Replace MultiPtHndlr.pointDistanceThreshold with PointerHandler.margin

It's not just useful for PinchHandler: TapHandler has a good use for it
too.  But unfortunately if the handler's parent Item has a custom mask,
we don't have a way to augment the mask with a margin; so if margin is
set, we assume the bounds are rectangular.

QQuickMultiPointHandler::eligiblePoints() now calls wantsEventPoint()
rather than bounds-checking the point directly: this adds flexibility,
potentially allowing an override in subclasses, if we need it later.

Task-number: QTBUG-68077
Change-Id: I65c95f00c532044a5862654e58c9c5f8c973df81
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 38 insertions, 1 deletions

diff --git a/src/quick/handlers/qquickpointerhandler.cpp b/src/quick/handlers/qquickpointerhandler.cpp
index 33c2acebe2..f222ac68f2 100644
--- a/src/quick/handlers/qquickpointerhandler.cpp
+++ b/src/quick/handlers/qquickpointerhandler.cpp
@@ -81,6 +81,39 @@ QQuickPointerHandler::~QQuickPointerHandler()
 }
 
 /*!
+     \qmlproperty real PointerHandler::margin
+
+     The margin beyond the bounds of the \l {PointerHandler::parent}{parent}
+     item within which an event point can activate this handler. For example, on
+     a PinchHandler where the \l {PointerHandler::target}{target} is also the
+     \c parent, it's useful to set this to a distance at least half the width
+     of a typical user's finger, so that if the \c parent has been scaled down
+     to a very small size, the pinch gesture is still possible.  Or, if a
+     TapHandler-based button is placed near the screen edge, it can be used
+     to comply with Fitts's Law: react to mouse clicks at the screen edge
+     even though the button is visually spaced away from the edge by a few pixels.
+
+     The default value is 0.
+
+     \image pointDistanceThreshold.png
+*/
+qreal QQuickPointerHandler::margin() const
+{
+    Q_D(const QQuickPointerHandler);
+    return d->m_margin;
+}
+
+void QQuickPointerHandler::setMargin(qreal pointDistanceThreshold)
+{
+    Q_D(QQuickPointerHandler);
+    if (d->m_margin == pointDistanceThreshold)
+        return;
+
+    d->m_margin = pointDistanceThreshold;
+    emit marginChanged();
+}
+
+/*!
     Notification that the grab has changed in some way which is relevant to this handler.
     The \a grabber (subject) will be the PointerHandler whose state is changing,
     or null if the state change regards an Item.
@@ -318,7 +351,11 @@ bool QQuickPointerHandler::parentContains(const QQuickEventPoint *point) const
             if (!par->window()->geometry().contains(screenPosition))
                 return false;
         }
-        return par->contains(par->mapFromScene(point->scenePosition()));
+        QPointF p = par->mapFromScene(point->scenePosition());
+        qreal m = margin();
+        if (m > 0)
+            return p.x() >= -m && p.y() >= -m && p.x() <= par->width() + m && p.y() <= par->height() + m;
+        return par->contains(p);
     }
     return false;
 }