[FEATURE] New expression variables for legend items

Adds new variables for use in data defined expressions for layout legend items, including

- @legend_title
- @legend_column_count
- @legend_split_layers
- @legend_wrap_string
- @legend_filter_by_map
- @legend_filter_out_atlas

Additionally, if the legend is linked to a map, then expressions used in that legend will also have access to the linked variables, including @map_scale, @map_extent, etc.

python/core/auto_generated/layout/qgslayoutitemlegend.sip.in

@@ -488,6 +488,9 @@ Returns the legend's renderer settings object.
     virtual void finalizeRestoreFromXml();
 
 
+    virtual QgsExpressionContext createExpressionContext() const;
+
+
 
   public slots:
 

src/core/expression/qgsexpression.cpp

@@ -757,6 +757,15 @@ void QgsExpression::initVariableHelp()
   // map canvas item variables
   sVariableHelpTexts.insert( QStringLiteral( "canvas_cursor_point" ), QCoreApplication::translate( "variable_help", "Last cursor position on the canvas in the project's geographical coordinates." ) );
 
+  // legend canvas item variables
+  sVariableHelpTexts.insert( QStringLiteral( "legend_title" ), QCoreApplication::translate( "variable_help", "Title of the legend." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "legend_column_count" ), QCoreApplication::translate( "variable_help", "Number of column in the legend." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "legend_split_layers" ), QCoreApplication::translate( "variable_help", "Boolean indicating if layers can be split in the legend." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "legend_wrap_string" ), QCoreApplication::translate( "variable_help", "Characters used to wrap the legend text." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "legend_filter_by_map" ), QCoreApplication::translate( "variable_help", "Boolean indicating if the content of the legend is filtered by the map." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "legend_filter_out_atlas" ), QCoreApplication::translate( "variable_help", "Boolean indicating if the Atlas is filtered out of the legend." ) );
+
+
   // map tool capture variables
   sVariableHelpTexts.insert( QStringLiteral( "snapping_results" ), QCoreApplication::translate( "variable_help",
                              "<p>An array with an item for each snapped point.</p>"

src/core/layout/qgslayoutitemlegend.cpp

@@ -818,6 +818,34 @@ void QgsLayoutItemLegend::onAtlasEnded()
   updateFilterByMap();
 }
 
+QgsExpressionContext QgsLayoutItemLegend::createExpressionContext() const
+{
+  QgsExpressionContext context = QgsLayoutItem::createExpressionContext();
+
+  // We only want the last scope from the map's expression context, as this contains
+  // the map specific variables. We don't want the rest of the map's context, because that
+  // will contain duplicate global, project, layout, etc scopes.
+
+  if ( mMap )
+  {
+    context.appendScope( mMap->createExpressionContext().popScope() );
+  }
+
+
+  QgsExpressionContextScope *scope = new QgsExpressionContextScope( tr( "Legend Settings" ) );
+
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_title" ), title(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_column_count" ), columnCount(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_split_layers" ), splitLayer(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_wrap_string" ), wrapString(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_filter_by_map" ), legendFilterByMapEnabled(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_filter_out_atlas" ), legendFilterOutAtlas(), true ) );
+
+  context.appendScope( scope );
+
+  return context;
+}
+
 // -------------------------------------------------------------------------
 #include "qgslayertreemodellegendnode.h"
 #include "qgsvectorlayer.h"

src/core/layout/qgslayoutitemlegend.h

@@ -436,6 +436,8 @@ class CORE_EXPORT QgsLayoutItemLegend : public QgsLayoutItem
 
     void finalizeRestoreFromXml() override;
 
+    QgsExpressionContext createExpressionContext() const override;
+
 
   public slots:
 

tests/src/python/test_qgslayoutlegend.py

@@ -29,7 +29,8 @@ from qgis.core import (QgsLayoutItemLegend,
                        QgsLayoutMeasurement,
                        QgsLayoutItem,
                        QgsLayoutPoint,
-                       QgsLayoutSize)
+                       QgsLayoutSize,
+                       QgsExpression)
 from qgis.testing import (start_app,
                           unittest
                           )
@@ -269,6 +270,41 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
         self.assertEqual(legend.columnCount(), 2)
         self.assertEqual(legend.legendSettings().columnCount(), 5)
 
+    def testLegendScopeVariables(self):
+        layout = QgsLayout(QgsProject.instance())
+        layout.initializeDefaults()
+
+        legend = QgsLayoutItemLegend(layout)
+        legend.setTitle("Legend")
+        layout.addLayoutItem(legend)
+
+        legend.setColumnCount(2)
+        legend.setWrapString('d')
+        legend.setLegendFilterOutAtlas(True)
+
+        expc = legend.createExpressionContext()
+        exp1 = QgsExpression("@legend_title")
+        self.assertEqual(exp1.evaluate(expc), "Legend")
+        exp2 = QgsExpression("@legend_column_count")
+        self.assertEqual(exp2.evaluate(expc), 2)
+        exp3 = QgsExpression("@legend_wrap_string")
+        self.assertEqual(exp3.evaluate(expc), 'd')
+        exp4 = QgsExpression("@legend_split_layers")
+        self.assertEqual(exp4.evaluate(expc), False)
+        exp5 = QgsExpression("@legend_filter_out_atlas")
+        self.assertEqual(exp5.evaluate(expc), True)
+
+        map = QgsLayoutItemMap(layout)
+        map.attemptSetSceneRect(QRectF(20, 20, 80, 80))
+        map.setFrameEnabled(True)
+        map.setExtent(QgsRectangle(781662.375, 3339523.125, 793062.375, 3345223.125))
+        layout.addLayoutItem(map)
+        map.setScale(15000)
+        legend.setLinkedMap(map)
+        expc2 = legend.createExpressionContext()
+        exp6 = QgsExpression("@map_scale")
+        self.assertAlmostEqual(exp6.evaluate(expc2), 15000, 2)
+
 
 if __name__ == '__main__':
     unittest.main()