[Interest] QThread and QPixmap::grabWindow()

[Alexander Syvak]

It's done as following, the thread wrapper (the class holding the QThread
object and the pointer to be moved into the thread-worker) connects the
class to be moved into the worker-thread with the main thread slot
'make_screenshot(pointer to the class which emits the signal)'. The main
thread slot puts into the pointer the made screenshot.
However, pictures are received are half damaged.
When there's only one thread worker, the screenshots are OK.
Any variants?


2013/8/8 Rainer Wiesenfarth <Rainer_Wiesenfarth at trimble.com>

> Am 08.08.2013 00:11, schrieb Alexander Syvak:
>
>> [...]
>>
>>     auto const this_thread = this->thread ();
>>     moveToThread (QApplication::instance()->**thread());
>>
>>     screenshot = QPixmap::grabWindow( QApplication::desktop()->**winId()
>> );
>>     moveToThread(this_thread);
>> [...]
>>
>
> I think you might have a problem in understanding threads and the thread
> context as used in Qt. moveToThread() does NOT change the thread the
> current code runs in! It only changes the target for events sent to this
> object.
>
> In your example, the thread that executes this->thread() is identical to
> the one that executes QPixmap::grabWindow().
>
> You will have to implement synchronization between the main (GUI) thread
> and your workers. The workers send an event to the main thread each time
> they need a screenshot, the main thread picks up the event, takes the
> screenshot and sends an event back to the "caller". The addressed worker
> thread then picks up this event and processes the screenshot.
>
> If you have only one worker, you might wrap up your processing in a single
> method and use QtConcurrent::run().
>
> Best Regards / Mit freundlichen Grüßen
> Rainer Wiesenfarth
>
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