[Interest] QPrintPreviewWidget

[Dan Allen]

Hi Bill,

What you've listed below is pretty much how my printing mechanism 
works.  This works perfectly and I'm happy with it.

I'm still a little confused as to why the preview can't operate in the 
same way.  If another thread paints into a QImage and then informs the 
dialog using a signal such that the GUI thread can then display it on 
the screen, why is this an issue?  Why does the GUI thread have to do 
the time consuming task?  The Qt documentation for QImage states "When 
using QPainter on a QImage, the painting can be performed in another 
thread than the current GUI thread."

Thanks,

Dan Allen.


On 03/04/17 14:49, william.crocker at analog.com wrote:
> On 04/01/2017 09:48 AM, Dan Allen wrote:
>> Hi Bill,
>>
>> I agree, however when the print is quite complex it makes the GUI very
>> unresponsive.
>>
>> Painting into an off screen image is not GUI related. The print 
>> preview could
>> show something like "Generating Preview" while a background thread 
>> generates it
>> and then show it when it's ready for example. This would prevent the 
>> GUI from
>> seeming unresponsive to the user. I wasn't sure if there was a 
>> current method to
>> do something like this.
>>
>
> Yes. We agree that the actual printing is non GUI, but if you want
> to show a print preview or progress meter, that is GUI and needs
> to stay in the main thread.
>
> So, something like:
> - Create a progress meter dialog.
> - Start your printing in a sub thread.
> - The main thread waits, perhaps in a sub-QEventLoop, servicing events.
> - After a few seconds, if printing has not completed, show the dialog.
> - The sub-event loop monitors the printing process with a thread safe
>   communication mechanism, like signals and slots.
> - When the printing completes, close the dialog.
> - Exit your sub-event loop.
>
>> Thanks,
>>
>> Dan Allen.
>>
>>
>> On 01/04/17 13:43, william.crocker at analog.com wrote:
>>> On 04/01/2017 03:41 AM, Dan Allen wrote:
>>>> Hi,
>>>>
>>>> Could anyone tell me if it's possible to paint into a 
>>>> QPrintPreviewWidget in
>>>> another thread, and if so, how?
>>>>
>>>> I have created a class to perform printing and this runs in another 
>>>> thread.
>>>> When used for printing it works fine.  However when used for the 
>>>> print preview,
>>>> nothing is shown unless I force the signal to either 
>>>> Qt::DirectConnection (i.e.
>>>> use the GUI thread) or Qt ::BlockingQueuedConnection (i.e. block 
>>>> the GUI
>>>> thread).
>>>>
>>> Dan:
>>>
>>> Conventional wisdom, if not the rule of law, states that all GUI 
>>> activity
>>> should be done from the main thread. You may just be experiencing
>>> the Qt library enforcing this rule.
>>>
>>> Bill
>>>
>>>> Thanks,
>>>>
>>>> Dan Allen.
>>>>
>>>>
>>>> _______________________________________________
>>>> Interest mailing list
>>>> Interest at qt-project.org
>>>> http://lists.qt-project.org/mailman/listinfo/interest
>>>>
>>>>
>>>>
>>> _______________________________________________
>>> Interest mailing list
>>> Interest at qt-project.org
>>> http://lists.qt-project.org/mailman/listinfo/interest
>>
>>
>> _______________________________________________
>> Interest mailing list
>> Interest at qt-project.org
>> http://lists.qt-project.org/mailman/listinfo/interest
>>
>>
>>
> _______________________________________________
> Interest mailing list
> Interest at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/interest