Expose drag threshold in MouseArea

In several cases such as for creating a Slider, it is useful to  have a
0 pixel threshold in order to initiate a drag operation. We have
previously hardcoded this to a platform dependent (usually 10 pixel)
value for MouseArea.

Note that we have no way of indicating the version/revision number
in a grouped property for documentation at the moment.

In addition we deliberately had to remove the REVISION from the
property because it is blocked by QTBUG-33179. However, since
this is not a user-creatable type it should not cause any
issues in practice.

This patch adds MouseArea.drag.threshold in order to improve on this.

Change-Id: Ia4871e64fab39e30c4494f00be99ad38cdd630df
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

5 files changed, 42 insertions, 10 deletions

diff --git a/src/quick/items/qquickdrag.cpp b/src/quick/items/qquickdrag.cpp
index 0ae26cb5c3..166b40aaf2 100644
--- a/src/quick/items/qquickdrag.cpp
+++ b/src/quick/items/qquickdrag.cpp
@@ -45,11 +45,12 @@
 #include <QtQuick/private/qquickevents_p_p.h>
 #include <private/qquickitemchangelistener_p.h>
 #include <private/qv8engine_p.h>
-#include <QtCore/qcoreapplication.h>
 #include <QtCore/qmimedata.h>
 #include <QtQml/qqmlinfo.h>
 #include <QtGui/qdrag.h>
 #include <QtGui/qevent.h>
+#include <QtGui/qstylehints.h>
+#include <QtGui/qguiapplication.h>
 
 #ifndef QT_NO_DRAGANDDROP
 
@@ -785,7 +786,8 @@ 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)
+_xmax(FLT_MAX), _ymin(-FLT_MAX), _ymax(FLT_MAX), _active(false), _filterChildren(false),
+  _threshold(qApp->styleHints()->startDragDistance())
 {
 }
 
@@ -879,6 +881,25 @@ void QQuickDrag::setYmax(qreal m)
     emit maximumYChanged();
 }
 
+
+qreal QQuickDrag::threshold() const
+{
+    return _threshold;
+}
+
+void QQuickDrag::setThreshold(qreal value)
+{
+    if (_threshold != value) {
+        _threshold = value;
+        emit thresholdChanged();
+    }
+}
+
+void QQuickDrag::resetThreshold()
+{
+    setThreshold(qApp->styleHints()->startDragDistance());
+}
+
 bool QQuickDrag::active() const
 {
     return _active;
diff --git a/src/quick/items/qquickdrag_p.h b/src/quick/items/qquickdrag_p.h
index 098fcc61b9..22829350fe 100644
--- a/src/quick/items/qquickdrag_p.h
+++ b/src/quick/items/qquickdrag_p.h
@@ -157,6 +157,9 @@ 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)
+    // 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)
     //### consider drag and drop
 
 public:
@@ -182,6 +185,10 @@ public:
     qreal ymax() const;
     void setYmax(qreal);
 
+    qreal threshold() const;
+    void setThreshold(qreal);
+    void resetThreshold();
+
     bool active() const;
     void setActive(bool);
 
@@ -199,6 +206,7 @@ Q_SIGNALS:
     void maximumYChanged();
     void activeChanged();
     void filterChildrenChanged();
+    void thresholdChanged();
 
 private:
     QQuickItem *_target;
@@ -209,6 +217,7 @@ private:
     qreal _ymax;
     bool _active : 1;
     bool _filterChildren: 1;
+    qreal _threshold;
     Q_DISABLE_COPY(QQuickDrag)
 };
 
diff --git a/src/quick/items/qquickmousearea.cpp b/src/quick/items/qquickmousearea.cpp
index 8a73a5d73d..39f07f2afd 100644
--- a/src/quick/items/qquickmousearea.cpp
+++ b/src/quick/items/qquickmousearea.cpp
@@ -48,10 +48,7 @@
 #include <private/qqmldata_p.h>
 
 #include <QtGui/private/qguiapplication_p.h>
-
 #include <QtGui/qevent.h>
-#include <QtGui/qguiapplication.h>
-#include <QtGui/qstylehints.h>
 
 #include <float.h>
 
@@ -695,8 +692,8 @@ void QQuickMouseArea::mouseMoveEvent(QMouseEvent *event)
             d->drag->target()->setPosition(dragPos);
 
         if (!keepMouseGrab()
-                && (QQuickWindowPrivate::dragOverThreshold(dragPos.x() - startPos.x(), Qt::XAxis, event)
-                || QQuickWindowPrivate::dragOverThreshold(dragPos.y() - startPos.y(), Qt::YAxis, event))) {
+                && (QQuickWindowPrivate::dragOverThreshold(dragPos.x() - startPos.x(), Qt::XAxis, event, d->drag->threshold())
+                || QQuickWindowPrivate::dragOverThreshold(dragPos.y() - startPos.y(), Qt::YAxis, event, d->drag->threshold()))) {
             setKeepMouseGrab(true);
             d->stealMouse = true;
             d->startScene = event->windowPos();
@@ -1189,6 +1186,7 @@ void QQuickMouseArea::setCursorShape(Qt::CursorShape shape)
     \qmlproperty real QtQuick2::MouseArea::drag.minimumY
     \qmlproperty real QtQuick2::MouseArea::drag.maximumY
     \qmlproperty bool QtQuick2::MouseArea::drag.filterChildren
+    \qmlproperty real QtQuick2::MouseArea::drag.threshold
 
     \c drag provides a convenient way to make an item draggable.
 
@@ -1213,6 +1211,10 @@ void QQuickMouseArea::setCursorShape(Qt::CursorShape shape)
     If \c drag.filterChildren is set to true, a drag can override descendant MouseAreas.  This
     enables a parent MouseArea to handle drags, for example, while descendants handle clicks:
 
+    \c drag.threshold determines the threshold in pixels of when the drag operation should
+    start. By default this is bound to a platform dependent value. This property was added in
+    Qt Quick 2.2.
+
     \snippet qml/mousearea/mouseareadragfilter.qml dragfilter
 
 */
diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index c8bc026868..513c3da0a2 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -2077,13 +2077,13 @@ bool QQuickWindowPrivate::sendFilteredMouseEvent(QQuickItem *target, QQuickItem
     return false;
 }
 
-bool QQuickWindowPrivate::dragOverThreshold(qreal d, Qt::Axis axis, QMouseEvent *event)
+bool QQuickWindowPrivate::dragOverThreshold(qreal d, Qt::Axis axis, QMouseEvent *event, int startDragThreshold)
 {
     QStyleHints *styleHints = qApp->styleHints();
     int caps = QGuiApplicationPrivate::mouseEventCaps(event);
     bool dragVelocityLimitAvailable = (caps & QTouchDevice::Velocity)
         && styleHints->startDragVelocity();
-    bool overThreshold = qAbs(d) > styleHints->startDragDistance();
+    bool overThreshold = qAbs(d) > (startDragThreshold >= 0 ? startDragThreshold : styleHints->startDragDistance());
     if (dragVelocityLimitAvailable) {
         QVector2D velocityVec = QGuiApplicationPrivate::mouseEventVelocity(event);
         qreal velocity = axis == Qt::XAxis ? velocityVec.x() : velocityVec.y();
diff --git a/src/quick/items/qquickwindow_p.h b/src/quick/items/qquickwindow_p.h
index 12925cbbca..e29ceb521f 100644
--- a/src/quick/items/qquickwindow_p.h
+++ b/src/quick/items/qquickwindow_p.h
@@ -221,7 +221,7 @@ public:
 
     static bool defaultAlphaBuffer;
 
-    static bool dragOverThreshold(qreal d, Qt::Axis axis, QMouseEvent *event);
+    static bool dragOverThreshold(qreal d, Qt::Axis axis, QMouseEvent *event, int startDragThreshold = -1);
 
     // data property
     static void data_append(QQmlListProperty<QObject> *, QObject *);