Pinchzoom example zooms more smooth

Remember the scale factor so it won't reset each time a new touch sequence
starts.

Reviewed-by: Bradley T. Hughes
(cherry picked from commit 791c4035c28257e5a8426e6519866546faaf5286)

2 files changed, 18 insertions, 5 deletions

diff --git a/examples/multitouch/pinchzoom/graphicsview.cpp b/examples/multitouch/pinchzoom/graphicsview.cpp
index 0c79073..00e620a 100644
--- a/examples/multitouch/pinchzoom/graphicsview.cpp
+++ b/examples/multitouch/pinchzoom/graphicsview.cpp
@@ -45,7 +45,7 @@
 #include <QTouchEvent>
 
 GraphicsView::GraphicsView(QGraphicsScene *scene, QWidget *parent)
-    : QGraphicsView(scene, parent)
+    : QGraphicsView(scene, parent), totalScaleFactor(1)
 {
     viewport()->setAttribute(Qt::WA_AcceptTouchEvents);
     setDragMode(ScrollHandDrag);
@@ -58,14 +58,24 @@ bool GraphicsView::viewportEvent(QEvent *event)
     case QEvent::TouchUpdate:
     case QEvent::TouchEnd:
     {
-        QList<QTouchEvent::TouchPoint> touchPoints = static_cast<QTouchEvent *>(event)->touchPoints();
+        QTouchEvent *touchEvent = static_cast<QTouchEvent *>(event);
+        QList<QTouchEvent::TouchPoint> touchPoints = touchEvent->touchPoints();
         if (touchPoints.count() == 2) {
             // determine scale factor
             const QTouchEvent::TouchPoint &touchPoint0 = touchPoints.first();
             const QTouchEvent::TouchPoint &touchPoint1 = touchPoints.last();
-            const qreal scaleFactor = QLineF(touchPoint0.pos(), touchPoint1.pos()).length()
-                                      / QLineF(touchPoint0.startPos(), touchPoint1.startPos()).length();
-            setTransform(QTransform().scale(scaleFactor, scaleFactor));
+            qreal currentScaleFactor =
+                    QLineF(touchPoint0.pos(), touchPoint1.pos()).length()
+                    / QLineF(touchPoint0.startPos(), touchPoint1.startPos()).length();
+            if (touchEvent->touchPointStates() & Qt::TouchPointReleased) {
+                // if one of the fingers is released, remember the current scale
+                // factor so that adding another finger later will continue zooming
+                // by adding new scale factor to the existing remembered value.
+                totalScaleFactor *= currentScaleFactor;
+                currentScaleFactor = 1;
+            }
+            setTransform(QTransform().scale(totalScaleFactor * currentScaleFactor,
+                                            totalScaleFactor * currentScaleFactor));
         }
         return true;
     }
diff --git a/examples/multitouch/pinchzoom/graphicsview.h b/examples/multitouch/pinchzoom/graphicsview.h
index c5195cd..81a115d 100644
--- a/examples/multitouch/pinchzoom/graphicsview.h
+++ b/examples/multitouch/pinchzoom/graphicsview.h
@@ -50,4 +50,7 @@ public:
     GraphicsView(QGraphicsScene *scene = 0, QWidget *parent = 0);
 
     bool viewportEvent(QEvent *event);
+
+private:
+    qreal totalScaleFactor;
 };