Add DragHandler.activeTranslation and persistentTranslation

If you want to set target: null and then bind translation to some
object's x and y properties directly (perhaps an Item, a Qt Quick 3D
Model object, etc.), it's a lot less trouble to use a translation
property that does not keep changing back to 0,0 every time a gesture
begins. In hindsight, the translation property should have been the
persistent one (for consistency with the fix for QTBUG-68941,
in which we made PinchHandler.scale persistent and added activeScale:
b4d31c9ff5f0c5821ea127c663532d9fc2cae43e). But for several years, the
translation property has been restarting with each gesture; so now we
add a persistentTranslation property. The new activeTranslation property
has the same value as the translation property (which is deprecated).

Also, the persistentTranslation property is settable, because
in some UIs there may be multiple ways to move the same object,
and there needs to be a way to sync them up.

Also fixed a bug: when minimumPointCount == 2,
QQuickMultiPointHandler::wantsPointerEvent() doesn't initialize
d->currentPoints until two points are pressed. But often, one point is
pressed, and in the next event, the second point is pressed while the
first is held Stationary. So QQuickHandlerPoint::reset() needs to set
pressPosition and scenePressPosition on both points at the same time,
because it is called on each HandlerPoint in d->currentPoints at that
time when both points are pressed. So if any point is pressed, act as if
they all were freshly pressed. Without this fix, the centroid's
scenePressPosition is wrong (based on the average of 0,0 and the second
point), therefore a "jump" was occurring when persistentTranslation
is used to directly drive a binding (like the tilt in map.qml).

[ChangeLog][QtQuick][Event Handlers] DragHandler.activeTranslation now
holds the amount of movement since the drag gesture began.
DragHandler.persistentTranslation holds the accumulated sum of
movement that has occurred during subsequent drag gestures, and can
be set to arbitrary values between gestures.

Task-number: QTBUG-94168
Change-Id: I1b2f8ea31d0f6ff55ccffe393bc9ba28c1a71d09
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

1 files changed, 38 insertions, 5 deletions

diff --git a/src/quick/handlers/qquickdraghandler.cpp b/src/quick/handlers/qquickdraghandler.cpp
index 809adfd043..233516c3a3 100644
--- a/src/quick/handlers/qquickdraghandler.cpp
+++ b/src/quick/handlers/qquickdraghandler.cpp
@@ -165,6 +165,7 @@ void QQuickDragHandler::onActiveChanged()
             // mouse grab too, whenever dragging occurs in an enabled direction
             parent->setKeepMouseGrab(true);
         }
+        m_startTranslation = m_persistentTranslation;
     } else {
         m_pressTargetPos = QPointF();
         m_pressedInsideTarget = false;
@@ -210,7 +211,7 @@ void QQuickDragHandler::handlePointerEventImpl(QPointerEvent *event)
             accumulatedDragDelta.setX(0);
         if (!m_yAxis.enabled())
             accumulatedDragDelta.setY(0);
-        setTranslation(accumulatedDragDelta);
+        setActiveTranslation(accumulatedDragDelta);
     } else {
         // Check that all points have been dragged past the drag threshold,
         // to the extent that the constraints allow,
@@ -316,11 +317,24 @@ void QQuickDragHandler::enforceAxisConstraints(QPointF *localPos)
         localPos->setY(qBound(m_yAxis.minimum(), localPos->y(), m_yAxis.maximum()));
 }
 
-void QQuickDragHandler::setTranslation(const QVector2D &trans)
+void QQuickDragHandler::setPersistentTranslation(const QVector2D &trans)
 {
-    if (trans == m_translation) // fuzzy compare?
+    if (trans == m_persistentTranslation)
         return;
-    m_translation = trans;
+
+    m_persistentTranslation = trans;
+    emit translationChanged();
+}
+
+void QQuickDragHandler::setActiveTranslation(const QVector2D &trans)
+{
+    if (trans == m_activeTranslation)
+        return;
+
+    m_activeTranslation = trans;
+    m_persistentTranslation = m_startTranslation + trans;
+    qCDebug(lcDragHandler) << "translation: start" << m_startTranslation
+                           << "active" << m_activeTranslation << "accumulated" << m_persistentTranslation;
     emit translationChanged();
 }
 
@@ -357,8 +371,27 @@ void QQuickDragHandler::setTranslation(const QVector2D &trans)
 /*!
     \readonly
     \qmlproperty QVector2D QtQuick::DragHandler::translation
+    \deprecated Use activeTranslation
+*/
+
+/*!
+    \qmlproperty QVector2D QtQuick::DragHandler::persistentTranslation
+
+    The translation to be applied to the \l target if it is not \c null.
+    Otherwise, bindings can be used to do arbitrary things with this value.
+    While the drag gesture is being performed, \l activeTranslation is
+    continuously added to it; after the gesture ends, it stays the same.
+*/
+
+/*!
+    \readonly
+    \qmlproperty QVector2D QtQuick::DragHandler::activeTranslation
 
-    The translation since the gesture began.
+    The translation while the drag gesture is being performed.
+    It is \c {0, 0} when the gesture begins, and increases as the event
+    point(s) are dragged downward and to the right. After the gesture ends, it
+    stays the same; and when the next drag gesture begins, it is reset to
+    \c {0, 0} again.
 */
 
 QT_END_NAMESPACE