Add Flickable.synchronousDrag property

When it is set true, Flickable begins dragging by making the content
jump to the position where it would have been if there was no drag
threshold: that is, the content moves exactly in sync with the mouse
cursor or finger (as long as it's not hitting the bounds).

[ChangeLog][QtQuick][Flickable] Added a synchronousDrag property that
makes the content jump to the position it would have had if there was
no drag threshold, as soon as dragging begins.

Task-number: QTBUG-62902
Change-Id: I5f3b530956363172167896b0f19aec4a41bf82b3
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

4 files changed, 37 insertions, 2 deletions

diff --git a/src/quick/items/qquickflickable.cpp b/src/quick/items/qquickflickable.cpp
index d892e3b490..5a3db5e1eb 100644
--- a/src/quick/items/qquickflickable.cpp
+++ b/src/quick/items/qquickflickable.cpp
@@ -248,6 +248,7 @@ QQuickFlickablePrivate::QQuickFlickablePrivate()
     , stealMouse(false), pressed(false)
     , scrollingPhase(false), interactive(true), calcVelocity(false)
     , pixelAligned(false)
+    , syncDrag(false)
     , lastPosTime(-1)
     , lastPressTime(0)
     , deceleration(QML_FLICK_DEFAULTDECELERATION)
@@ -959,6 +960,33 @@ void QQuickFlickable::setPixelAligned(bool align)
     }
 }
 
+/*!
+    \qmlproperty bool QtQuick::Flickable::synchronousDrag
+    \since 5.12
+
+    If this property is set to true, then when the mouse or touchpoint moves
+    far enough to begin dragging the content, the content will jump, such that
+    the content pixel which was under the cursor or touchpoint when pressed
+    remains under that point.
+
+    The default is \c false, which provides a smoother experience (no jump)
+    at the cost that some of the drag distance is "lost" at the beginning.
+*/
+bool QQuickFlickable::synchronousDrag() const
+{
+    Q_D(const QQuickFlickable);
+    return d->syncDrag;
+}
+
+void QQuickFlickable::setSynchronousDrag(bool v)
+{
+    Q_D(QQuickFlickable);
+    if (v != d->syncDrag) {
+        d->syncDrag = v;
+        emit synchronousDragChanged();
+    }
+}
+
 qint64 QQuickFlickablePrivate::computeCurrentTime(QInputEvent *event) const
 {
     if (0 != event->timestamp())
@@ -1075,7 +1103,7 @@ void QQuickFlickablePrivate::drag(qint64 currentTimestamp, QEvent::Type eventTyp
         if (overThreshold || elapsedSincePress > 200) {
             if (!vMoved)
                 vData.dragStartOffset = dy;
-            qreal newY = dy + vData.pressPos - vData.dragStartOffset;
+            qreal newY = dy + vData.pressPos - (syncDrag ? 0 : vData.dragStartOffset);
             // Recalculate bounds in case margins have changed, but use the content
             // size estimate taken at the start of the drag in case the drag causes
             // the estimate to be altered
@@ -1151,7 +1179,7 @@ void QQuickFlickablePrivate::drag(qint64 currentTimestamp, QEvent::Type eventTyp
         if (overThreshold || elapsedSincePress > 200) {
             if (!hMoved)
                 hData.dragStartOffset = dx;
-            qreal newX = dx + hData.pressPos - hData.dragStartOffset;
+            qreal newX = dx + hData.pressPos - (syncDrag ? 0 : hData.dragStartOffset);
             const qreal minX = hData.dragMinBound + hData.startMargin;
             const qreal maxX = hData.dragMaxBound - hData.endMargin;
             if (!(boundsBehavior & QQuickFlickable::DragOverBounds)) {
diff --git a/src/quick/items/qquickflickable_p.h b/src/quick/items/qquickflickable_p.h
index 939e3af698..45921c8b86 100644
--- a/src/quick/items/qquickflickable_p.h
+++ b/src/quick/items/qquickflickable_p.h
@@ -106,6 +106,7 @@ class Q_QUICK_PRIVATE_EXPORT QQuickFlickable : public QQuickItem
     Q_PROPERTY(QQuickFlickableVisibleArea *visibleArea READ visibleArea CONSTANT)
 
     Q_PROPERTY(bool pixelAligned READ pixelAligned WRITE setPixelAligned NOTIFY pixelAlignedChanged)
+    Q_PROPERTY(bool synchronousDrag READ synchronousDrag WRITE setSynchronousDrag NOTIFY synchronousDragChanged REVISION 12)
 
     Q_PROPERTY(qreal horizontalOvershoot READ horizontalOvershoot NOTIFY horizontalOvershootChanged REVISION 9)
     Q_PROPERTY(qreal verticalOvershoot READ verticalOvershoot NOTIFY verticalOvershootChanged REVISION 9)
@@ -213,6 +214,9 @@ public:
     bool pixelAligned() const;
     void setPixelAligned(bool align);
 
+    bool synchronousDrag() const;
+    void setSynchronousDrag(bool v);
+
     qreal horizontalOvershoot() const;
     qreal verticalOvershoot() const;
 
@@ -259,6 +263,7 @@ Q_SIGNALS:
     void dragStarted();
     void dragEnded();
     void pixelAlignedChanged();
+    Q_REVISION(12) void synchronousDragChanged();
     Q_REVISION(9) void horizontalOvershootChanged();
     Q_REVISION(9) void verticalOvershootChanged();
 
diff --git a/src/quick/items/qquickflickable_p_p.h b/src/quick/items/qquickflickable_p_p.h
index 08f069e830..dfb943d710 100644
--- a/src/quick/items/qquickflickable_p_p.h
+++ b/src/quick/items/qquickflickable_p_p.h
@@ -221,6 +221,7 @@ public:
     bool interactive : 1;
     bool calcVelocity : 1;
     bool pixelAligned : 1;
+    bool syncDrag : 1;
     QElapsedTimer timer;
     QBasicTimer movementEndingTimer;
     qint64 lastPosTime;
diff --git a/src/quick/items/qquickitemsmodule.cpp b/src/quick/items/qquickitemsmodule.cpp
index 96927dd37a..806fd9e465 100644
--- a/src/quick/items/qquickitemsmodule.cpp
+++ b/src/quick/items/qquickitemsmodule.cpp
@@ -425,6 +425,7 @@ static void qt_quickitems_defineModule(const char *uri, int major, int minor)
 
     qmlRegisterType<QQuickAnimatedSprite, 12>("QtQuick", 2, 12, "AnimatedSprite");
     qmlRegisterType<QQuickGradient, 12>(uri, 2, 12, "Gradient");
+    qmlRegisterType<QQuickFlickable, 12>(uri, 2, 12, "Flickable");
 }
 
 static void initResources()