Improve docs and python bindings for feature count

python/core/qgsvectorlayer.sip

@@ -843,9 +843,17 @@ Return the provider type for this layer
 
     bool countSymbolFeatures();
 %Docstring
- Count features for symbols. Feature counts may be get by featureCount().
- \param showProgress show progress dialog
- :return: true if calculated, false if failed or was canceled by user
+ Count features for symbols.
+ The method will return immediately. You will need to connect to the
+ symbolFeatureCountMapChanged() signal to be notified when the freshly updated
+ feature counts are ready.
+
+.. note::
+
+   If you need to wait for the results, create and start your own QgsVectorLayerFeatureCounter
+       task and call waitForFinished().
+
+.. versionadded:: 3.0
  :rtype: bool
 %End
 

python/core/qgsvectorlayerfeaturecounter.sip

@@ -32,6 +32,13 @@ class QgsVectorLayerFeatureCounter : QgsTask
     virtual bool run();
 
 
+    long featureCount( const QString &legendKey ) const;
+%Docstring
+ Get the feature count for a particular ``legendKey``.
+ If the key has not been found, -1 will be returned.
+ :rtype: long
+%End
+
   signals:
 
     void symbolsCounted();

src/core/qgsvectorlayer.h

@@ -807,9 +807,15 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
     void setDataSource( const QString &dataSource, const QString &baseName, const QString &provider, bool loadDefaultStyleFlag = false );
 
     /**
-     * Count features for symbols. Feature counts may be get by featureCount().
-     * \param showProgress show progress dialog
-     * \returns true if calculated, false if failed or was canceled by user
+     * Count features for symbols.
+     * The method will return immediately. You will need to connect to the
+     * symbolFeatureCountMapChanged() signal to be notified when the freshly updated
+     * feature counts are ready.
+     *
+     * \note If you need to wait for the results, create and start your own QgsVectorLayerFeatureCounter
+     *       task and call waitForFinished().
+     *
+     * \since This is asynchroneous since QGIS 3.0
      */
     bool countSymbolFeatures();
 

src/core/qgsvectorlayerfeaturecounter.cpp

@@ -77,3 +77,8 @@ QHash<QString, long> QgsVectorLayerFeatureCounter::symbolFeatureCountMap() const
 {
   return mSymbolFeatureCountMap;
 }
+
+long QgsVectorLayerFeatureCounter::featureCount( const QString &legendKey ) const
+{
+  return mSymbolFeatureCountMap.value( legendKey, -1 );
+}

src/core/qgsvectorlayerfeaturecounter.h

@@ -36,6 +36,12 @@ class CORE_EXPORT QgsVectorLayerFeatureCounter : public QgsTask
      */
     QHash<QString, long> symbolFeatureCountMap() const SIP_SKIP;
 
+    /**
+     * Get the feature count for a particular \a legendKey.
+     * If the key has not been found, -1 will be returned.
+     */
+    long featureCount( const QString &legendKey ) const;
+
   signals:
 
     /**