How to use separate threads for pushbuttons in QT (like SwingWorker in Java)

[artt]

I do not understand how link this 2 connect statements:
f.i. connect(thread, SIGNAL(updateText(QString)), label, SLOT(setText(QString)));
what should be (updateText(QString)) - should it be wrapper of run() or start(), why to use Qstring argument?
I do not clearly grasp the linking of signal-slot (if button-click() --> trigger the SLOT), but here I want to signal() return the whole Qstring, then SLOT take it and assign to textview. Or you mean (updateText(QString)) is just for emit signal() so it takes the Qstring?
Why I cannot just use (thread, SIGNAL(start(), textview, SLOT(setText()) and or Qstring argument is exact argument that link signal and slot?
Anyway I need use button after clicking which I settext() but I cannot use it in aforementioned example.
So if just use it -- the textview would be filled just after launching the application, but I need use it just after clicking.
So as I understand I need some intermediate variable as listed2 where I can put the results of xml processing, and then put it in Qtextedit yet in main-gui...

[anda_skoa]

I do not understand how link this 2 connect statements:
f.i. connect(thread, SIGNAL(updateText(QString)), label, SLOT(setText(QString)));

This is only one connect statement.

what should be (updateText(QString)) - should it be wrapper of run() or start()

It is a signal, it doesn't wrap anything.

why to use Qstring argument?

You want to send text from the thread to the GUI, QString is Qt's class for text.

Or you mean (updateText(QString)) is just for emit signal() so it takes the Qstring?

Yes, signals are emitted.

Why I cannot just use (thread, SIGNAL(start(), textview, SLOT(setText())

Who would have thought?
Could it be that thread does not have a start() signal?
Could it be that textview does not have a setText() slot?

Anyway I need use button after clicking which I settext() but I cannot use it in aforementioned example.

If you need to set a text when a button is clicked, create a slot that you can connect to the button's signal and call widget setters from there.

So as I understand I need some intermediate variable as listed2 where I can put the results of xml processing, and then put it in Qtextedit yet in main-gui...

I have no idea what you mean.

Cheers,
_

[artt]

Our last spin of discussion is probably the most worthless and long -- of course I need simple action triggeed by button.
As all of the others. So I see just transient variable, that got qtext variable after thread processing, and then assigning it to Qtextview in connect() statement by button. Notheing more... Despite processing xml, then assigning to variable, despite using some aditional resources as virtual memory would be justified, as I just put this variable to Qtextview in readxml() and maybe free the virtual memory in the same method.
So my SLOT(readxmlinthread()) would like as:
readxmlinthread() {
thread.start();//process to xml Qstring listed2
textview.setText(Lister::listed2);
//use destructor to free listed2 on heap;
}

[artt]

Here I defined such variable and slot method in Lister.cpp:

QList <Filewalker*> Lister::listed;
QString Lister::listed2("dddddd"); //I assigned such simple text for simple testing
...
void Lister::readthread() { //As slot function
    RThread* R=new RThread();
    R->start();
    textview->setText(Lister::listed2); //It doesnt render dddddd
    textview->setText("Lister::listed2"); //But it render "Lister::listed2" --So did I incorrectly assign listed2 to textview with setText()? 
}

QList <Filewalker*> Lister::listed;
QString Lister::listed2("dddddd"); //I assigned such simple text for simple testing
...
void Lister::readthread() { //As slot function
    RThread* R=new RThread();
    R->start();
    textview->setText(Lister::listed2); //It doesnt render dddddd
    textview->setText("Lister::listed2"); //But it render "Lister::listed2" --So did I incorrectly assign listed2 to textview with setText()? 
}

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[anda_skoa]

So I see just transient variable, that got qtext variable after thread processing, and then assigning it to Qtextview in connect() statement by button

A connect doesn't assign anything.
It provides a connection between a signal and a slot.
When the signal is emitted, the slot is invoked.
If the signal carries arguments, then the slot is called with these arguments.

So my SLOT(readxmlinthread()) would like as:
readxmlinthread() {
thread.start();//process to xml Qstring listed2
textview.setText(Lister::listed2);
//use destructor to free listed2 on heap;
}

That is obviously not going to work, as the thread is running in parallel so that variable might or might not have been set at the point of reading it.
Also obviously not thread-safe.

Hence the suggetion to use a signal:
- is thread-safe
- sets the widget's value then the value has been calculated
- avoids hacks such as this static variable

Cheers,
_

[artt]

I see that it doesnt work - but I ask probably 7-8 time here - how connect Button, signal from thread with xmlprocessing results and textview it main gui? i have no suggestion as there is nessecity of transient variable.

[anda_skoa]

I see that it doesnt work - but I ask probably 7-8 time here - how connect Button, signal from thread with xmlprocessing results and textview it main gui?

So what is the problem with the examples that were already given?

Comment #7 shows how to connect a button to a slot from within the receiver class.
Comment #13 shows a connect of a button signal to a slot from outside both sender and receiver.
Comment #44 has several examples of connecting signals that carry arguments.
Comment #100 shows a declaration of a custom signal and how to use it as a cross-thread data exchange mechanism.
Comment #104 shows how to emit such a signal.

Cheers,
_

[artt]

I have looked through your references: but if signal() is just declared, not defined what is the sense of its argument.
If I declare:
signals: setuptext(Qstring);
and then emit...
How I can link void readxmldirectly() [that shapes these mega QString as the result of xml processing] to this signal?
How I can link as well button with clicked() signal??
Maybe it is impossible to realize such option -- maybe it needs some queue or some nested signal-slot connection?
As you cannot provide some blueprint, I think you have no idea too, or I am not correct -- as the task not simply connect the signal with slot, but connect signal from button with the output of thread processing to slot??
Maybe I could override in such way -- button,clicked(Qstring) and clicked is triggered when I emit signal with the thread::run() output?

[anda_skoa]

The button's clicked() signal gets connected to a slot.
The slot creates the thread instance, connects the thread's signal and starts the thread.
The thread emits the signal when it is done.

Cheers,
_

[artt]

void Lister::readthread() {
//RThread* R=new RThread();
//R->start();
textview->setText(Lister:ath0);
textview->setText("Lister::listed2");
} -- And returning to my last SLOT -- it also do not work with path0, so the issue is not in thread Rthread as the path0i defined... Maybe I need use there signal finished() of Qthread as it would be like join() in Java, then I can assign listed2 variable, but textview->setText(Lister:ath0) with predefined do not work.
So I need connect(button,clicked(), f, readthreadxml());
Inside readthreadxml() i need use another slot()? Or how -- but it should emit some signal
connecting with some event --should it be the craetion of Lister f object or start of application?
...The thread emits the signal when it is done -- shoukld this signal to be void without nothing or it should be the readxnldirectly() content?

[anda_skoa]

So I need connect(button,clicked(), f, readthreadxml());

Yes

Inside readthreadxml() i need use another slot()?

Inside that method you have another connect().
Whether you connect to a slot in Lister or directly to the textview is up to you.

Or how -- but it should emit some signal

No, the thread emits the signal when it has finished creating the content you want to show

...The thread emits the signal when it is done -- shoukld this signal to be void without nothing or it should be the readxnldirectly() content?

Well, it is easier if the signal transports the content because then you can directly connect to textview.

Cheers,
_

[artt]

"Inside that method you have another connect() -- I do understand as slot without connect() nothing means.
Whether you connect to a slot in Lister or directly to the textview is up to you."
--Indeed I need to define the slot of nested connect() in Lister.cpp, so I need to create Lister object again(or beforehand to be strict), and it will be out of created in main() so it would be the error as ever, or I need static slot with static textview -that is impossible.
Ot I should simply use textview->setText(...), no Lister->textview->setText(...).
Anyway --there is build in finished() signal in qthread as I wrote early, maybe it would be option.
And I do not understand why

void Lister::readthread() {
textview->setText(Lister:path0);
textview->setText("Lister::listed2");
}

void Lister::readthread() {
textview->setText(Lister:path0);
textview->setText("Lister::listed2");
}

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do not render path0 that is not empty and predefined in code. If it would work maybe I would check my previous option

[anda_skoa]

--Indeed I need to define the slot of nested connect() in Lister.cpp, so I need to create Lister object again

Why would you need another instance of Lister?

Anyway --there is build in finished() signal in qthread as I wrote early, maybe it would be option.

Of course.
You could also store the result in a member of the thread and retrieve it from the slot connected to finished().

And I do not understand why

void Lister::readthread() {
textview->setText(Lister:path0);
textview->setText("Lister::listed2");
}

void Lister::readthread() {
textview->setText(Lister:path0);
textview->setText("Lister::listed2");
}

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do not render path0 that is not empty and predefined in code. If it would work maybe I would check my previous option

That should always display "Lister::listed2", as this is the value that is set on the textview.

Cheers,
_

[artt]

void Lister::readthread() {
    textview->setText("Lister::listed2");
    textview->setText(Lister:path0); //In this case 2nd line render but no first one, so the first line do not //render anyway
    }

void Lister::readthread() {
    textview->setText("Lister::listed2");
    textview->setText(Lister:path0); //In this case 2nd line render but no first one, so the first line do not //render anyway
    }

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I have found the solution but not very fine as it works but for big folders like C:\Windows it use too much memory for the whole Windows XP system.

void Lister::slot() {
        textview->setText("Lister::listed2");
        textview->setText(Lister::listed2);
}
 
void Lister::readthread() {
    RThread* R=new RThread();
    R->start();
    QObject::connect(R, SIGNAL(finished()),this, SLOT(slot()));
    //textview->setText("Lister::listed2");
    //textview->setText(Lister::listed2);
}

void Lister::slot() {
        textview->setText("Lister::listed2");
        textview->setText(Lister::listed2);
}

void Lister::readthread() {
    RThread* R=new RThread();
    R->start();
    QObject::connect(R, SIGNAL(finished()),this, SLOT(slot()));
    //textview->setText("Lister::listed2");
    //textview->setText(Lister::listed2);
}

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-- So I can do the conclusion about inefficiency of using threads for rendering a big text areas to Qtextedit.

[anda_skoa]

You probably want to connect before you start the thread.
Otherwise the thread could be finished before you establish the connection.

Cheers,
_

[artt]

No it render small folder xml immediately, but for big folder I need wait about minute to see the xml in textview, despite hunging began before it, and after rendering continues and deteriorate.
QObject::connect(R->start(), SIGNAL(finished()),this, SLOT(slot())); using R->start() instead R lead to compile error.

[anda_skoa]

QObject::connect(R->start(), SIGNAL(finished()),this, SLOT(slot())); using R->start() instead R lead to compile error.

Why would you even try that?

It has nothing to do with what I wrote.
I pointed you to a potential race condition and suggested a fix for it.

Cheers,
_

[artt]

Why do you state that I connect before the start of thread as it is obvious from that excerpt of code that R->start(); then connect(R,finished...). And it works in general but with memory leak or so. Anyway what would you suggest just with your version of emitting signal despite here is built-in finished() signal. And it seems is the simpliest way

[anda_skoa]

Why do you state that I connect before the start of thread

No, I said you should connect before starting.

as it is obvious from that excerpt of code that R->start(); then connect(R,finished...)

Exactly.
Start before connect, i.e. the wrong way around.

Cheers,
_

[artt]

"terminate called after throwing an instance of 'std::bad_alloc'
what(): std::bad_alloc
terminate called recursively..."
if I put connect before start of thread (I need click button twice so it incorrect in principle as it was without nested concnet())
Anyway as in other way. there is bad_alloc, despite I put Lister::listed2 on heap memory: bad_alloc is when I put in the end
delete Lister::listed2; I need probably uqual it to Null. But in my first variant-- evrything works fine despite temporal freezing.
But after 2-3 minutes the whole buncg of xml from WINDOWS folder is shown in textview.