Input events that originate from actual device interaction should reflect the
device's state, and device and events need to be kept in sync so that event
sequences (such as multi-touch events, where we have begin/update/end cycles
spanning multiple events) are working correctly.
For that reason, the event point data in pointer events is explicitly shared,
and we only detach in exceptional situations. This saves us memory allocations,
and makes sure that the event point data carried by events, and the event point
data stored persistently in the device, are kept in sync.
Cloned pointer events do not originate from device interactions, and should
therefore not sync back to the device. E.g. accepting a clone should not modify
the original event data stored in the device. There are exceptions here as
well, e.g. when cloning an event in Qt in order to deliver a translated version
of it to a different scene. Different points might even get delivered to
different scenes or windows, or at least different items in the same scene. For
that reason, we explicitly detach, and then explicitly write back the relevant
states after the cloned event has been delivered.
But in general, we should assume that cloned events do not write back to the
device. Since QEventPoint is an explicitly shared data type that never detaches
itself, we have to explicitly detach it when making copies that should not be
shared.
The ideal implementation of this would be to do the detach in the copy
constructor of QPointerEvent, which is called when cloning. However, Qt itself
makes copies of QPointerEvent without using clone, e.g. when assembling lists
of touch events for the different subscenes or windows in
QGuiApplicationPrivate::processTouchEvent, where event objects are added to a
QVarLengthArray<QMutableTouchEvent>. This makes copies, and those copies must
not detach.
So we have to implement the special cloning behavior in each override of
QPointerEvent::clone(). For this, introduce a dedicated macro for the common
member functions. This macro must be used for QPointerEvent subclasses.
Fixes: QTBUG-107560
Change-Id: I4b56f9e71c7d067ba9054a2a631e8ba5bc7b1ab9
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
12b7eab4d2