[Qt-interest] QWidget and QThread ?

Fri Jun 5 22:12:48 CEST 2009

Are you sure that it is coming from a different thread? Try hiding the widget. What type of widget is it? Is it your own special widget, or is it a QLineEdit or something?

-----Original Message-----
From: Patric [mailto:userqt at gmail.com] 
Sent: Friday, June 05, 2009 3:01 PM
To: Eric Clark; qt-interest at trolltech.com
Subject: Re: [Qt-interest] QWidget and QThread ?

Hm, it's strange. I just did it 5 minutes ago.
I set the text of a line in my widget from a different thread trought the 
*ui variable.
There was so error message and the text changed...

Patric
----- Original Message ----- 
From: "Eric Clark" <eclark at ara.com>
To: <qt-interest at trolltech.com>
Sent: Friday, June 05, 2009 10:50 PM
Subject: Re: [Qt-interest] QWidget and QThread ?

> It really depends on what you are doing. Most of the time, you cannot 
> manipulate a QWidget anywhere other than the GUI thread. This is a 
> requirement of Qt and if you attempt to do it, Qt will crash and report an 
> error saying something along the lines of "Cannot manipulate QWidget 
> outside of the GUI thread," it's been a while since I have had one because 
> I know better now. It is difficult to know whether or not the action is 
> allowed, but I would say that you are best off never allowing manipulation 
> to happen on a different thread. I know, for example, that you cannot 
> hide/show/enable/disable widgets from any thread other than the GUI 
> thread. Setting text, checking boxes, creating widgets!!! Is not allowed 
> outside of the GUI thread. If you make an attempt at doing it, you will 
> know that you have done something wrong because Qt will tell you.
>
> I think this limitation is common among GUI frameworks, including MFC. I 
> am not a platform developer like those that work at Trolltech, so I do not 
> know the software or hardware reasons for this limitation, you would have 
> to ask someone on Trolltech's team to know the details.
>
> Eric
>
> -----Original Message-----
> From: Patric [mailto:userqt at gmail.com]
> Sent: Friday, June 05, 2009 1:44 PM
> To: Eric Clark; qt-interest at trolltech.com
> Subject: Re: [Qt-interest] QWidget and QThread ?
>
> Hm, I see. But Eric, why is that needed ? Why the data should be 
> manipulated
> only from the main thread ?
>
> Patric
>
> ----- Original Message ----- 
> From: "Eric Clark" <eclark at ara.com>
> To: <qt-interest at trolltech.com>
> Sent: Friday, June 05, 2009 6:21 PM
> Subject: Re: [Qt-interest] QWidget and QThread ?
>
>
>> Patric,
>>
>> We do some similar stuff in our application. This can be pretty tricky
>> though and you need to be very careful with the functions that manipulate
>> the data. Basically, when a function needs to manipulate the QWidget's
>> data, it needs to be done from the GUI thread. Here is our approach:
>>
>> void Foo()
>> {
>>   If this is not the main thread
>>   {
>>     Push the call to Foo onto the GUI thread
>>     Return;
>>   }
>>
>>   // Operate on the data of the widget
>> }
>>
>> Basically, if you have a function, Foo, that operates on the widget, when
>> the function is called, check to see if it was called from the main GUI
>> thread. If it was not, then you need to post an event to the GUI thread 
>> to
>> call this function. How you post the event is up to you. We have a 
>> special
>> Custom event to handle these calls and we also implemented a macro called
>> ThreadCheck that checks to see if the call was made on the GUI thread, 
>> and
>> if it wasn't, it posts the event to the GUI thread, and returns out of 
>> the
>> function.
>>
>> This approach can be somewhat tricky, and sometimes you may need to wait
>> for the event to get processed before returning from the function. If you
>> need to wait, use some kind of static or global variable that gets set
>> when the function is done running and sit in a while loop until the
>> variable says it has been called. Inside the while loop you can sleep the
>> thread periodically.
>>
>> Eric
>>
>> -----Original Message-----
>> From: qt-interest-bounces at trolltech.com
>> [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Patric
>> Sent: Friday, June 05, 2009 5:11 AM
>> To: Sean Harmer; qt-interest at trolltech.com
>> Subject: Re: [Qt-interest] QWidget and QThread ?
>>
>> Well, I have an application. With tabs. Every tab have some fields and
>> every
>> tab is different widget, created with QT Designer. I use the multiple
>> inheritance approach here.
>>
>> The GUI is generated from the main thread. But I want the data in each 
>> tab
>> (i.e. the fields in the tabs) to be managed by different threads, so the
>> GUI
>> not freeze if there is a problem or a slower operation.
>> And I'm wondering how to design this situation. It's all about.
>>
>> I didn't know I can't multiple inherite QObject.
>>
>> Regards,
>> Patric
>>
>>
>> ----- Original Message ----- 
>> From: "Sean Harmer" <sean.harmer at maps-technology.com>
>> To: <qt-interest at trolltech.com>
>> Sent: Friday, June 05, 2009 1:03 PM
>> Subject: Re: [Qt-interest] QWidget and QThread ?
>>
>>
>>> Hi,
>>>
>>> On Friday 05 Jun 2009 10:56:35 Patric wrote:
>>>> Hi all,
>>>> I have a form, which i generate in a widget using the multiple
>>>> inheritance
>>>> approach. I need a thread that operates on the data of this widget. I'm
>>>> wondering, is it a good idea to make my widget thread also (i.e.
>>>> multiple
>>>> inheritance from QWidget and QThread), so it'll be more convinient to
>>>> operate on the data. What do you think about it ?
>>> In general no. You can only inherit from QObject once and both QWidget
>>> and
>>> QThread are derived from QObject.
>>>
>>> Also all GUI operations must be performed in the main thread (the one in
>>> which
>>> QApplication lives).
>>>
>>> I would go even further and suggest that you redesign your application 
>>> to
>>> separate out the data and processing side from the GUI side. It will 
>>> make
>>> things cleaner in the long run.
>>>
>>> What is it you are trying to achieve? We may be able to give you some
>>> nudges
>>> in suitable directions?
>>>
>>> Cheers,
>>>
>>> Sean
>>>
>>> _______________________________________________
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>>> Qt-interest at trolltech.com
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>>>
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