Polling of available printers in now asynchronous (#264,#256)

- Increased polling interval from 1s to 10s
- Use QtConcurrent::run to initiate a poll asynchronously to main event loop
- Do not initiate a poll if the previous poll has not completed
- Use copies instead of references to mCurrentAvailablePrinters in public API

glabels/CMakeLists.txt

@@ -138,6 +138,7 @@ target_include_directories (glabels-qt
 
 target_link_libraries (glabels-qt
   Model
+  Qt6::Concurrent
   Qt6::Widgets
 )
 

glabels/PrintView.cpp

@@ -48,8 +48,8 @@ namespace glabels
 		
 		auto* printerMonitor = PrinterMonitor::instance();
 		loadDestinations( printerMonitor->availablePrinters() );
-		connect( printerMonitor, SIGNAL(availablePrintersChanged(const QStringList&)),
-		         this, SLOT(onAvailablePrintersChanged(const QStringList&)) );
+		connect( printerMonitor, SIGNAL(availablePrintersChanged(QStringList)),
+		         this, SLOT(onAvailablePrintersChanged(QStringList)) );
 		
 		setDestination( model::Settings::recentPrinter() );
 
@@ -74,7 +74,7 @@ namespace glabels
 	///
 	/// Available printers changed handler
 	///
-	void PrintView::onAvailablePrintersChanged( const QStringList& printers )
+	void PrintView::onAvailablePrintersChanged( QStringList printers )
 	{
 		auto savedSelection = destinationCombo->currentText();
 		loadDestinations( printers );

glabels/PrintView.h

@@ -57,7 +57,7 @@ namespace glabels
 		// Slots
 		/////////////////////////////////
 	private slots:
-		void onAvailablePrintersChanged( const QStringList& printers );
+		void onAvailablePrintersChanged( QStringList printers );
 		void onModelChanged();
 		void updateView();
 		void onFormChanged();

glabels/PrinterMonitor.cpp

@@ -21,8 +21,10 @@
 
 #include "PrinterMonitor.h"
 
-#include <QPrinterInfo>
 #include <QDebug>
+#include <QMutexLocker>
+#include <QPrinterInfo>
+#include <QtConcurrentRun>
 
 
 namespace glabels
@@ -39,11 +41,13 @@ namespace glabels
 	///
 	PrinterMonitor::PrinterMonitor()
 	{
+		using namespace std::chrono_literals;
+
 		mCurrentAvailablePrinters = QPrinterInfo::availablePrinterNames();
 
 		mTimer.reset( new QTimer( this ) );
 		connect( mTimer.get(), SIGNAL(timeout()), this, SLOT(onTimerTimeout()) );
-		mTimer->start( 1000 );
+		mTimer->start( 10s );
 	}
 
 
@@ -64,8 +68,9 @@ namespace glabels
 	///
 	/// Get available printers
 	///
-	const QStringList& PrinterMonitor::availablePrinters() const
+	QStringList PrinterMonitor::availablePrinters()
 	{
+		QMutexLocker mutex( &mCurrentAvailablePrintersMutex );
 		return mCurrentAvailablePrinters;
 	}
 
@@ -74,10 +79,25 @@ namespace glabels
 	/// On timer timeout
 	///
 	void PrinterMonitor::onTimerTimeout()
+	{
+		// Make sure previous poll is complete before starting a new one
+		if ( mPollStatus.isFinished() )
+		{
+			mPollStatus = QtConcurrent::run( &PrinterMonitor::asyncPoll, this );
+		}
+	}
+
+	
+	///
+	/// Asynchronous poll
+	///
+	void PrinterMonitor::asyncPoll()
 	{
 		auto newAvailablePrinters = QPrinterInfo::availablePrinterNames();
 		if ( newAvailablePrinters != mCurrentAvailablePrinters )
 		{
+			QMutexLocker mutex( &mCurrentAvailablePrintersMutex );
+
 			mCurrentAvailablePrinters = newAvailablePrinters;
 
 			emit availablePrintersChanged( mCurrentAvailablePrinters );

glabels/PrinterMonitor.h

@@ -22,6 +22,8 @@
 #define PrinterMonitor_h
 
 
+#include <QFuture>
+#include <QMutex>
 #include <QObject>
 #include <QStringList>
 #include <QTimer>
@@ -53,7 +55,7 @@ namespace glabels
 		// Public methods
 		/////////////////////////////////
 	public:
-		const QStringList& availablePrinters() const;
+		QStringList availablePrinters();
 
 
 		/////////////////////////////////
@@ -67,7 +69,14 @@ namespace glabels
 		// Signals
 		/////////////////////////////////
 	signals:
-		void availablePrintersChanged( const QStringList& availablePrinters );
+		void availablePrintersChanged( QStringList availablePrinters );
+
+
+		/////////////////////////////////
+		// Private methods
+		/////////////////////////////////
+	private:
+		void asyncPoll();
 
 
 		/////////////////////////////////
@@ -77,8 +86,11 @@ namespace glabels
 		static std::unique_ptr<PrinterMonitor> mInstance;
 
 		std::unique_ptr<QTimer> mTimer;
+		
 		QStringList mCurrentAvailablePrinters;
+		QMutex      mCurrentAvailablePrintersMutex;
 
+		QFuture<void> mPollStatus;
 	};
 
 }