Prevent image buffer exhaustion on Android using Qt (#938)

m7913d · web-flow · commit 7d6d8bb070bd · 2025-04-28T10:06:11.000+02:00
* Prevent image buffer exhaustion on Android using Qt This commit fixes the following exception: java.lang.IllegalStateException: maxImages (10) has already been acquired, call #close before acquiring more. This exception is thrown when the device cannot process the images fast enough. Note that Qt captures the images in a separate (Java) thread (on Android) and sends them asynchronously to the main (GUI) thread. This fix moves the processing to a separate thread and discards all new images while another image (frame) is still being processed. This allows Qt to close the Android resources before the buffer is full. In recent versions of Qt, Qt catches this exception itself (see https:// bugreports.qt.io/browse/QTBUG-116526) by restarting the camera, but frequent restarts may degrade performance. On non-Android platforms, this fix has the advantage that the GUI remains responsive even if the barcode processing is slow, and that always the most recently captured frame is processed. Related to #743 * Support multiple threads to process video frames in Qt * Do not start processing new frames during shutdown.
diff --git a/example/ZXingQtReader.h b/example/ZXingQtReader.h
@@ -16,6 +16,7 @@
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 #include <QAbstractVideoFilter>
 #else
+#include <QThreadPool>
 #include <QVideoFrame>
 #include <QVideoSink>
 #endif
@@ -333,7 +334,16 @@ class BarcodeReader : public QObject, private ReaderOptions
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 	BarcodeReader(QObject* parent = nullptr) : QAbstractVideoFilter(parent) {}
 #else
-	BarcodeReader(QObject* parent = nullptr) : QObject(parent) {}
+	BarcodeReader(QObject* parent = nullptr) : QObject(parent)
+	{
+		_pool.setMaxThreadCount(1);
+	}
+	~BarcodeReader()
+	{
+		_pool.setMaxThreadCount(0);
+		_pool.waitForDone(-1);
+	}
+
 #endif
 
 	// TODO: find out how to properly expose QFlags to QML
@@ -373,10 +383,11 @@ class BarcodeReader : public QObject, private ReaderOptions
 	ZQ_PROPERTY(bool, isPure, setIsPure)
 
 	// For debugging/development
-	int runTime = 0;
+	QAtomicInt runTime = 0;
 	Q_PROPERTY(int runTime MEMBER runTime)
 
 public slots:
+	// Function should be thread safe, as it may be called from a separate thread.
 	ZXingQt::Barcode process(const QVideoFrame& image)
 	{
 		QElapsedTimer t;
@@ -403,6 +414,7 @@ public slots:
 #else
 private:
 	QVideoSink *_sink = nullptr;
+	QThreadPool _pool;
 
 public:
 	void setVideoSink(QVideoSink* sink) {
@@ -413,9 +425,29 @@ public slots:
 			disconnect(_sink, nullptr, this, nullptr);
 
 		_sink = sink;
-		connect(_sink, &QVideoSink::videoFrameChanged, this, &BarcodeReader::process);
+		connect(_sink, &QVideoSink::videoFrameChanged, this, &BarcodeReader::onVideoFrameChanged, Qt::DirectConnection);
+	}
+	void onVideoFrameChanged(const QVideoFrame& frame)
+	{
+		if (_pool.activeThreadCount() >= _pool.maxThreadCount())
+			return; // we are busy => skip the frame
+
+		_pool.start([this, frame](){process(frame);});
 	}
 	Q_PROPERTY(QVideoSink* videoSink MEMBER _sink WRITE setVideoSink)
+	Q_PROPERTY(int maxThreadCount READ maxThreadCount WRITE setMaxThreadCount)
+	int maxThreadCount () const
+	{
+		return _pool.maxThreadCount();
+	}
+	void setMaxThreadCount (int maxThreadCount)
+	{
+		if (_pool.maxThreadCount() != maxThreadCount) {
+			_pool.setMaxThreadCount(maxThreadCount);
+			emit maxThreadCountChanged();
+		}
+	}
+	Q_SIGNAL void maxThreadCountChanged();
 #endif
 
 };
