[Qt-interest] QWidget::setCursor() and qApp->processEvents()

[william.crocker at analog.com]

Joshua Grauman wrote:
> Thanks!
> 
> This indeed did the trick, but I must admit it is a bit baffling to me. I 
> didn't think the cursor was drawn by Qt, but by the OS.

If you are running under X11 then the cursor is drawn by the
X11 server. It will not change the cursor until the buffer of
commands which contains the change-cursor command is flushed
from the client (your app) to the server. This will not happen
until the buffer is full, the buffer is explicitly flushed
or an exchange is triggered which requires a round trip.

That is my understanding.

Bill

> That's why I 
> didn't think of doing a repaint although I understand how the event system 
> works. Indeed, my cursor is animated and it still animates smoothly while 
> I am doing my calculations while the rest of my app is completely 
> unresponsive... This seems to suggest that indeed the OS is drawing the 
> cursor, but then I'm still confused as to why a repaint is necessary to 
> change the cursor???
> 
> Josh
> 
>> Josh:
>>
>>  Does some_widget_or_other->repaint() work for you?
>>
>>                     Atlant
>>
>> -----Original Message-----
>> From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Josh
>> Sent: Wednesday, July 28, 2010 23:45 PM
>> To: qt-interest at trolltech.com
>> Subject: [Qt-interest] QWidget::setCursor() and qApp->processEvents()
>>
>> Hello all,
>>
>> Before I do some calculations in my program I want to set the cursor
>> using: viewport()->setCursor(Qt::WaitCursor);
>>
>> However, the cursor doesn't change because of the calculations. I don't
>> want to call qApp->processEvents() during the calculations because I
>> actually want the ui *not* to respond while the calculations are taking
>> place. So I was able to get the cursor to update BEFORE the calculations
>> as follows:
>>
>> qApp->processEvents();
>> qApp->processEvents();
>> viewport()->setCursor(Qt::WaitCursor);
>> for(int i=0; i<20; i++)
>>   qApp->processEvents();
>> do my calculations...
>>
>> This is obviously a hack. I needed to call processEvents 20 times for the
>> cursor to update. Any less and the cursor wouldn't update before my
>> calculations. I would assume that on different machines I might need
>> different number of calls to processEvents(), which is undesirable. Anyone
>> have any ideas on the correct way to do this?
>>
>> So in summary:
>> How can I make sure the cursor is changed before doing my calculations
>> when I don't want to call processEvents() during my calculations?
>>
>> Thanks!
>>
>> Josh
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