Introduce MouseArea.drag.smoothed

[ChangeLog][QtQuick] Added property MouseArea.drag.smoothed for toggling
whether the drag target is moved to the current mouse position after a
drag operation has started.

Task-number: QTBUG-38539
Change-Id: I989b51bc83fc24c64b84ac8e9a50b352e674e8f8
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>

3 files changed, 26 insertions, 2 deletions

diff --git a/src/quick/items/qquickdrag.cpp b/src/quick/items/qquickdrag.cpp
index 6c36032d3c..7e5b357e9c 100644
--- a/src/quick/items/qquickdrag.cpp
+++ b/src/quick/items/qquickdrag.cpp
@@ -791,7 +791,7 @@ void QQuickDragAttached::startDrag(QQmlV4Function *args)
 QQuickDrag::QQuickDrag(QObject *parent)
 : QObject(parent), _target(0), _axis(XAndYAxis), _xmin(-FLT_MAX),
 _xmax(FLT_MAX), _ymin(-FLT_MAX), _ymax(FLT_MAX), _active(false), _filterChildren(false),
-  _threshold(qApp->styleHints()->startDragDistance())
+  _smoothed(true), _threshold(qApp->styleHints()->startDragDistance())
 {
 }
 
@@ -885,6 +885,18 @@ void QQuickDrag::setYmax(qreal m)
     emit maximumYChanged();
 }
 
+bool QQuickDrag::smoothed() const
+{
+    return _smoothed;
+}
+
+void QQuickDrag::setSmoothed(bool smooth)
+{
+    if (_smoothed != smooth) {
+        _smoothed = smooth;
+        emit smoothedChanged();
+    }
+}
 
 qreal QQuickDrag::threshold() const
 {
diff --git a/src/quick/items/qquickdrag_p.h b/src/quick/items/qquickdrag_p.h
index d9021d0f6d..eebe07469a 100644
--- a/src/quick/items/qquickdrag_p.h
+++ b/src/quick/items/qquickdrag_p.h
@@ -157,6 +157,7 @@ class Q_AUTOTEST_EXPORT QQuickDrag : public QObject
     Q_PROPERTY(qreal maximumY READ ymax WRITE setYmax NOTIFY maximumYChanged)
     Q_PROPERTY(bool active READ active NOTIFY activeChanged)
     Q_PROPERTY(bool filterChildren READ filterChildren WRITE setFilterChildren NOTIFY filterChildrenChanged)
+    Q_PROPERTY(bool smoothed READ smoothed WRITE setSmoothed NOTIFY smoothedChanged)
     // Note, threshold was added in QtQuick 2.2 but REVISION is not supported (or needed) for grouped
     // properties See QTBUG-33179
     Q_PROPERTY(qreal threshold READ threshold WRITE setThreshold NOTIFY thresholdChanged RESET resetThreshold)
@@ -185,6 +186,9 @@ public:
     qreal ymax() const;
     void setYmax(qreal);
 
+    bool smoothed() const;
+    void setSmoothed(bool smooth);
+
     qreal threshold() const;
     void setThreshold(qreal);
     void resetThreshold();
@@ -206,6 +210,7 @@ Q_SIGNALS:
     void maximumYChanged();
     void activeChanged();
     void filterChildrenChanged();
+    void smoothedChanged();
     void thresholdChanged();
 
 private:
@@ -217,6 +222,7 @@ private:
     qreal _ymax;
     bool _active : 1;
     bool _filterChildren: 1;
+    bool _smoothed : 1;
     qreal _threshold;
     Q_DISABLE_COPY(QQuickDrag)
 };
diff --git a/src/quick/items/qquickmousearea.cpp b/src/quick/items/qquickmousearea.cpp
index 2d84660b6d..01d1305260 100644
--- a/src/quick/items/qquickmousearea.cpp
+++ b/src/quick/items/qquickmousearea.cpp
@@ -723,7 +723,9 @@ void QQuickMouseArea::mouseMoveEvent(QMouseEvent *event)
                 || QQuickWindowPrivate::dragOverThreshold(dragPos.y() - startPos.y(), Qt::YAxis, event, d->drag->threshold()))) {
             setKeepMouseGrab(true);
             d->stealMouse = true;
-            d->startScene = event->windowPos();
+
+            if (d->drag->smoothed())
+                d->startScene = event->windowPos();
         }
 
         d->moved = true;
@@ -1250,6 +1252,10 @@ void QQuickMouseArea::setCursorShape(Qt::CursorShape shape)
     start. By default this is bound to a platform dependent value. This property was added in
     Qt Quick 2.2.
 
+    If \c drag.smoothed is \c true, the target will be moved only after the drag operation has
+    started. If set to \c false, the target will be moved straight to the current mouse position.
+    By default, this property is \c true. This property was added in Qt Quick 2.4
+
     \snippet qml/mousearea/mouseareadragfilter.qml dragfilter
 
 */