[Qt-interest] QWidget and QThread ?

Patric userqt at gmail.com
Fri Jun 5 22:40:00 CEST 2009 

Thank you, Eric. I'll have to think about how exactly to do it...

Best regards,
Patric

----- Original Message ----- 
From: "Eric Clark" <eclark at ara.com>
To: <qt-interest at trolltech.com>
Sent: Friday, June 05, 2009 11:37 PM
Subject: Re: [Qt-interest] QWidget and QThread ?


> Ok... like I said... not all functions cause errors. However, many do, and 
> if you want to do this kind of thing, just pay close attention to the docs 
> when doing it; make sure the function is thread-safe. Be careful in any 
> slots connected to the textChanged signal or any signal you may be 
> connected to. Use mutexes when necessary. Just take note, that if you see 
> the error you just saw, the function cannot be called outside the GUI 
> thread and you need to do something to cause it to run on the GUI thread 
> like the example I gave. Good luck!
>
> Eric
>
> -----Original Message-----
> From: Patric [mailto:userqt at gmail.com]
> Sent: Friday, June 05, 2009 3:32 PM
> To: Eric Clark; qt-interest at trolltech.com
> Subject: Re: [Qt-interest] QWidget and QThread ?
>
> Yes... runtime error...
>
> ----- Original Message ----- 
> From: "Eric Clark" <eclark at ara.com>
> To: <qt-interest at trolltech.com>
> Sent: Friday, June 05, 2009 11:25 PM
> Subject: Re: [Qt-interest] QWidget and QThread ?
>
>
>> Did you try to hide it?
>>
>> -----Original Message-----
>> From: Patric [mailto:userqt at gmail.com]
>> Sent: Friday, June 05, 2009 3:24 PM
>> To: Eric Clark; qt-interest at trolltech.com
>> Subject: Re: [Qt-interest] QWidget and QThread ?
>>
>> 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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