[composer] Use largest map in composition as reference map if no

reference map is explicitly set

src/core/composer/qgscomposition.cpp

@@ -791,7 +791,24 @@ void QgsComposition::setPrintResolution( const int dpi )
 
 QgsComposerMap* QgsComposition::referenceMap() const
 {
-  return dynamic_cast< QgsComposerMap* >( const_cast< QgsComposerItem* >( getComposerItemByUuid( mWorldFileMapId ) ) );
+  // prefer explicitly set reference map
+  if ( QgsComposerMap* map = dynamic_cast< QgsComposerMap* >( const_cast< QgsComposerItem* >( getComposerItemByUuid( mWorldFileMapId ) ) ) )
+    return map;
+
+  // else try to find largest map
+  QList< const QgsComposerMap* > maps = composerMapItems();
+  const QgsComposerMap* largestMap = nullptr;
+  double largestMapArea = 0;
+  Q_FOREACH ( const QgsComposerMap* map, maps )
+  {
+    double area = map->rect().width() * map->rect().height();
+    if ( area > largestMapArea )
+    {
+      largestMapArea = area;
+      largestMap = map;
+    }
+  }
+  return const_cast< QgsComposerMap* >( largestMap );
 }
 
 void QgsComposition::setReferenceMap( QgsComposerMap* map )

src/core/composer/qgscomposition.h

@@ -424,7 +424,8 @@ class CORE_EXPORT QgsComposition : public QGraphicsScene, public QgsExpressionCo
     void setGenerateWorldFile( bool enabled ) { mGenerateWorldFile = enabled; }
 
     /** Returns the map item which will be used to generate corresponding world files when the
-     * composition is exported, or nullptr if no corresponding map is set.
+     * composition is exported. If no map was explicitly set via setReferenceMap(), the largest
+     * map in the composition will be returned (or nullptr if there are no maps in the composition).
      * @see setReferenceMap()
      * @see generateWorldFile()
      */

tests/src/core/testqgscomposition.cpp

@@ -57,6 +57,7 @@ class TestQgsComposition : public QObject
     void georeference();
     void variablesEdited();
     void itemVariablesFunction();
+    void referenceMap();
 
   private:
     QgsComposition *mComposition;
@@ -639,5 +640,37 @@ void TestQgsComposition::itemVariablesFunction()
   delete composition;
 }
 
+void TestQgsComposition::referenceMap()
+{
+  QgsRectangle extent( 2000, 2800, 2500, 2900 );
+  QgsMapSettings ms;
+  ms.setExtent( extent );
+  QgsComposition* composition = new QgsComposition( ms, QgsProject::instance() );
+
+  // no maps
+  QVERIFY( !composition->referenceMap() );
+
+  QgsComposerMap* map = new QgsComposerMap( composition );
+  map->setNewExtent( extent );
+  map->setSceneRect( QRectF( 30, 60, 200, 100 ) );
+  composition->addComposerMap( map );
+
+  QCOMPARE( composition->referenceMap(), map );
+
+  // add a larger map
+  QgsComposerMap* map2 = new QgsComposerMap( composition );
+  map2->setNewExtent( extent );
+  map2->setSceneRect( QRectF( 30, 60, 250, 150 ) );
+  composition->addComposerMap( map2 );
+
+  QCOMPARE( composition->referenceMap(), map2 );
+
+  // explicitly set reference map
+  composition->setReferenceMap( map );
+  QCOMPARE( composition->referenceMap(), map );
+
+  delete composition;
+}
+
 QGSTEST_MAIN( TestQgsComposition )
 #include "testqgscomposition.moc"