[Qt-interest] Drag and Drop when source is deleted

[Martin Grossberger]

> Den 20-05-2011 14:35, Martin Grossberger skrev:
>>> Den 20-05-2011 11:48, Martin Grossberger skrev:
>>>>> Den 20-05-2011 11:40, Martin Grossberger skrev:
>>>>>>> Hi
>>>>>>>
>>>>>>> Am 20.05.2011 11:04, schrieb Martin Grossberger:
>>>>>>>> However, the program still crashes before I can even check if the
>>>>>>>> widget
>>>>>>>> still exists on dropping.
>>>>>>> Maybe it's not what you are dragin, but the drag object itself. 
>>>>>>> Is it
>>>>>>> created on the heap or is there a possibility that it gets
>>>>>>> destroyed if
>>>>>>> the corresponding widget gets deleted while you are dragging?
>>>>>>>
>>>>>>> Alex
>>>>>>> _______________________________________________
>>>>>>> Qt-interest mailing list
>>>>>>> Qt-interest at qt.nokia.com
>>>>>>> http://lists.qt.nokia.com/mailman/listinfo/qt-interest
>>>>>> Hi,
>>>>>>
>>>>>> It is created on the heap. I've hacked together a minimalistic 
>>>>>> program
>>>>>> to reproduce the issue, and it has the same issue.
>>>>>> To reproduce the issue:
>>>>>>
>>>>>> 1. Compile the code attached
>>>>>> 2. Start dragging the DragWidget and hold it for 5 seconds
>>>>>> 3. After those 5 seconds the DragWidget gets deleted, if you drop
>>>>>> now, the program will crash
>>>>>
>>>>> That's because you have created the drag object with the widget as
>>>>> QObject parent. So when the widget is deleted, so is the drag object.
>>>>>
>>>>> Don't do that :)
>>>>>
>>>>> Bo Thorsen,
>>>>> Fionia Software.
>>>>>
>>>> Can you send me a line of code how this should work?
>>>> If I change the creation of the drag to:
>>>> QDrag *drag = new QDrag(0);
>>>> then no drag is actually happening.
>>>
>>> Sorry, that was the wrong direction. Apparently, you need to make sure
>>> the widget is alive for as long as the drag is active. I didn't know
>>> this. And it's IMO a Qt bug not to allow the drag source to be deleted.
>>>
>>> Bo Thorsen,
>>> Fionia Software.
>>>
>> I was hoping there would be another solution, because I have found no
>> way to tell if a drag&drop operation is occuring.
>> If I understand the documentation correctly, the best approach would be
>> to hide the widget instead of deleting it, and delete it once the
>> drag&drop operation is done (in mouseMoveEvent? according to the docs
>> QDrag::start or exec blocks the event loop).
>
> One thing you can do is use the destroyed signal on all QObject 
> subclasses plus a QPointer.
>
> So where you start the drag, you do this:
>
> class ... {
>
> private:
>   QPointer<QDrag*> mDrag;
> }
>
> startDrag() {
>   // do the drag here
>   mDrag.set(new QDrag);
>   ...
> }
>
> wantToDelete() {
>   if (mDrag.isNull()) {
>     delete this;
>   } else {
>     connect(mDrag.data(), SIGNAL(destroyed()), SLOT(deleteLater()));
>     hide();
>   }
> }
>
> I hope you can figure it out based on this.
>
> Bo Thorsen,
> Fionia Software.
>
That's a very nice idea. It works in my sample project. Big Thanks :)

Kind Regards,
Martin Grossberger

-- 
Martin Grossberger

Phone : +49 811 95949612
E-Mail : mgrossberger at andtek.com

ANDTEK GmbH
Am Söldnermoos 17
85399 Hallbergmoos
Deutschland/Germany

Phone: +49 811 9594960
FAX: +49 811 95949676
E-Mail: info at andtek.com

ANDTEK GmbH, Am Söldnermoos 17, 85399 Hallbergmoos, Deutschland, 
Geschäftsführer:Stefan Grossberger, Roland Russwurm, Telefon +49 (0) 811 
9594960, Fax +49(0) 811 95949676, Internet: http://www.andtek.com; 
Handelsregister: Gericht München, HRB 133279, USt-ID: DE813064648
The information transmitted is intended only for the person or entity to 
which it is addressed and may contain confidential and/or privileged 
material. Any review, retransmission, dissemination or other use of, or 
taking of any action in reliance upon, this information by persons or 
entities other than the intended recipient is prohibited. Any opinions 
expressed in this email are those of the individual and not necessarily 
the Company.
If you receive this transmission in error, please email to 
info at andtek.com, including a copy of this message. Please then delete 
this email and destroy any copies of it.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.qt-project.org/pipermail/qt-interest-old/attachments/20110520/039ba7e6/attachment.html