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

[John McClurkin]

Josh wrote:
> 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?
> 

AFAIR, processEvents() collapses all pending requests into a single 
call. Your mulitple calls on qApp->processEvents() essentially 
constitute a busy loop that halts your program until qApp processes 
events. You might play around with QTimer, setting a timeout after 
setCursor() and calling processEvents() in the timer slot.