Merge pull request #521 from tiresiasfromthebai/imageviewer-wheel-zoom

Add Ctrl+wheel zoom to the Image Visualizer

src/QImageViewer.cpp

@@ -9,6 +9,7 @@
 #include <QtGui/QImageReader>
 #include <QtGui/QImageWriter>
 #include <QtGui/QPainter>
+#include <QtGui/QWheelEvent>
 #include <QtCore/QDebug>
 #include <QtPrintSupport/QPrintDialog>
 
@@ -19,6 +20,8 @@
 
 QImageViewer::QImageViewer (QWidget* parent) : QWidget(parent) {
 
+    setFocusPolicy(Qt::StrongFocus); // So the widget can take focus and receive the +/-/ESC zoom keys.
+
     _zoomFactor = 1.0;
 
     // Setup the widgets
@@ -34,6 +37,10 @@ QImageViewer::QImageViewer (QWidget* parent) : QWidget(parent) {
     _scrollArea->setWidget(_imageLabel);
     _scrollArea->setWidgetResizable(false);
 
+    // The scroll area consumes wheel events for scrolling, so watch its
+    // viewport to catch Ctrl+wheel for zooming.
+    _scrollArea->viewport()->installEventFilter(this);
+
     layout->addWidget(_scrollArea);
 
     clearImage();
@@ -174,7 +181,30 @@ void QImageViewer::keyPressEvent (QKeyEvent* event) {
     }
 }
 
-void QImageViewer::enterEvent (QEvent* event) {
+bool QImageViewer::eventFilter (QObject* object, QEvent* event) {
+
+    // Ctrl+wheel over the image zooms in or out. A plain wheel keeps
+    // scrolling the image as usual.
+    if (object == _scrollArea->viewport() && event->type() == QEvent::Wheel) {
+
+        QWheelEvent* wheelEvent = static_cast<QWheelEvent*>(event);
+
+        if (wheelEvent->modifiers() & Qt::ControlModifier) {
+
+            if (wheelEvent->angleDelta().y() > 0) {
+                zoomIn();
+            }else if (wheelEvent->angleDelta().y() < 0) {
+                zoomOut();
+            }
+
+            return true;
+        }
+    }
+
+    return QWidget::eventFilter(object, event);
+}
+
+void QImageViewer::enterEvent (QEnterEvent* event) {
 
     Q_UNUSED(event);
 

src/QImageViewer.h

@@ -38,8 +38,9 @@ class QImageViewer : public QWidget {
 
     protected slots:
         void                    keyPressEvent           (QKeyEvent* event);
-        void                    enterEvent              (QEvent*    event);
-        void                    leaveEvent              (QEvent*    event);
+        bool                    eventFilter             (QObject* object, QEvent* event) override;
+        void                    enterEvent              (QEnterEvent* event) override;
+        void                    leaveEvent              (QEvent*    event) override;
 
     private:
         QImage                  _image;

src/resources/help/ImageVisualizer.md

@@ -78,5 +78,6 @@ Available Quick keys while in the Image Visualizer:
     '+'   zoom in
     '-'   zoom out
     ESC   reset to default zoom level.
+    Ctrl+mouse wheel   zoom in or out.
 ```