Add empty() and emptyUnknown() to QgsFeatureSource

Author: m-kuhn

python/core/auto_generated/qgsfeaturesource.sip.in

@@ -66,6 +66,27 @@ if the feature count is unknown.
 %Docstring
 Returns the number of features contained in the source, or -1
 if the feature count is unknown.
+%End
+
+    virtual bool empty() const;
+%Docstring
+Determines if there are any features available in the source.
+
+In case it is not known if there are features available, false is returned.
+Use ``emptyUnknown()`` to determine if this value is reliable.
+
+.. seealso:: :py:func:`QgsVectorDataProvider.empty`
+information.
+
+.. versionadded:: 3.2
+%End
+
+    virtual bool emptyUnknown() const;
+%Docstring
+Returns true if the value returned by ``empty()`` may be wrong.
+This depends on the dataprovider.
+
+.. versionadded:: 3.2
 %End
 
     virtual QSet<QVariant> uniqueValues( int fieldIndex, int limit = -1 ) const;
@@ -139,7 +160,6 @@ for its ownership.
 
 .. versionadded:: 3.0
 %End
-
 };
 
 

src/core/qgsfeaturesource.cpp

@@ -23,6 +23,16 @@
 #include "qgsvectorlayer.h"
 #include "qgsvectordataprovider.h"
 
+bool QgsFeatureSource::empty() const
+{
+  return featureCount();
+}
+
+bool QgsFeatureSource::emptyUnknown() const
+{
+  return featureCount() == -1;
+}
+
 QSet<QVariant> QgsFeatureSource::uniqueValues( int fieldIndex, int limit ) const
 {
   if ( fieldIndex < 0 || fieldIndex >= fields().count() )

src/core/qgsfeaturesource.h

@@ -85,6 +85,27 @@ class CORE_EXPORT QgsFeatureSource
      */
     virtual long featureCount() const = 0;
 
+    /**
+     * Determines if there are any features available in the source.
+     *
+     * In case it is not known if there are features available, false is returned.
+     * Use ``emptyUnknown()`` to determine if this value is reliable.
+     *
+     * \see QgsVectorDataProvider::empty() for data provider specific
+     *      information.
+     *
+     * \since QGIS 3.2
+     */
+    virtual bool empty() const;
+
+    /**
+     * Returns true if the value returned by ``empty()`` may be wrong.
+     * This depends on the dataprovider.
+     *
+     * \since QGIS 3.2
+     */
+    virtual bool emptyUnknown() const;
+
     /**
      * Returns the set of unique values contained within the specified \a fieldIndex from this source.
      * If specified, the \a limit option can be used to limit the number of returned values.
@@ -150,7 +171,6 @@ class CORE_EXPORT QgsFeatureSource
      */
     QgsVectorLayer *materialize( const QgsFeatureRequest &request,
                                  QgsFeedback *feedback = nullptr ) SIP_FACTORY;
-
 };
 
 Q_DECLARE_METATYPE( QgsFeatureSource * )