New utility class QgsAggregateCalculator, simplifies calculating

aggregates from vector layer fields and expressions

python/core/core.sip

@@ -21,6 +21,7 @@
 %Include qgsapplication.sip
 %Include qgsaction.sip
 %Include qgsactionmanager.sip
+%Include qgsaggregatecalculator.sip
 %Include qgsattributeaction.sip
 %Include qgsattributetableconfig.sip
 %Include qgsbrowsermodel.sip

python/core/qgsaggregatecalculator.sip

@@ -0,0 +1,72 @@
+/** \ingroup core
+ * \class QgsAggregateCalculator
+ * \brief Utility class for calculating aggregates for a field (or expression) over the features
+ * from a vector layer.
+ * \note added in QGIS 2.16
+ */
+class QgsAggregateCalculator
+{
+
+%TypeHeaderCode
+#include <qgsaggregatecalculator.h>
+%End
+
+  public:
+
+    //! Available aggregates to calculate. Not all aggregates are available for all field
+    //! types.
+    enum Aggregate
+    {
+      Count,  //!< Count
+      CountDistinct,  //!< Number of distinct values
+      CountMissing,  //!< Number of missing (null) values
+      Min,  //!< Min of values
+      Max,  //!< Max of values
+      Sum,  //!< Sum of values
+      Mean,  //!< Mean of values (numeric fields only)
+      Median, //!< Median of values (numeric fields only)
+      StDev, //!< Standard deviation of values (numeric fields only)
+      StDevSample, //!< Sample standard deviation of values (numeric fields only)
+      Range, //!< Range of values (max - min) (numeric and datetime fields only)
+      Minority, //!< Minority of values (numeric fields only)
+      Majority, //!< Majority of values (numeric fields only)
+      FirstQuartile, //!< First quartile (numeric fields only)
+      ThirdQuartile, //!< Third quartile (numeric fields only)
+      InterQuartileRange, //!< Inter quartile range (IQR) (numeric fields only)
+      StringMinimumLength, //!< Minimum length of string (string fields only)
+      StringMaximumLength, //!< Maximum length of string (string fields only)
+    };
+
+    /** Constructor for QgsAggregateCalculator.
+     * @param layer vector layer to calculate aggregate from
+     */
+    QgsAggregateCalculator( QgsVectorLayer* layer );
+
+    /** Returns the associated vector layer.
+     */
+    QgsVectorLayer* layer() const;
+
+    /** Sets a filter to limit the features used during the aggregate calculation.
+     * @param filterExpression expression for filtering features, or empty string to remove filter
+     * @see filter()
+     */
+    void setFilter( const QString& filterExpression );
+
+    /** Returns the filter which limits the features used during the aggregate calculation.
+     * @see setFilter()
+     */
+    QString filter() const;
+
+    /** Calculates the value of an aggregate.
+     * @param aggregate aggregate to calculate
+     * @param fieldOrExpression source field or expression to use as basis for aggregated values.
+     * If an expression is used, then the context parameter must be set.
+     * @param context expression context for evaluating expressions
+     * @param ok if specified, will be set to true if aggregate calculation was successful
+     * @returns calculated aggregate value
+     */
+    QVariant calculate( Aggregate aggregate, const QString& fieldOrExpression,
+                        QgsExpressionContext* context = nullptr, bool* ok = nullptr ) const;
+
+
+};

src/core/CMakeLists.txt

@@ -79,6 +79,7 @@ SET(QGIS_CORE_SRCS
   qgsapplication.cpp
   qgsaction.cpp
   qgsactionmanager.cpp
+  qgsaggregatecalculator.cpp
   qgsattributetableconfig.cpp
   qgsbrowsermodel.cpp
   qgscachedfeatureiterator.cpp
@@ -597,6 +598,7 @@ SET(QGIS_CORE_HDRS
 
   qgis.h
   qgsaction.h
+  qgsaggregatecalculator.h
   qgsattributetableconfig.h
   qgsattributeaction.h
   qgscachedfeatureiterator.h