[Qt-interest] QWidget and QThread ?

Fri Jun 5 22:23:40 CEST 2009

It's QLineEdit. But I don't use signals&slots, I'm just calling the method 
of the object from different thread. It's QLineEdit in my custom widget.

Patric

----- Original Message ----- 
From: "Eric Clark" <eclark at ara.com>
To: <qt-interest at trolltech.com>
Sent: Friday, June 05, 2009 11:12 PM
Subject: Re: [Qt-interest] QWidget and QThread ?

> 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
>>>> http://lists.trolltech.com/mailman/listinfo/qt-interest
>>>>
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