Add QQuickWheelEvent::inverted()

When handling wheel events, it's useful to know whether or not
"natural" scrolling was enabled on platforms like OSX where
the scrolling direction can be reversed. It's especially useful
when implementing components like Slider (see QTBUG-35972).
This will soon be needed in QtQuick.Controls because we plan
to remove the WheelArea.

Change-Id: I2770e0aa39e7e54511d7f9f7cc12760580a55e5c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

4 files changed, 29 insertions, 4 deletions

diff --git a/src/imports/qtquick2/plugins.qmltypes b/src/imports/qtquick2/plugins.qmltypes
index a1a3723600..c3a14254f3 100644
--- a/src/imports/qtquick2/plugins.qmltypes
+++ b/src/imports/qtquick2/plugins.qmltypes
@@ -4628,6 +4628,7 @@ Module {
         Property { name: "pixelDelta"; type: "QPoint"; isReadonly: true }
         Property { name: "buttons"; type: "int"; isReadonly: true }
         Property { name: "modifiers"; type: "int"; isReadonly: true }
+        Property { name: "inverted"; type: "bool" }
         Property { name: "accepted"; type: "bool" }
     }
     Component {
diff --git a/src/quick/items/qquickevents.cpp b/src/quick/items/qquickevents.cpp
index 5061c19f1e..14c0adf393 100644
--- a/src/quick/items/qquickevents.cpp
+++ b/src/quick/items/qquickevents.cpp
@@ -415,4 +415,26 @@ Item {
     \endqml
 */
 
+/*!
+    \qmlproperty int QtQuick::WheelEvent::inverted
+
+    Returns whether the delta values delivered with the event are inverted.
+
+    Normally, a vertical wheel will produce a WheelEvent with positive delta
+    values if the top of the wheel is rotating away from the hand operating it.
+    Similarly, a horizontal wheel movement will produce a QWheelEvent with
+    positive delta values if the top of the wheel is moved to the left.
+
+    However, on some platforms this is configurable, so that the same
+    operations described above will produce negative delta values (but with the
+    same magnitude). For instance, in a QML component (such as a tumbler or a
+    slider) where it is appropriate to synchronize the movement or rotation of
+    an item with the direction of the wheel, regardless of the system settings,
+    the wheel event handler can use the inverted property to decide whether to
+    negate the angleDelta or pixelDelta values.
+
+    \note Many platforms provide no such information. On such platforms
+    \l inverted always returns false.
+*/
+
 QT_END_NAMESPACE
diff --git a/src/quick/items/qquickevents_p_p.h b/src/quick/items/qquickevents_p_p.h
index b28ab555b0..3eafce4d7e 100644
--- a/src/quick/items/qquickevents_p_p.h
+++ b/src/quick/items/qquickevents_p_p.h
@@ -153,13 +153,14 @@ class QQuickWheelEvent : public QObject
     Q_PROPERTY(QPoint pixelDelta READ pixelDelta)
     Q_PROPERTY(int buttons READ buttons)
     Q_PROPERTY(int modifiers READ modifiers)
+    Q_PROPERTY(bool inverted READ inverted)
     Q_PROPERTY(bool accepted READ isAccepted WRITE setAccepted)
 
 public:
     QQuickWheelEvent(qreal x, qreal y, const QPoint& angleDelta, const QPoint& pixelDelta,
-                     Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers)
+                     Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers, bool inverted)
         : _x(x), _y(y), _angleDelta(angleDelta), _pixelDelta(pixelDelta), _buttons(buttons),
-          _modifiers(modifiers), _accepted(true) {}
+          _modifiers(modifiers), _inverted(inverted), _accepted(true) {}
 
     qreal x() const { return _x; }
     qreal y() const { return _y; }
@@ -167,7 +168,7 @@ public:
     QPoint pixelDelta() const { return _pixelDelta; }
     int buttons() const { return _buttons; }
     int modifiers() const { return _modifiers; }
-
+    bool inverted() const { return _inverted; }
     bool isAccepted() { return _accepted; }
     void setAccepted(bool accepted) { _accepted = accepted; }
 
@@ -178,6 +179,7 @@ private:
     QPoint _pixelDelta;
     Qt::MouseButtons _buttons;
     Qt::KeyboardModifiers _modifiers;
+    bool _inverted;
     bool _accepted;
 };
 
diff --git a/src/quick/items/qquickmousearea.cpp b/src/quick/items/qquickmousearea.cpp
index 920a86881b..f940df59ed 100644
--- a/src/quick/items/qquickmousearea.cpp
+++ b/src/quick/items/qquickmousearea.cpp
@@ -866,7 +866,7 @@ void QQuickMouseArea::wheelEvent(QWheelEvent *event)
     }
 
     QQuickWheelEvent we(event->posF().x(), event->posF().y(), event->angleDelta(),
-                        event->pixelDelta(), event->buttons(), event->modifiers());
+                        event->pixelDelta(), event->buttons(), event->modifiers(), event->inverted());
     we.setAccepted(d->isWheelConnected());
     emit wheel(&we);
     if (!we.isAccepted())