[FEATURE] ctrl+wheel on canvas results in smaller zoom

Holding down ctrl while using the mouse wheel to zoom in or
out now results in a finer zoom. This behaviour brings canvas
into line with composer.

Author: nyalldawson

src/gui/qgsmapcanvas.cpp

@@ -1488,15 +1488,18 @@ void QgsMapCanvas::wheelEvent( QWheelEvent *e )
   if ( mMapTool )
   {
     mMapTool->wheelEvent( e );
+    if ( e->isAccepted() )
+      return;
   }
 
-  if ( QgsApplication::keyboardModifiers() )
+  double zoomFactor = mWheelZoomFactor;
+  if ( e->modifiers() & Qt::ControlModifier )
   {
-    // leave the wheel for map tools if any modifier pressed
-    return;
+    //holding ctrl while wheel zooming results in a finer zoom
+    zoomFactor = 1.0 + ( zoomFactor - 1.0 ) / 20.0;
   }
 
-  double signedWheelFactor = e->delta() > 0 ? 1 / mWheelZoomFactor : mWheelZoomFactor;
+  double signedWheelFactor = e->delta() > 0 ? 1 / zoomFactor : zoomFactor;
 
   // zoom map to mouse cursor by scaling
   QgsPoint oldCenter = center();

src/gui/qgsmapcanvas.h

@@ -789,6 +789,8 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     //! @note added in 2.16
     void updateMapSize();
 
+    friend class TestQgsMapCanvas;
+
 }; // class QgsMapCanvas
 Q_NOWARN_DEPRECATED_POP
 

src/gui/qgsmaptool.cpp

@@ -164,7 +164,7 @@ void QgsMapTool::canvasReleaseEvent( QgsMapMouseEvent* e )
 
 void QgsMapTool::wheelEvent( QWheelEvent *e )
 {
-  Q_UNUSED( e );
+  e->ignore();
 }
 
 void QgsMapTool::keyPressEvent( QKeyEvent *e )

tests/src/gui/testqgsmapcanvas.cpp

@@ -38,7 +38,7 @@ class TestQgsMapCanvas : public QObject
     Q_OBJECT
   public:
     TestQgsMapCanvas()
-        : mCanvas( 0 )
+        : mCanvas( nullptr )
     {}
 
   private slots:
@@ -50,6 +50,7 @@ class TestQgsMapCanvas : public QObject
     void testMagnification();
     void testMagnificationExtent();
     void testMagnificationScale();
+    void testZoomByWheel();
 
   private:
     QgsMapCanvas* mCanvas;
@@ -360,5 +361,39 @@ void TestQgsMapCanvas::testMagnificationScale()
   QCOMPARE( initialScale, mCanvas->scale() );
 }
 
+void TestQgsMapCanvas::testZoomByWheel()
+{
+  mCanvas->setExtent( QgsRectangle( 0, 0, 10, 10 ) );
+  QgsRectangle initialExtent = mCanvas->extent();
+  double originalWidth = initialExtent.width();
+  double originalHeight = initialExtent.height();
+
+  mCanvas->setWheelFactor( 2 );
+
+  //test zoom out
+  QWheelEvent e( QPoint( 0, 0 ), -1, Qt::NoButton, Qt::NoModifier );
+  mCanvas->wheelEvent( &e );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().width(), originalWidth * 2.0, 0.1 ) );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().height(), originalHeight * 2.0, 0.1 ) );
+
+  //test zoom in
+  e = QWheelEvent( QPoint( 0, 0 ), 1, Qt::NoButton, Qt::NoModifier );
+  mCanvas->wheelEvent( &e );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().width(), originalWidth, 0.1 ) );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().height(), originalHeight, 0.1 ) );
+
+  // test zoom out with ctrl
+  e = QWheelEvent( QPoint( 0, 0 ), -1, Qt::NoButton, Qt::ControlModifier );
+  mCanvas->wheelEvent( &e );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().width(), 1.05 * originalWidth, 0.1 ) );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().height(), 1.05 * originalHeight, 0.1 ) );
+
+  //test zoom in with ctrl
+  e = QWheelEvent( QPoint( 0, 0 ), 1, Qt::NoButton, Qt::ControlModifier );
+  mCanvas->wheelEvent( &e );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().width(), originalWidth, 0.1 ) );
+  QVERIFY( qgsDoubleNear( mCanvas->extent().height(), originalHeight, 0.1 ) );
+}
+
 QTEST_MAIN( TestQgsMapCanvas )
 #include "testqgsmapcanvas.moc"