[FEATURE] New expression variables for map settings

- map_crs, map_crs_definition: retrieves crs details for map
- map_units: retrieves units for map (eg 'meters','degrees')

Author: nyalldawson

src/core/composer/qgscomposermap.cpp

@@ -1834,6 +1834,13 @@ QgsExpressionContext QgsComposerMap::createExpressionContext() const
   QgsGeometry centerPoint = QgsGeometry::fromPoint( extent.center() );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_extent_center" ), QVariant::fromValue( centerPoint ), true ) );
 
+  if ( mComposition )
+  {
+    scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_crs" ), mComposition->mapSettings().destinationCrs().authid(), true ) );
+    scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_crs_definition" ), mComposition->mapSettings().destinationCrs().toProj4(), true ) );
+    scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_units" ), QgsUnitTypes::toString( mComposition->mapSettings().mapUnits() ), true ) );
+  }
+
   context.appendScope( scope );
 
   return context;

src/core/qgsexpression.cpp

@@ -5825,6 +5825,9 @@ void QgsExpression::initVariableHelp()
   gVariableHelpTexts.insert( QStringLiteral( "map_extent_center" ), QCoreApplication::translate( "variable_help", "Center of map." ) );
   gVariableHelpTexts.insert( QStringLiteral( "map_extent_width" ), QCoreApplication::translate( "variable_help", "Width of map." ) );
   gVariableHelpTexts.insert( QStringLiteral( "map_extent_height" ), QCoreApplication::translate( "variable_help", "Height of map." ) );
+  gVariableHelpTexts.insert( QStringLiteral( "map_crs" ), QCoreApplication::translate( "variable_help", "Coordinate reference system of map (e.g., 'EPSG:4326')." ) );
+  gVariableHelpTexts.insert( QStringLiteral( "map_crs_definition" ), QCoreApplication::translate( "variable_help", "Coordinate reference system of map (full definition)." ) );
+  gVariableHelpTexts.insert( QStringLiteral( "map_units" ), QCoreApplication::translate( "variable_help", "Units for map measurements." ) );
 
   gVariableHelpTexts.insert( QStringLiteral( "row_number" ), QCoreApplication::translate( "variable_help", "Stores the number of the current row." ) );
   gVariableHelpTexts.insert( QStringLiteral( "grid_number" ), QCoreApplication::translate( "variable_help", "Current grid annotation value." ) );

src/core/qgsexpressioncontext.cpp

@@ -803,6 +803,10 @@ QgsExpressionContextScope* QgsExpressionContextUtils::mapSettingsScope( const Qg
   QgsGeometry centerPoint = QgsGeometry::fromPoint( mapSettings.visibleExtent().center() );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_extent_center" ), QVariant::fromValue( centerPoint ), true ) );
 
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_crs" ), mapSettings.destinationCrs().authid(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_crs_definition" ), mapSettings.destinationCrs().toProj4(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "map_units" ), QgsUnitTypes::toString( mapSettings.mapUnits() ), true ) );
+
   return scope;
 }
 

tests/src/core/testqgscomposition.cpp

@@ -624,6 +624,18 @@ void TestQgsComposition::itemVariablesFunction()
   r = e.evaluate( &c );
   QGSCOMPARENEAR( r.toDouble(), 1.38916e+08, 100 );
 
+  QgsExpression e2( "map_get( item_variables( 'map_id' ), 'map_crs' )" );
+  r = e2.evaluate( &c );
+  QCOMPARE( r.toString(), QString( "EPSG:4326" ) );
+
+  QgsExpression e3( "map_get( item_variables( 'map_id' ), 'map_crs_definition' )" );
+  r = e3.evaluate( &c );
+  QCOMPARE( r.toString(), QString( "+proj=longlat +datum=WGS84 +no_defs" ) );
+
+  QgsExpression e4( "map_get( item_variables( 'map_id' ), 'map_units' )" );
+  r = e4.evaluate( &c );
+  QCOMPARE( r.toString(), QString( "degrees" ) );
+
   delete composition;
 }