[Qt-interest] QTouchEvent/QGestureEvent propagation

[Sean Hayes]

On Mon, Apr 11, 2011 at 10:48 AM, Sean Hayes <sean.t.hayes at vanderbilt.edu>wrote:

> On Mon, Apr 11, 2011 at 10:09 AM, Stephen M. Webb <
> stephen.webb at bregmasoft.ca> wrote:
>
>> On Sun, 2011-04-10 at 22:33 +0200, Wathek LOUED wrote:
>> > Hi all,
>> >
>> > I've noticed that the Events in Qt using the
>> > QGraphicsView/QGraphicsScene/QGraphicsItem are spread from the
>> QGraphicsView
>> > then to the QGraphicsScene and the to the item that got the lowest
>> Z-Index
>> > to the most top item.
>> >
>> > I think that that's a bit weird cause generally when using QTouchEvent
>> and
>> > QGesture we look at the most top element so imagine that you programmed
>> a
>> > gesture and that that gesture occured at position (x, y) and in that
>> > position there's 4 items that are grabbing the same gesture, so it'll be
>> > grabbed by all the item and not the top element won't it ?
>>
>> But imagine the scenario in which a multi-touch gesture is made with
>> four fingers spread across the scene?  Which element should receive the
>> event?  Should all elements receive a single-finger gesture or does a
>> 4-finger gesture have some significance?  Is the QGraphicsScene not the
>> right place to determine that?
>
>
> I would argue that to proper place is dependent on your design. If you have
> two separate gesture types (one for single finger gestures and one for
> 4-finger gestures), then the scene could grab 4-fingered gestures and
> QGraphicsObjects could grab the single-finger gestures. If that is what you
> want, they should receive "simultaneous" but different events.
>
> "Ignoring the gesture will mean the gesture will never be offered to you
> again. It *will *be offered to a parent widget or item as well." - from Qt
> 4.7 Gesture Overview Documentation<http://doc.qt.nokia.com/latest/gestures-overview.html#standard-procedure-for-event-handling>
> .
>
> Therefore, if a QGraphicsObject ignores a gesture and has
> a QGraphicsObject, the gesture event should be passed to the
> parent QGraphicsObject. Likewise, if the parent of the QGraphicsObject is a
> QGraphicsScene, it should then be passed to the parent scene. If the scene
> then ignores the event, it should then be passed to the scenes parent (which
> might be the QGraphicsView). As I understand the documentation, that should
> be the order of the event propagation. It does not make sense for the scene
> or the view to receive the gesture event on both sides of the QGraphicObject
> handling of the event. Therefore, if scene should not receive the event
> until after the QGraphicsObject ignores the event. That is my just opinion
> of how it should be based on my limited knowledge of the Qt event loop
> documentation. Any flows with that logic?
>

I just realized that a QGraphicsObject's parent must another QGraphicsItem.
Therefore, I was incorrect to say that the parent could be the scene.
However, because QGraphicsObjects are within the scene. Isn't the scene in
most ways acting as the "parent" of QGraphicsObjects with no parent? The
scene "takes ownership of the item" like a parent normally would.

Sean
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.qt-project.org/pipermail/qt-interest-old/attachments/20110411/0e424f6f/attachment.html