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.....