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fix(desktop qr): show an error on Linux when the camera can't be opened #1488

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Merged
julian-CStack merged 1 commit into from
Nov 3, 2026
Merged

fix(desktop qr): show an error on Linux when the camera can't be opened #1488

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81 changes: 68 additions & 13 deletions lib/widgets/desktop/qr_code_scanner_dialog.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -36,9 +36,9 @@ class QrCodeScannerDialog extends StatefulWidget {
}

class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
final CameraLinux? _cameraLinuxPlugin = Platform.isLinux
? CameraLinux()
: null;
// Created in _initializeCamera: the constructor loads libcamera_linux.so
// and throws if it can't.
CameraLinux? _cameraLinuxPlugin;
final CameraWindows? _cameraWindowsPlugin = Platform.isWindows
? CameraWindows()
: null;
Expand All @@ -49,6 +49,10 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
String? _macOSDeviceId;
final int _imageDelayInMs = Platform.isLinux ? 500 : 250;
int _consecutiveFailures = 0;
// Time since the last usable frame, to notice a camera that opened but
// stopped (or never started) returning images.
final Stopwatch _sinceLastFrame = Stopwatch();
static const _frameTimeout = Duration(seconds: 10);
String? _cameraError;
String? _initError;

Expand Down Expand Up @@ -87,8 +91,17 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {

Future<bool> _initializeCamera() async {
try {
if (Platform.isLinux && _cameraLinuxPlugin != null) {
await _cameraLinuxPlugin.initializeCamera();
if (Platform.isLinux) {
final plugin = _cameraLinuxPlugin = CameraLinux();
// This only starts a capture thread. If OpenCV can't open a camera
// the thread just exits, and captureImage() keeps returning nothing,
// so wait for a first frame before calling the camera open.
await plugin.initializeCamera();
if (!await _waitForFirstLinuxFrame(plugin)) {
plugin.stopCamera();
if (!mounted) return false;
throw TimeoutException("No image from the camera", _frameTimeout);
}
Logging.instance.d("Linux Camera initialized");
} else if (Platform.isWindows && _cameraWindowsPlugin != null) {
final List<CameraDescription> cameras = await _cameraWindowsPlugin
Expand Down Expand Up @@ -181,7 +194,11 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {

return true;
} catch (e, s) {
_initError = e is CameraException ? e.description ?? e.code : "$e";
_initError = switch (e) {
CameraException() => e.description ?? e.code,
TimeoutException() => e.message ?? "$e",
_ => "$e",
};
Logging.instance.e(
"Failed to initialize camera",
error: e,
Expand All @@ -191,12 +208,48 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
}
}

/// Polls [plugin] until it returns a frame. Returns false on timeout or if
/// the dialog closes first.
Future<bool> _waitForFirstLinuxFrame(CameraLinux plugin) async {
final stopwatch = Stopwatch()..start();
Object? lastError;
while (mounted && stopwatch.elapsed < _frameTimeout) {
try {
if ((await plugin.captureImage()).isNotEmpty) {
return true;
}
} catch (e) {
// captureImage() can throw on the null pointer it gets when there is
// no frame yet.
lastError = e;
}
await Future<void>.delayed(const Duration(milliseconds: 250));
}
if (lastError != null) {
Logging.instance.w("No frame from Linux camera", error: lastError);
}
return false;
}

/// Shows an error once no usable frame has arrived for [_frameTimeout].
void _checkFrameTimeout() {
if (_sinceLastFrame.elapsed >= _frameTimeout &&
_cameraError == null &&
mounted) {
setState(() {
_cameraError =
"The camera is not returning images. You can still "
"use \"Select file\" to scan a QR code image.";
});
}
}

Future<void> _stopCamera() async {
_isScanning = false;

try {
if (Platform.isLinux && _cameraLinuxPlugin != null) {
_cameraLinuxPlugin.stopCamera();
_cameraLinuxPlugin!.stopCamera();
Logging.instance.d("Linux Camera stopped");
} else if (Platform.isWindows && _cameraWindowsPlugin != null) {
// if (_cameraId >= 0) {
Expand Down Expand Up @@ -289,25 +342,28 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {

Future<void> _captureAndScanImage() async {
_isScanning = true;
_sinceLastFrame.start();
while (_isScanning) {
try {
String? base64Image;
img.Image? windowsFrame;
if (Platform.isLinux && _cameraLinuxPlugin != null) {
base64Image = await _cameraLinuxPlugin.captureImage();
base64Image = await _cameraLinuxPlugin!.captureImage();
} else if (Platform.isWindows) {
windowsFrame = await _captureWindowsPreviewFrame();
} else if (Platform.isMacOS) {
final macOSimg = await CameraMacOS.instance.takePicture();
if (macOSimg == null) {
Logging.instance.w("Failed to capture image");
await Future<void>.delayed(Duration(milliseconds: _imageDelayInMs));
_checkFrameTimeout();
continue;
}
final img.Image? image = img.decodeImage(macOSimg.bytes!);
if (image == null) {
Logging.instance.w("Failed to capture image");
await Future<void>.delayed(Duration(milliseconds: _imageDelayInMs));
_checkFrameTimeout();
continue;
}
base64Image = base64Encode(img.encodePng(image));
Expand All @@ -318,6 +374,7 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
// .log("Failed to capture image", level: LogLevel.Error);
// Spammy.
await Future<void>.delayed(Duration(milliseconds: _imageDelayInMs));
_checkFrameTimeout();
continue;
}
final img.Image? image =
Expand All @@ -328,10 +385,12 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
if (image == null) {
Logging.instance.w("Failed to decode image");
await Future<void>.delayed(Duration(milliseconds: _imageDelayInMs));
_checkFrameTimeout();
continue;
}

_consecutiveFailures = 0;
_sinceLastFrame.reset();
if (mounted) {
setState(() {
_cameraError = null;
Expand Down Expand Up @@ -375,11 +434,7 @@ class _QrCodeScannerDialogState extends State<QrCodeScannerDialog> {
stackTrace: s,
);
}
if (_consecutiveFailures == 20 && mounted) {
setState(() {
_cameraError = "The camera is not returning images.";
});
}
_checkFrameTimeout();
// Without this a failing capture spins the loop with no delay.
await Future<void>.delayed(Duration(milliseconds: _imageDelayInMs));
}
Expand Down
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