[Qt-interest] Widget does not update content after some time

[John McClurkin]

Damien R wrote:
>  > Seeing two locks, I bet you have deadlock situation -- you've already
>  > acquired the lock in setImage(), then you invoke update. However, you
>  > still hold the lock. What happens if update() calls your paintEvent()
>  > method for the same object? You attempt to lock again, but you already
>  > hold the mutex. Deadlock.
> 
> No, it is not a deadlock. update() add an event in the process event 
> list of Qt so update() do not directly call paintEvent() as I understand 
> the documentation of Qt. So this code is never equivalent to the 
> following code :
> void setImage(std::size_t i)
> {
>   boost::mutex::scoped_lock lock(mutex_);
>   uchar * imageBuffer = image.bits();
>   memset(imageBuffer, i * 25 % 255, WIDTH * HEIGHT * 3);
>   paintEvent(0);
> }
> 
> Moreover, the program is not frozen. Indeed, if I hide the widget and 
> show it again, the content of the widget is updated.
> 
> --
> Damien R
Well, you can test Brad's assumption easily enough

void setImage(std::size_t i)
{
	{
		boost::mutex::scoped_lock lock(mutex_);
		uchar * imageBuffer = image.bits();
		memset(imageBuffer, i * 25 % 255, WIDTH * HEIGHT * 3);
	}
	update();
}

This will release the mutex before update is called.