fix: Ignore additional calls to start() when the controller is starting #1086

navaronbracke · 2024-06-03T12:10:11Z

Fixes: #1078
Fixes: #502

simplenotezy · 2024-06-15T17:24:07Z

Nice!

You probably need to update the examples folder as well, as I had to now manually call start() after setting the controller in initState().

navaronbracke · 2024-06-15T17:27:07Z

@simplenotezy Ah right, I'll probably have to tweak the automatic start in the samples.

Just to be clear, you want automatic start to be enabled for the samples? Or not?

simplenotezy · 2024-06-15T17:29:12Z

Sounds good! After trying your version, I needed to call start manually. Otherwise the example code would not work.

class _TicketScannerScreenState extends State<TicketScannerScreen> {
  late MobileScannerController controller;

  @override
  void initState() {
    super.initState();
    controller = MobileScannerController(
      formats: const [BarcodeFormat.qrCode],
    );

+    controller.start();
  }

  @override
  Future<void> dispose() async {
    super.dispose();
    await controller.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final scanWindow = Rect.fromCenter(
      center: MediaQuery.sizeOf(context).center(Offset.zero),
      width: 400,
      height: 200,
    );

simplenotezy · 2024-06-15T17:31:55Z

Hmm - something strange is going on in the repository code. Same issue I had before, but this time, there's just no error. It just wont work - black screen.

import 'package:auto_route/auto_route.dart';
import 'package:flutter/material.dart';
import 'package:mobile_scanner/mobile_scanner.dart';

@RoutePage()
class TicketScannerScreen extends StatefulWidget {
  const TicketScannerScreen({super.key});

  @override
  State<TicketScannerScreen> createState() => _TicketScannerScreenState();
}

class _TicketScannerScreenState extends State<TicketScannerScreen> {
  late MobileScannerController controller;

  @override
  void initState() {
    super.initState();
    controller = MobileScannerController(
      formats: const [BarcodeFormat.qrCode],
    );

    controller.start();
  }

  @override
  Future<void> dispose() async {
    super.dispose();
    await controller.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final scanWindow = Rect.fromCenter(
      center: MediaQuery.sizeOf(context).center(Offset.zero),
      width: 400,
      height: 200,
    );

    return Scaffold(
      backgroundColor: Colors.black,
      appBar: AppBar(
        title: Text('Scan'.hardcoded),
      ),
      body: Stack(
        fit: StackFit.expand,
        children: [
          Center(
            child: MobileScanner(
              fit: BoxFit.contain,
              controller: controller,
              scanWindow: scanWindow,
              errorBuilder: (context, error, child) {
                return ScannerErrorWidget(error: error);
              },
              overlayBuilder: (context, constraints) {
                return Padding(
                  padding: const EdgeInsets.all(16.0),
                  child: Align(
                    alignment: Alignment.bottomCenter,
                    child: ScannedBarcodeLabel(barcodes: controller.barcodes),
                  ),
                );
              },
            ),
          ),
          ValueListenableBuilder(
            valueListenable: controller,
            builder: (context, value, child) {
              if (!value.isInitialized ||
                  !value.isRunning ||
                  value.error != null) {
                return const SizedBox();
              }

              return CustomPaint(
                painter: ScannerOverlay(scanWindow: scanWindow),
              );
            },
          ),
          Align(
            alignment: Alignment.bottomCenter,
            child: Padding(
              padding: const EdgeInsets.all(16.0),
              child: Row(
                mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                children: [
                  ToggleFlashlightButton(controller: controller),
                  SwitchCameraButton(controller: controller),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

navaronbracke · 2024-06-15T17:34:13Z

Hmm, what device is this on? Do you see anything in XCode / Logcat / browser logs?

simplenotezy · 2024-06-15T17:37:07Z

It's a physical device. I see the following exception being thrown:

simplenotezy · 2024-06-15T17:38:35Z

And it works initially. If I start a new debug session it will work just fine. If I then hot reload/restart the app, and go to the route, it won't work. Which smells like something is off in the underlying APIs.

navaronbracke · 2024-06-15T17:39:00Z

Hmm, it's precisely that error that we should ignore when already starting. Is autoStart false in your case?

Also, do you have a stack trace? I can test with the sample above to see where it occurs, though.

simplenotezy · 2024-06-15T17:40:03Z

The error is this: when the widget is disposed*, the underlying service is not being stopped. When you restart your application, it is technically still running, so when you go to the route that starts the controller, it won't work.

simplenotezy · 2024-06-15T17:42:58Z

Here's a video showing the issue:

RPReplay_Final1718473315.mov

navaronbracke · 2024-06-15T17:48:43Z

when the widget is disposed*, the underlying service is not being stopped

Could you clarify that? We should be disposing the underlying camera.

https://github.com/navaronbracke/mobile_scanner/blob/fix_already_started_bug/android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScanner.kt#L398

Perhaps there is a case where this isn't true?

simplenotezy · 2024-06-15T17:50:26Z

@navaronbracke Not sure how I can clarify further apart from the video I sent above. The code won't work if you use flutters hot restart functionality (note: hot reload works, but not hot restart)

navaronbracke · 2024-06-15T17:56:49Z

So this exception only happens when you press r (not R) in the terminal? In that case, I think it has to do with the plugin not handling hot restart properly. We have an open bug for that as well, although I would have hoped that this fix would have fixed that issue as well.

I'll have to specifically address the use case for hot reload then, perhaps by just short circuiting a return value instead of an error, if we do not have hot reload callbacks.

Another idea would be to see if stopping the controller in reassemble() works around this issue?

navaronbracke · 2024-06-18T08:13:13Z

analysis_options.yaml

+    - unnecessary_library_directive
+    - prefer_single_quotes


We had one violation for this rule. I do personally prefer single quotes, so I turned this on.

navaronbracke · 2024-06-18T08:19:40Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScannerPermissions.kt

@@ -25,7 +21,7 @@ class MobileScannerPermissions {
    }

    interface ResultCallback {
-        fun onResult(errorCode: String?, errorDescription: String?)
+        fun onResult(errorCode: String?)


The second argument was unused, since we return a bool to the interface

navaronbracke · 2024-06-18T08:36:47Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScannerHandler.kt

@@ -28,7 +29,7 @@ class MobileScannerHandler(

    private val analyzeImageErrorCallback: AnalyzerErrorCallback = {
        Handler(Looper.getMainLooper()).post {
-            analyzerResult?.error("MobileScanner", it, null)
+            analyzerResult?.error(MobileScannerErrorCodes.BARCODE_ERROR, it, null)


To be consistent with the results from the scanning stream, I used the same barcode error constant

navaronbracke · 2024-06-18T08:40:59Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScannerHandler.kt

@@ -103,21 +104,21 @@ class MobileScannerHandler(

    @ExperimentalGetImage
    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
-        if (mobileScanner == null) {


This check isn't needed. The scanner is set up when attached to the activity

navaronbracke · 2024-06-18T08:41:50Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScannerHandler.kt

        when (call.method) {
            "state" -> result.success(permissions.hasCameraPermission(activity))
            "request" -> permissions.requestPermission(
                activity,
                addPermissionListener,
                object: MobileScannerPermissions.ResultCallback {
-                    override fun onResult(errorCode: String?, errorDescription: String?) {
+                    override fun onResult(errorCode: String?) {


Plumbing through the new error codes

navaronbracke · 2024-06-18T08:42:23Z

ios/Classes/MobileScanner.swift

@@ -127,7 +124,6 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
    /// Gets called when a new image is added to the buffer
    public func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        guard let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else {
-            print("Failed to get image buffer from sample buffer.")


The print isn't helpful to users :)

navaronbracke · 2024-06-18T08:43:13Z

ios/Classes/MobileScanner.swift

@@ -25,9 +25,6 @@ public class MobileScanner: NSObject, AVCaptureVideoDataOutputSampleBufferDelega
    /// Barcode scanner for results
    var scanner = BarcodeScanner.barcodeScanner()

-    /// Return image buffer with the Barcode event
-    var returnImage: Bool = false


This field was moved to fix a bug

navaronbracke · 2024-06-18T08:46:26Z

lib/src/mobile_scanner_exception.dart

-///
-/// This exception type is only used internally,
-/// and is not part of the public API.
-class PermissionRequestPendingException implements Exception {}


This class is no longer used, as the workaround is removed.

navaronbracke · 2024-06-18T08:48:46Z

ios/Classes/MobileScannerPlugin.swift

-                            return nil
-                        }
-                    } else {
+            if (error != nil) {


Small refactor to use early returns for readability + plumbing of the new error codes

navaronbracke · 2024-06-18T09:29:29Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScanner.kt

-                            barcodeMap.add(barcode.data)
-                        }
-                    } else {
+                    if (scanWindow == null) {


Refactoring with early returns

fulstadev · 2024-09-24T13:58:46Z

As I've found multiple github links related to this issue and was now able to solve it thanks to the sequence of your posts, trying to sum everything up here, if that may be beneficial for further debuggers:

@navaronbracke the behaviour @simplenotezy presents in his video (thanks for that @simplenotezy) happens for me in the exact same way!

Flutter 3.24.2 • channel stable Tools
Dart 3.5.2
Android (API Level 33 and above) 
when testing on a physical (Android) device

when it happens, the screen showing up is simply plain white (Scaffold's background screen, which in @simplenotezy most likely is set to black), with nothing inside it. That is, visually. in the widget tree, all the widgets are built normally, which is somewhat weird (The QR-Code Scanner / Camera simply does not show up and the entire screen stays in the single color of the Scaffold background). The pace at which we open and close the screen multiple times quickly or slowly one after the other (waiting a few seconds before re-opening the screen after closing it) did not seem to have an impact on the outcome.
So all of this seems to have little to nothing to do with hot reload / restart. We've even completely initialized an app, and could provoke the same issue, at least in our case.

Regarding the controller, I copy-pasted your methods provided in the example https://github.com/juliansteenbakker/mobile_scanner/blob/master/example/lib/barcode_scanner_controller.dart. Interestingly, when we tried to never call controller.start() within our mobile scanner widget while trying to debug it, our app reported the same error: MobileScannerException: code genericError, message: Called start() while already started.
In that case however successfully displaying the error screen instead of the fully white screen. This made me think; are you eventually calling start() somewhere else, under the hood, and that is being called without controller.start()? That's when I read through your hints again, and actually, adding:

autoStart: false to the MobileScannerController constructor call

comepletely fixed the issue for me (@simplenotezy, are you sure you've set that to false too?).

But your proposed additional permission check still makes absolute sense to me. Without it, when the mobile scanner is opened for the first time and permission was not granted, there's another problem: The app keeps endlessly running into the error:

MobileScannerException: code genericError, message: Another request is ongoing and multiple requests cannot be handled at once.

until the permission dialog is dismissed.

To act against that, I'm additionally adding your permission check within initState too, even though disabling the autoStart for the MobileScannerController already solved the already called start() thing.

Related to your proposed permission check, if I may suggest:

controller.value.error?.errorCode == MobileScannerErrorCode.permissionDenied

@navaronbracke you may want to change the example's lifecycle handler init check, as there is no controller.value.hasCameraPermission property, but I guess that's exactly what you were aiming to do in that example?

navaronbracke · 2024-09-24T14:03:26Z

controller.value.error?.errorCode == MobileScannerErrorCode.permissionDenied
@navaronbracke you may want to change the example's lifecycle handler init check, as there is no controller.value.hasCameraPermission property, but I guess that's exactly what you were aiming to do in that example?

That is a new getter that I added in 1ab7f5a, which does exactly this check.

navaronbracke · 2024-09-28T09:21:09Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/BarcodeHandler.kt

@@ -24,6 +24,12 @@ class BarcodeHandler(binaryMessenger: BinaryMessenger) : EventChannel.StreamHand
        }
    }

+    fun publishError(errorCode: String, errorMessage: String, errorDetails: Any?) {


Like publishEvent above, but for result.error()

navaronbracke · 2024-09-28T09:22:01Z

android/src/main/kotlin/dev/steenbakker/mobile_scanner/MobileScannerHandler.kt

-            "name" to "error",
-            "data" to error,
-        ))
+        barcodeHandler.publishError(MobileScannerErrorCodes.BARCODE_ERROR, error, null)


This is an actual error event now.

navaronbracke · 2024-09-28T09:24:14Z

ios/Classes/MobileScannerPlugin.swift

@@ -42,7 +42,10 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin {
    init(barcodeHandler: BarcodeHandler, registry: FlutterTextureRegistry) {
        self.mobileScanner = MobileScanner(registry: registry, mobileScannerCallback: { barcodes, error, image in
            if error != nil {
-                barcodeHandler.publishEvent(["name": "error", "data": error!.localizedDescription])
+                barcodeHandler.publishError(


This is an actual error event now

navaronbracke · 2024-09-28T09:25:11Z

lib/src/web/zxing/zxing_exception.dart

+/// The JS static interop class for the Result class in the ZXing library.
+///
+/// See also: https://github.com/zxing-js/library/blob/master/src/core/Exception.ts
+@JS('ZXing.Exception')


ZXing had an error type, so I prefer to use it, so we can forward the message.

lib/src/method_channel/mobile_scanner_method_channel.dart

navaronbracke · 2024-09-28T10:35:17Z

I have started testing this patch and it seems to fix the underlying issue.

I still need to test it on iOS and I'm having a bit of trouble testing MacOS hot reload/hot restart due to a tool bug,
but once those have been tested I will land this patch.

… for sample

navaronbracke · 2024-09-28T11:55:28Z

example/.gitignore

 .buildlog/
 .history
 .svn/
+.swiftpm/


This was added by a project migrator when running on the master branch of Flutter. This is probably needed for SwiftPM stuff.

navaronbracke · 2024-09-28T11:55:51Z

example/web/index.html

@@ -21,7 +21,7 @@
  <meta name="description" content="Demonstrates how to use the mobile_scanner plugin.">

  <!-- iOS meta tags & icons -->
-  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="mobile-web-app-capable" content="yes">


The old tag is deprecated

navaronbracke · 2024-09-28T11:56:36Z

lib/src/web/zxing/zxing_barcode_reader.dart

-            ),
-          );
+          // Skip the event if no code was detected.
+          if (error != null && error.message != kNoCodeDetectedErrorMessage) {


When introducing the forwarding of the errors on web, this was apparently breaking our usage. Added a check to fix it.

navaronbracke · 2024-09-28T11:57:24Z

example/macos/Runner/DebugProfile.entitlements

@@ -10,5 +10,7 @@
 	<true/>
 	<key>com.apple.security.network.server</key>
 	<true/>
+  	<key>com.apple.security.files.user-selected.read-only</key>


Not having this broke the example app on MacOS for analyze image.

This was documented in image_picker

navaronbracke · 2024-09-28T11:57:41Z

example/macos/Runner/AppDelegate.swift

@@ -6,4 +6,8 @@ class AppDelegate: FlutterAppDelegate {
  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
    return true
  }
+
+  override func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool {


This fixes a warning for MacOS

navaronbracke · 2024-09-30T07:00:51Z

example/ios/Runner.xcodeproj/project.pbxproj

@@ -488,14 +488,14 @@
 				CODE_SIGN_IDENTITY = "Apple Development";
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = "$(FLUTTER_BUILD_NUMBER)";
-				DEVELOPMENT_TEAM = 75Y2P2WSQQ;
+				DEVELOPMENT_TEAM = "";


When trying to test the iOS example app, XCode kept complaining about the development team & bundle identifier.

Presumably the old development team was Julian's personal team or something.
Likewise, the bundle identifier was already registered (which really should not be the case for an exampel app, that isn't intended for publishing)

I updated the bundle identifier to something that is not registered, by adding the -example string at the end.
I changed the development team to None in XCode, which is better than a team that isn't yours, in my opinion.

navaronbracke · 2024-09-30T07:03:38Z

This fix was now verified for iOS, MacOS, Android and the web. With this fix, during hot restart, the app restarts fully (as expected) and with hot reload the camera continues to work as if the hot reload never happened (since hot reload is only for Dart code on the client, not the plugin)

This fix will be available for the next release, which I plan to release later today.

navaronbracke changed the title ~~Fix: Ignore additional calls to start() when the controller is starting~~ fix: Ignore additional calls to start() when the controller is starting Jun 3, 2024

github-actions bot assigned navaronbracke Jun 3, 2024

This was referenced Jun 6, 2024

Doesn't work on Android 14 #1077

Closed

Error: Called start() while already started! #1091

Open

simplenotezy mentioned this pull request Jun 16, 2024

Stability issues #1098

Closed

navaronbracke force-pushed the fix_already_started_bug branch from 4fead48 to a7e67ba Compare June 18, 2024 07:51

navaronbracke commented Jun 18, 2024

View reviewed changes

navaronbracke force-pushed the fix_already_started_bug branch from e48a689 to b50b565 Compare June 18, 2024 08:44

navaronbracke commented Jun 18, 2024

View reviewed changes

navaronbracke added 2 commits September 24, 2024 15:18

fix MacOS leftover error codes

53a4134

remove outdated line in changelog

4d92d5d

navaronbracke force-pushed the fix_already_started_bug branch from 3fdae25 to 4d92d5d Compare September 24, 2024 13:18

publis barcode error event for Android

3ec0dc1

navaronbracke added 5 commits September 28, 2024 10:38

forward barcode error events as FlutterError()'s on iOS & MacOS

eda540f

forward errors from ZXing

8bc967c

handle barcode error events in the method channel implementation

e9b3ad4

sort methods

d346a45

use const for barcode event name

e967f54

navaronbracke commented Sep 28, 2024

View reviewed changes

navaronbracke added 2 commits September 28, 2024 11:50

handle no code detected for ZXing

447996d

fix macos entitlements for example & use raw value for sample

ed3f54f

navaronbracke added 2 commits September 28, 2024 13:07

fix deprecated tag

f5dd22d

add swiftpm dir to macos example gitignore; fix secure coding warning…

c4bddc3

… for sample

navaronbracke commented Sep 28, 2024

View reviewed changes

navaronbracke mentioned this pull request Sep 28, 2024

Camera is not working #1078

Closed

adjust iOS example development team & bundle identifier

bb999fe

navaronbracke commented Sep 30, 2024

View reviewed changes

navaronbracke merged commit 17255d4 into juliansteenbakker:master Sep 30, 2024
5 checks passed

navaronbracke deleted the fix_already_started_bug branch September 30, 2024 07:03

This was referenced Sep 30, 2024

Hot restart - camera no longer usable #1037

Open

Better support for Hot Reload #773

Open

fix: Ignore additional calls to start() when the controller is starting #1086

fix: Ignore additional calls to start() when the controller is starting #1086

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