[Backport release-4_1] Add a new LayerUtils invokable function to save a vector layer as an on-disk dataset

src/core/utils/layerutils.cpp

@@ -42,6 +42,7 @@
 #include <qgssymbol.h>
 #include <qgssymbollayer.h>
 #include <qgstextbuffersettings.h>
+#include <qgsvectorfilewriter.h>
 #include <qgsvectorlayer.h>
 #include <qgsvectorlayerlabeling.h>
 #include <qgsvectorlayerutils.h>
@@ -545,6 +546,16 @@ bool LayerUtils::hasMValue( QgsVectorLayer *layer )
   return QgsWkbTypes::hasM( layer->wkbType() );
 }
 
+QSet<QVariant> LayerUtils::uniqueValuesForVectorLayerFieldIndex( QgsVectorLayer *layer, int fieldIndex )
+{
+  if ( !layer )
+  {
+    return QSet<QVariant>();
+  }
+
+  return layer->uniqueValues( fieldIndex );
+}
+
 QgsVectorLayer *LayerUtils::loadVectorLayer( const QString &uri, const QString &name, const QString &provider )
 {
   QgsVectorLayer *layer = new QgsVectorLayer( uri, name, provider );
@@ -595,3 +606,64 @@ FeatureIterator LayerUtils::createFeatureIteratorFromRectangle( QgsVectorLayer *
   const QgsFeatureRequest request = QgsFeatureRequest( rectangle );
   return FeatureIterator( layer, request );
 }
+
+QString LayerUtils::saveVectorLayerAs( QgsVectorLayer *layer, const QString &filePath, const QString &driverName, const QString &filterExpression )
+{
+  if ( !layer || filePath.isEmpty() )
+  {
+    return QString();
+  }
+
+  QFileInfo fileInfo( filePath );
+  const QString finalDriverName = driverName.isEmpty() ? QgsVectorFileWriter::driverForExtension( fileInfo.suffix() ) : driverName;
+  if ( finalDriverName.isEmpty() )
+  {
+    return QString();
+  }
+  QDir dir;
+  if ( !dir.mkpath( fileInfo.absolutePath() ) )
+  {
+    return QString();
+  }
+
+  QStringList datasetOptions = QgsVectorFileWriter::defaultDatasetOptions( finalDriverName );
+  if ( finalDriverName == QStringLiteral( "GPX" ) )
+  {
+    datasetOptions.removeAll( QStringLiteral( "GPX_USE_EXTENSIONS=NO" ) );
+    datasetOptions << QStringLiteral( "GPX_USE_EXTENSIONS=YES" );
+  }
+
+  QString finalFileName;
+  QString finalLayerName;
+  QgsVectorFileWriter::SaveVectorOptions saveOptions;
+  saveOptions.fileEncoding = QStringLiteral( "UTF8" );
+  saveOptions.layerName = fileInfo.completeBaseName();
+  saveOptions.driverName = finalDriverName;
+  saveOptions.datasourceOptions = datasetOptions;
+  saveOptions.layerOptions = QgsVectorFileWriter::defaultLayerOptions( finalDriverName );
+  saveOptions.symbologyExport = Qgis::FeatureSymbologyExport::NoSymbology;
+  saveOptions.actionOnExistingFile = QgsVectorFileWriter::CreateOrOverwriteFile;
+
+  std::unique_ptr<QgsVectorFileWriter> writer( QgsVectorFileWriter::create( filePath, layer->fields(), layer->wkbType(), layer->crs(), QgsProject::instance()->transformContext(), saveOptions, QgsFeatureSink::RegeneratePrimaryKey, &finalFileName, &finalLayerName ) );
+  if ( writer->hasError() )
+  {
+    qInfo() << QStringLiteral( "Vector layer file writer error: %1" ).arg( writer->errorMessage() );
+    return QString();
+  }
+
+  QgsFeatureRequest request;
+  if ( !filterExpression.isEmpty() )
+  {
+    request.setFilterExpression( filterExpression );
+    request.setExpressionContext( layer->createExpressionContext() );
+  }
+
+  QgsFeatureIterator it = layer->getFeatures( request );
+  QgsFeature feature;
+  while ( it.nextFeature( feature ) )
+  {
+    writer->addFeature( feature, QgsFeatureSink::FastInsert );
+  }
+
+  return finalFileName;
+}

src/core/utils/layerutils.h

@@ -180,6 +180,11 @@ class LayerUtils : public QObject
      */
     Q_INVOKABLE static bool hasMValue( QgsVectorLayer *layer );
 
+    /**
+     * Returns a list of unique values for a given \a fieldIndex from the \a layer.
+     */
+    Q_INVOKABLE QSet<QVariant> uniqueValuesForVectorLayerFieldIndex( QgsVectorLayer *layer, int fieldIndex );
+
     /**
      * Loads a vector layer.
      * \param uri the data source uri
@@ -226,6 +231,16 @@ class LayerUtils : public QObject
      * Returns a feature iterator to get features overlapping a given \a rectangle within the provided \a layer.
      */
     Q_INVOKABLE static FeatureIterator createFeatureIteratorFromRectangle( QgsVectorLayer *layer, const QgsRectangle &rectangle );
+
+    /**
+     * Saves a vector layer into an on-disk dataset a given path using the OGR provider.
+     * \param layer the vector layer to save features from
+     * \param filePath the file path where the dataset will be writen
+     * \param driverName an optional OGR driver name (if left empty, the file path extension will drive the OGR driver)
+     * \param filterExpression an optional filter expression used to save a subset of features from the layer (note that only the global, project, and layer expression context scopes are used)
+     * \returns If successful, finalized file path will be returned, otherwise an empty string will be returned
+     */
+    Q_INVOKABLE static QString saveVectorLayerAs( QgsVectorLayer *layer, const QString &filePath, const QString &driverName = QString(), const QString &filterExpression = QString() );
 };
 
 #endif // LAYERUTILS_H