I need to call start() again, when the user returns to the ScanPage.

What I tried: The CameraController implements WidgetsBindingObserver and reacts to didChangeAppLifecycleState(). This works perfectly when I press the home button, but not when I push a new Route to the navigation stack.

Question: Is there an equivalent for viewDidAppear() and viewWillDisappear() on iOS or onPause() and onResume() on Android for Widgets in Flutter? If not, how can I start and stop my CameraController so that it stops scanning for barcodes when another Widget is on top of the navigation stack?

class ScanPage extends StatefulWidget {

  ScanPage({ Key key} ) : super(key: key);

  @override
  _ScanPageState createState() => new _ScanPageState();

}

class _ScanPageState extends State<ScanPage> {

  //implements WidgetsBindingObserver
  CameraController controller;

  @override
  void initState() {

    controller = new CameraController(this.didDetectBarcode);
    WidgetsBinding.instance.addObserver(controller);

    controller.initialize().then((_) {
      if (!mounted) {
        return;
      }
      setState(() {});
    });
  }

  //navigate to new page
  void didDetectBarcode(String barcode) {
      Navigator.of(context).push(
          new MaterialPageRoute(
            builder: (BuildContext buildContext) {
              return new SearchProductPage(barcode);
            },
          )
      );    
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(controller);
    controller?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {

    if (!controller.value.initialized) {
      return new Center(
        child: new Text("Lade Barcodescanner..."),
      );
    }

    return new CameraPreview(controller);
  }
}

Edit:

/// Controls a device camera.
///
///
/// Before using a [CameraController] a call to [initialize] must complete.
///
/// To show the camera preview on the screen use a [CameraPreview] widget.
class CameraController extends ValueNotifier<CameraValue> with WidgetsBindingObserver {

  int _textureId;
  bool _disposed = false;

  Completer<Null> _creatingCompleter;
  BarcodeHandler handler;

  CameraController(this.handler) : super(const CameraValue.uninitialized());

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {

    switch(state){
      case AppLifecycleState.inactive:
        print("--inactive--");
        break;
      case AppLifecycleState.paused:
        print("--paused--");
        stop();
        break;
      case AppLifecycleState.resumed:
        print("--resumed--");
        start();
        break;
      case AppLifecycleState.suspending:
        print("--suspending--");
        dispose();
        break;
    }
  }

  /// Initializes the camera on the device.
  Future<Null> initialize() async {

    if (_disposed) {
      return;
    }
    try {
      _creatingCompleter = new Completer<Null>();
      _textureId = await BarcodeScanner.initCamera();

      print("TextureId: $_textureId");

      value = value.copyWith(
        initialized: true,
      );
      _applyStartStop();
    } on PlatformException catch (e) {
      value = value.copyWith(errorDescription: e.message);
      throw new CameraException(e.code, e.message);
    }

    BarcodeScanner._channel.setMethodCallHandler((MethodCall call){
      if(call.method == "barcodeDetected"){
        String barcode = call.arguments;
        debounce(2500, this.handler, [barcode]);
      }
    });

    _creatingCompleter.complete(null);
  }

  void _applyStartStop() {
    if (value.initialized && !_disposed) {
      if (value.isStarted) {
        BarcodeScanner.startCamera();
      } else {
        BarcodeScanner.stopCamera();
      }
    }
  }

  /// Starts the preview.
  ///
  /// If called before [initialize] it will take effect just after
  /// initialization is done.
  void start() {
    value = value.copyWith(isStarted: true);
    _applyStartStop();
  }

  /// Stops the preview.
  ///
  /// If called before [initialize] it will take effect just after
  /// initialization is done.
  void stop() {
    value = value.copyWith(isStarted: false);
    _applyStartStop();
  }

  /// Releases the resources of this camera.
  @override
  Future<Null> dispose() {
    if (_disposed) {
      return new Future<Null>.value(null);
    }
    _disposed = true;
    super.dispose();
    if (_creatingCompleter == null) {
      return new Future<Null>.value(null);
    } else {
      return _creatingCompleter.future.then((_) async {
        BarcodeScanner._channel.setMethodCallHandler(null);
        await BarcodeScanner.disposeCamera();
      });
    }
  }
}

Solution

I ended up stopping the controller before I navigate to the other page and restart it, when pop() is called.

//navigate to new page
void didDetectBarcode(String barcode) {
   controller.stop();
   Navigator.of(context)
       .push(...)
       .then(() => controller.start()); //future completes when pop() returns to this page

}

Another solution would be to set the maintainState property of the route that opens ScanPage to false.