Add snapMode to DragHandler

This changes snap behavior slightly. Basically, it does not snap
anymore if the target() item is an ancestor of the parentItem().

In addition, we add a property that enables users to change the behavior.
(SnapIfPressedOutsideTarget has the old behavior)

[ChangeLog][QtQuick][Event Handlers] Added DragHandler.snapMode which can
be used to configure under which conditions the dragged item is snapped
to be below the cursor. The default mode is SnapAuto. The old behavior
can be obtained through the SnapIfPressedOutsideTarget mode.

Fixes: QTBUG-75661
Change-Id: Ibc00e8fbe31b779f8e817af1505e76425467d27a
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 48 insertions, 2 deletions

diff --git a/src/quick/handlers/qquickdraghandler.cpp b/src/quick/handlers/qquickdraghandler.cpp
index 22fe3df4d0..61b66beff4 100644
--- a/src/quick/handlers/qquickdraghandler.cpp
+++ b/src/quick/handlers/qquickdraghandler.cpp
@@ -114,8 +114,14 @@ void QQuickDragHandler::onGrabChanged(QQuickPointerHandler *grabber, QQuickEvent
     QQuickMultiPointHandler::onGrabChanged(grabber, transition, point);
     if (grabber == this && transition == QQuickEventPoint::GrabExclusive && target()) {
         // In case the grab got handed over from another grabber, we might not get the Press.
-        if (!m_pressedInsideTarget) {
-            if (target() != parentItem())
+
+        auto isDescendant = [](QQuickItem *parent, QQuickItem *target) {
+            return (target != parent) && !target->isAncestorOf(parent);
+        };
+        if (m_snapMode == SnapAlways
+            || (m_snapMode == SnapIfPressedOutsideTarget && !m_pressedInsideTarget)
+            || (m_snapMode == SnapAuto && !m_pressedInsideTarget && isDescendant(parentItem(), target()))
+            ) {
                 m_pressTargetPos = QPointF(target()->width(), target()->height()) / 2;
         } else if (m_pressTargetPos.isNull()) {
             m_pressTargetPos = targetCentroidPosition();
@@ -123,6 +129,33 @@ void QQuickDragHandler::onGrabChanged(QQuickPointerHandler *grabber, QQuickEvent
     }
 }
 
+/*!
+    \qmlproperty enumeration QtQuick.DragHandler::snapMode
+
+    This property holds the snap mode.
+
+    The snap mode configures snapping of the \l target item's center to the event point.
+
+    Possible values:
+    \value DragHandler.SnapNever Never snap
+    \value DragHandler.SnapAuto The \l target snaps if the event point was pressed outside of the \l target
+                                item \e and the \l target is a descendant of \l parentItem (default)
+    \value DragHandler.SnapWhenPressedOutsideTarget The \l target snaps if the event point was pressed outside of the \l target
+    \value DragHandler.SnapAlways Always snap
+*/
+QQuickDragHandler::SnapMode QQuickDragHandler::snapMode() const
+{
+    return m_snapMode;
+}
+
+void QQuickDragHandler::setSnapMode(QQuickDragHandler::SnapMode mode)
+{
+    if (mode == m_snapMode)
+        return;
+    m_snapMode = mode;
+    emit snapModeChanged();
+}
+
 void QQuickDragHandler::onActiveChanged()
 {
     QQuickMultiPointHandler::onActiveChanged();
diff --git a/src/quick/handlers/qquickdraghandler_p.h b/src/quick/handlers/qquickdraghandler_p.h
index 748026488a..18c1fd841d 100644
--- a/src/quick/handlers/qquickdraghandler_p.h
+++ b/src/quick/handlers/qquickdraghandler_p.h
@@ -62,8 +62,17 @@ class Q_QUICK_PRIVATE_EXPORT QQuickDragHandler : public QQuickMultiPointHandler
     Q_PROPERTY(QQuickDragAxis * xAxis READ xAxis CONSTANT)
     Q_PROPERTY(QQuickDragAxis * yAxis READ yAxis CONSTANT)
     Q_PROPERTY(QVector2D translation READ translation NOTIFY translationChanged)
+    Q_PROPERTY(SnapMode snapMode READ snapMode WRITE setSnapMode NOTIFY snapModeChanged)
 
 public:
+    enum SnapMode {
+        NoSnap = 0,
+        SnapAuto,
+        SnapIfPressedOutsideTarget,
+        SnapAlways
+    };
+    Q_ENUM(SnapMode)
+
     explicit QQuickDragHandler(QQuickItem *parent = nullptr);
 
     void handlePointerEventImpl(QQuickPointerEvent *event) override;
@@ -73,11 +82,14 @@ public:
 
     QVector2D translation() const { return m_translation; }
     void setTranslation(const QVector2D &trans);
+    QQuickDragHandler::SnapMode snapMode() const;
+    void setSnapMode(QQuickDragHandler::SnapMode mode);
 
     void enforceConstraints();
 
 Q_SIGNALS:
     void translationChanged();
+    void snapModeChanged();
 
 protected:
     void onActiveChanged() override;
@@ -97,6 +109,7 @@ private:
 
     QQuickDragAxis m_xAxis;
     QQuickDragAxis m_yAxis;
+    QQuickDragHandler::SnapMode m_snapMode = SnapAuto;
     bool m_pressedInsideTarget = false;
 
     friend class QQuickDragAxis;