Could not parse stylesheet of object QLabel

[oBOXPOH]

Hello everyone!

Now I am trying to use QThread with Widgets. And I want to change style of some label in widget from thread. But I have no success with it.

I have the thread, where I repeat some actions, and during of repeat I want to change style of some label in widget (in thread I have pointer to widget).

First I just created public method for changing of this stylesheet and called this method inside the thread, but I got error (or exception) from OS.

Second (as now) I connected in constructor of widget signal from thread to slot of widget. And now no exception, but string
pLabel->setStyleSheet(QString(pCChar_GreenStatusStyle))
doesn't work, because I get these messages:
Could not parse stylesheet of object QLabel

Hope for your help! Thank you in advance!

[oBOXPOH]

@mrjj said in Could not parse stylesheet of object QLabel:

@oBOXPOH

Hi
can you show the complete stylesheet?
as that errors says it cannot be parsed so it might just be a case of invalid syntax.

You are right, by the way! I need:

const char* *pCChar_GreenStatusStyle = "QLabel { color: green; font-weight: bold; }";

And now it works finally! By the way, I deleted this QLabel word because I tried to find an error.

Anyway, If someone has such problem, try to use only signals and slots like I used (image and description above) and check your styles. For serialPorts, for example, don't use threads, because else you will be in trouble.

[Christian Ehrlicher]

@oBOXPOH said in Could not parse stylesheet of object QLabel:

And I want to change style of some label in widget from thread.

You must not modify the gui from any other thread than the main gui thread!

[oBOXPOH]

@Christian-Ehrlicher said in Could not parse stylesheet of object QLabel:

@oBOXPOH said in Could not parse stylesheet of object QLabel:

And I want to change style of some label in widget from thread.

You must not modify the gui from any other thread than the main gui thread!

Sorry for my explaining. I just want to change style with some request from thread. ANd I don't know, how I need to do it correctly.

[Christian Ehrlicher]

To communicate between thread you should use Signals and Slots.
But based on your question I would ask that you think about if you really need a thread at all.

[JonB]

@oBOXPOH
@jsulm already answered your question for this over at https://forum.qt.io/topic/126942/qlabel-and-setstylesheet-exception/16

@oBOXPOH It is not allowed to change UI from other threads than main (GUI) thread. Use signals/slots to communicate between threads (for example to trigger actions/ui changes).

[oBOXPOH]

@JonB said in Could not parse stylesheet of object QLabel:

@oBOXPOH
@jsulm already answered your question for this over at https://forum.qt.io/topic/126942/qlabel-and-setstylesheet-exception/16

@oBOXPOH It is not allowed to change UI from other threads than main (GUI) thread. Use signals/slots to communicate between threads (for example to trigger actions/ui changes).

I tried to use signals and slot, but no success. Yes, now I haven't exception, but still haven't a style... I don't know, where and how I need to organize it for correct work.

[JonB]

@oBOXPOH
I assume you know how to write signals & slots already. If not, look at @Christian-Ehrlicher's link.

Define a signal in the thread. emit the signal from the thread, with whatever parameters are desired. Define a slot in the main UI thread. In the UI connect() the thread's signal to the UI's slot. There act on the signal and its parameters to execute the pLabel->setStyleSheet().

As @Christian-Ehrlicher said, whether you need or should have any non-UI thread is another question. Most newcomers seem to think they need threads, when they do not.

[Pl45m4]

@oBOXPOH said in Could not parse stylesheet of object QLabel:

I tried to use signals and slot, but no success

What did you try?
The idea is to send a signal from the second thread to trigger the style change in your GUI thread (so you avoid any conflict).

[oBOXPOH]

@JonB said in Could not parse stylesheet of object QLabel:

@oBOXPOH
I assume you know how to write signals & slots already. If not, look at @Christian-Ehrlicher's link.

Define a signal in the thread. emit the signal from the thread, with whatever parameters are desired. Define a slot in the main UI thread. In the UI connect() the thread's signal to the UI's slot. There act on the signal and its parameters to execute the pLabel->setStyleSheet().

As @Christian-Ehrlicher said, whether you need or should have any non-UI thread is another question. Most newcomers seem to think they need threads, when they do not.

Ok. But where can I find the main UI thread? Just I created slot for changing in MainWindow.cpp - and I still have my problem...

Let me explain, what I have. I have MainWindow class, where I create 2 widgets. In the second widget I have some another widgets. When I activate button of the first widget, I start the thread, where I get requests to change some properties of widgets in second widget. For example, I can change text for labels, but can't change styles of labels.

[Pradeep P N]

@oBOXPOH Can you have look at Signals/slots accross threads
Hope this might explain you a bit.

All the best.

[Pl45m4]

@oBOXPOH said in Could not parse stylesheet of object QLabel:

I start the thread

But do you really need the thread? You dont need an extra thread just to change widget properties

[oBOXPOH]

@Pl45m4 said in Could not parse stylesheet of object QLabel:

@oBOXPOH said in Could not parse stylesheet of object QLabel:

I start the thread

But do you really need the thread? You dont need an extra thread just to change widget properties

I think, I need the thread, because I use QSerialPort to send and read messages. If I won't use the thread, I will block the widget, where I create the thread.

[JonB]

@oBOXPOH
No, Qt classes like serial ports are asynchronous. So long as you use the signals/slots, not blocking calls. It seems unlikely you need a secondary thread, and without multiple threads your life will be much easier....

[Pl45m4]

@oBOXPOH

If you are currently using the blocking QSerialPort approach (with waitFor...()), have a look at the non-blocking example:

• https://doc.qt.io/qt-5/qtserialport-terminal-example.html

[JonB]

@Pl45m4 said in Could not parse stylesheet of object QLabel:

@JonB said in Could not parse stylesheet of object QLabel:

serial ports are asynchronous

Not always. There is a blocking and non-blocking QSerialPort appraoch.

I'm not following you? I wrote

No, Qt classes like serial ports are asynchronous. So long as you use the signals/slots, not blocking calls.

The calls are asynchronous. If you choose to waitFor..() you are choosing to block, by making your code synchronous. What are you saying any different from that?

[oBOXPOH]

@JonB said in Could not parse stylesheet of object QLabel:

@oBOXPOH
I assume you know how to write signals & slots already. If not, look at @Christian-Ehrlicher's link.

Define a signal in the thread. emit the signal from the thread, with whatever parameters are desired. Define a slot in the main UI thread. In the UI connect() the thread's signal to the UI's slot. There act on the signal and its parameters to execute the pLabel->setStyleSheet().

As @Christian-Ehrlicher said, whether you need or should have any non-UI thread is another question. Most newcomers seem to think they need threads, when they do not.

I can't to win my problem with styleSheet... I even deleted thread. Let me explain what I have done.

When I click to button1_1, I emit some signal from widget1 to slot of widget2. In slot of widget2 I emit some signal to slot of created some class inside widget2. Inside this class I have a loop with signals/slots. And during this loop I need to change styles of widgets inside widget2 (label2_1). When I need to change style of label, I emit signal from class to slot of widget2 and call method of, for example, widget2_1 in order to change the style of label, but, without exception, I get "Could not parse stylesheet of object QLabel(0xf66ad8)". I tried many things, but still no success.

Hope for your help! Thank you in advance!

[mrjj]

@oBOXPOH

Hi
can you show the complete stylesheet?
as that errors says it cannot be parsed so it might just be a case of invalid syntax.

[oBOXPOH]

@mrjj said in Could not parse stylesheet of object QLabel:

@oBOXPOH

Hi
can you show the complete stylesheet?
as that errors says it cannot be parsed so it might just be a case of invalid syntax.

pLabel->setStyleSheet(QString(pCChar_GreenStatusStyle));

const char *pCChar_GreenStatusStyle = "{ color: green; font-weight: bold; }";

[oBOXPOH]

@mrjj said in Could not parse stylesheet of object QLabel:

@oBOXPOH

Hi
can you show the complete stylesheet?
as that errors says it cannot be parsed so it might just be a case of invalid syntax.

You are right, by the way! I need:

const char* *pCChar_GreenStatusStyle = "QLabel { color: green; font-weight: bold; }";

And now it works finally! By the way, I deleted this QLabel word because I tried to find an error.

Anyway, If someone has such problem, try to use only signals and slots like I used (image and description above) and check your styles. For serialPorts, for example, don't use threads, because else you will be in trouble.

[JonB]

@oBOXPOH said in Could not parse stylesheet of object QLabel:

I need:
const char* *pCChar_GreenStatusStyle = "QLabel { color: green; font-weight: bold; }";

I am surprised at this! That is a char **. But if you say that is required to work.....