[composer] Ensure that extent of new map items is centered on current centre of map

Author: nyalldawson

src/app/composer/qgscomposermapwidget.cpp

@@ -185,7 +185,7 @@ void QgsComposerMapWidget::on_mSetToMapCanvasExtentButton_clicked()
         newExtent.setYMinimum( newExtent.yMinimum() - deltaHeight / 2 );
         newExtent.setYMaximum( newExtent.yMaximum() + deltaHeight / 2 );
       }
-      else if ( currentWidthHeightRatio > newWidthHeightRatio )
+      else
       {
         //enlarge width of new extent, ensuring the map center stays the same
         double newWidth = currentWidthHeightRatio * newExtent.height();

src/core/composer/qgscomposermap.cpp

@@ -85,7 +85,10 @@ QgsComposerMap::QgsComposerMap( QgsComposition *composition, int x, int y, int w
   //calculate mExtent based on width/height ratio and map canvas extent
   if ( mMapRenderer )
   {
-    mExtent = mMapRenderer->extent();
+    QgsRectangle mapExtent = mMapRenderer->extent();
+    //make extent make item shape while keeping centre unchanged
+    adjustExtentToItemShape( width, height, mapExtent );
+    mExtent = mapExtent;
   }
   setSceneRect( QRectF( x, y, width, height ) );
   setToolTip( tr( "Map %1" ).arg( mId ) );
@@ -126,6 +129,29 @@ QgsComposerMap::QgsComposerMap( QgsComposition *composition )
   initGridAnnotationFormatFromProject();
 }
 
+void QgsComposerMap::adjustExtentToItemShape( double itemWidth, double itemHeight, QgsRectangle& extent ) const
+{
+  double itemWidthHeightRatio = itemWidth / itemHeight;
+  double newWidthHeightRatio = extent.width() / extent.height();
+
+  if ( itemWidthHeightRatio <= newWidthHeightRatio )
+  {
+    //enlarge height of new extent, ensuring the map center stays the same
+    double newHeight = extent.width() / itemWidthHeightRatio;
+    double deltaHeight = newHeight - extent.height();
+    extent.setYMinimum( extent.yMinimum() - deltaHeight / 2 );
+    extent.setYMaximum( extent.yMaximum() + deltaHeight / 2 );
+  }
+  else
+  {
+    //enlarge width of new extent, ensuring the map center stays the same
+    double newWidth = itemWidthHeightRatio * extent.height();
+    double deltaWidth = newWidth - extent.width();
+    extent.setXMinimum( extent.xMinimum() - deltaWidth / 2 );
+    extent.setXMaximum( extent.xMaximum() + deltaWidth / 2 );
+  }
+}
+
 void QgsComposerMap::extentCenteredOnOverview( QgsRectangle& extent ) const
 {
   extent = mExtent;

src/core/composer/qgscomposermap.h

@@ -479,6 +479,10 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
     /**True if annotation items, rubber band, etc. from the main canvas should be displayed*/
     bool mDrawCanvasItems;
 
+    /**Adjusts an extent rectangle to match the provided item width and height, so that extent
+     * center of extent remains the same */
+    void adjustExtentToItemShape( double itemWidth, double itemHeight, QgsRectangle& extent ) const;
+
     /**Draws the map grid*/
     void drawGrid( QPainter* p );
     void drawGridFrame( QPainter* p, const QList< QPair< double, QLineF > >& hLines, const QList< QPair< double, QLineF > >& vLines );