Dynamically changing QLabel background colour

[wysota]

It worked for me without the auto fill, but I believe this may be required for WindowsXP style.

[jpn]

It worked for me without the auto fill, but I believe this may be required for WindowsXP style.

The label is a top level widget and therefore not transparent; "Windows are always filled with QPalette::Window, unless the WA_OpaquePaintEvent or WA_NoSystemBackground attributes are set."

[wysota]

Correct.
Here is a final working experiment:

#include <QLabel>
#include <QApplication>
#include <QVBoxLayout>
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QWidget wgt;
  QVBoxLayout *l = new QVBoxLayout(&wgt);
  QLabel *label = new QLabel;
  l->addWidget(label);
  label->setText("Testing");
// this:
//  label.setStyleSheet("background-color: red;");
// or this:
  QPalette pal = label->palette();
  pal.setColor(label->backgroundRole(), Qt::red);
  label->setPalette(pal);
  label->setAutoFillBackground(true); // rem this to see the difference
  wgt.show();
  return app.exec();
}

#include <QLabel>
#include <QApplication>
#include <QVBoxLayout>

int main(int argc, char **argv){
  QApplication app(argc, argv);
  QWidget wgt;
  QVBoxLayout *l = new QVBoxLayout(&wgt);
  QLabel *label = new QLabel;
  l->addWidget(label);
  label->setText("Testing");
// this:
//  label.setStyleSheet("background-color: red;");
// or this:
  QPalette pal = label->palette();
  pal.setColor(label->backgroundRole(), Qt::red);
  label->setPalette(pal);
  label->setAutoFillBackground(true); // rem this to see the difference
  wgt.show();
  return app.exec();
}

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[TheRonin]

I have a related issue and thought it might fit into this thread. I have a comboBox that's supposed to 'blink' red when an item is added to it.

Unfortunately, just getting something that works has proven to be a bit of a pain:

MyClass::MyClass()
{
  comboBox->setAutoFillBackground(true);
}
static bool useRed = true;
void MyClass::addItem(QString itemText)
{
  comboBox->insertItem(-1, itemText);
  comboBox->setCurrentIndex(0);
  QTimer::singleShot(1, this, SLOT(blink()));
}
 
void myClass::blink()
{
  if (useRed){
    comboBox->setStyleSheet("background-color: red;");
    QTimer::singleShot(500, this, SLOT(blink()));
  }else
    comboBox->setStyleSheet("background-color: yellow;");
 
  comboBox->update();  
  useRed = !useRed;  
}

MyClass::MyClass()
{
  comboBox->setAutoFillBackground(true);
}
static bool useRed = true;
void MyClass::addItem(QString itemText)
{
  comboBox->insertItem(-1, itemText);
  comboBox->setCurrentIndex(0);
  QTimer::singleShot(1, this, SLOT(blink()));
}

void myClass::blink()
{
  if (useRed){
    comboBox->setStyleSheet("background-color: red;");
    QTimer::singleShot(500, this, SLOT(blink()));
  }else
    comboBox->setStyleSheet("background-color: yellow;");
  
  comboBox->update();  
  useRed = !useRed;  
}

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first time blink is called the background color changes to red, just as expected. The second time around however, the color goes back to the standard grey and subsequent calls have no effect. Am i missing something here?

[TheRonin]

Perhaps i should have started a new thread instead of just appending an old one?

[jpn]

Could you wrap it as a compilable source file so we could test and see ourselves?

[TheRonin]

Could you wrap it as a compilable source file so we could test and see ourselves?

Sure thing Jpn!

header file:

#include <QMainWindow>
#include <QWidget>
#include <QPushButton>
#include <QComboBox>
#include <QVBoxLayout>
 
class Gui : public QMainWindow
{
	Q_OBJECT
public:
	Gui( QWidget * parent = 0 );
 
private slots:
	void blinkMe();
	void blinkStyle();
	void blinkPalette();
private:
    QWidget *centralwidget;
    QPushButton *pushButton;
    QComboBox *comboBox;
    QVBoxLayout *layout;
};

#include <QMainWindow>
#include <QWidget>
#include <QPushButton>
#include <QComboBox>
#include <QVBoxLayout>

class Gui : public QMainWindow
{
	Q_OBJECT
public:
	Gui( QWidget * parent = 0 );

private slots:
	void blinkMe();
	void blinkStyle();
	void blinkPalette();
private:
    QWidget *centralwidget;
    QPushButton *pushButton;
    QComboBox *comboBox;
    QVBoxLayout *layout;
};

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source file:

#include <iostream>
#include <QApplication>
#include <QString>
#include <QTimer>
 
#include "gui.h"
//
using namespace std;
//
int main(int argc, char ** argv)
{
	QApplication app( argc, argv );
	Gui gui;
	gui.show(); 		     
	app.connect( &app, SIGNAL( lastWindowClosed() ), &app, SLOT( quit() ) );
	return app.exec();
}
 
Gui::Gui( QWidget * parent ) : QMainWindow(parent)
{	
	this->setObjectName(QString::fromUtf8("MainWindow"));
	this->setWindowTitle("main window test");
	centralwidget = new QWidget(this);
	centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
   	this->setCentralWidget(centralwidget);
 
	pushButton = new QPushButton(centralwidget);
	pushButton->setObjectName(QString::fromUtf8("pushButton"));
   	pushButton->setText("blink?");
	pushButton->setAutoFillBackground(true);
 
   	comboBox = new QComboBox(centralwidget);
   	comboBox->setObjectName(QString::fromUtf8("comboBox"));	
	comboBox->setAutoFillBackground(true);
 
	layout = new QVBoxLayout(centralwidget);
	layout->addWidget(comboBox);
	layout->addWidget(pushButton);
 
	 QSize size(322, 149);
	size = size.expandedTo(this->minimumSizeHint());
	this->resize(size);
 
	connect(pushButton, SIGNAL(clicked()),
			this, SLOT(blinkMe()));
 
}
 
static int blinkCount = 0;
void Gui::blinkMe()
{
	cout << "trying to blink the comboBox..." << endl;;
 
	blinkStyle();
	//blinkPalette();
}
 
void Gui::blinkStyle()
{	
	if (++blinkCount > 6){
		blinkCount = 0;
		return;		
	}
 
	QWidget* blinkable = comboBox;		
 
	if (blinkCount % 2){
		//qApp->setStyleSheet("QComboBox#comboBox { background-color: yellow }");
		blinkable->setStyleSheet("background-color: yellow");
	}else{
		//qApp->setStyleSheet("QComboBox#comboBox {}");
		blinkable->setStyleSheet("");
	}
	blinkable->update();
 
	QTimer::singleShot(200, this, SLOT(blinkStyle()));
 
	cout << "blink style! " << blinkable->styleSheet().toStdString() << endl;
}
 
void Gui::blinkPalette()
{	
	static QColor oldBaseColor;
	static QColor oldHighlightColor;
 
	if (++blinkCount > 6){
		blinkCount = 0;
		return;
	}		
 
	QWidget* blinkable = comboBox;
	QPalette palette = blinkable->palette();
 
	if (blinkCount % 2){
		oldBaseColor = blinkable->palette().color(QPalette::Base);		
		palette.setColor(QPalette::Base, Qt::yellow);
 
		oldHighlightColor = blinkable->palette().color(QPalette::Highlight);
		palette.setColor(QPalette::Highlight, Qt::yellow);
	}else{
		palette.setColor(QPalette::Base, oldBaseColor);
		palette.setColor(QPalette::Highlight, oldHighlightColor);
	}
	blinkable->setPalette(palette);
	blinkable->update();
 
	QTimer::singleShot(200, this, SLOT(blinkPalette()));
 
	cout << "blink palette! " << endl;
}
//

#include <iostream>
#include <QApplication>
#include <QString>
#include <QTimer>

#include "gui.h"
//
using namespace std;
//
int main(int argc, char ** argv)
{
	QApplication app( argc, argv );
	Gui gui;
	gui.show(); 		     
	app.connect( &app, SIGNAL( lastWindowClosed() ), &app, SLOT( quit() ) );
	return app.exec();
}

Gui::Gui( QWidget * parent ) : QMainWindow(parent)
{	
	this->setObjectName(QString::fromUtf8("MainWindow"));
	this->setWindowTitle("main window test");
	centralwidget = new QWidget(this);
	centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
   	this->setCentralWidget(centralwidget);
   	
	pushButton = new QPushButton(centralwidget);
	pushButton->setObjectName(QString::fromUtf8("pushButton"));
   	pushButton->setText("blink?");
	pushButton->setAutoFillBackground(true);
   	
   	comboBox = new QComboBox(centralwidget);
   	comboBox->setObjectName(QString::fromUtf8("comboBox"));	
	comboBox->setAutoFillBackground(true);
	
	layout = new QVBoxLayout(centralwidget);
	layout->addWidget(comboBox);
	layout->addWidget(pushButton);

	 QSize size(322, 149);
	size = size.expandedTo(this->minimumSizeHint());
	this->resize(size);
	
	connect(pushButton, SIGNAL(clicked()),
			this, SLOT(blinkMe()));
			
}

static int blinkCount = 0;
void Gui::blinkMe()
{
	cout << "trying to blink the comboBox..." << endl;;

	blinkStyle();
	//blinkPalette();
}

void Gui::blinkStyle()
{	
	if (++blinkCount > 6){
		blinkCount = 0;
		return;		
	}
	
	QWidget* blinkable = comboBox;		
	
	if (blinkCount % 2){
		//qApp->setStyleSheet("QComboBox#comboBox { background-color: yellow }");
		blinkable->setStyleSheet("background-color: yellow");
	}else{
		//qApp->setStyleSheet("QComboBox#comboBox {}");
		blinkable->setStyleSheet("");
	}
	blinkable->update();

	QTimer::singleShot(200, this, SLOT(blinkStyle()));
	
	cout << "blink style! " << blinkable->styleSheet().toStdString() << endl;
}

void Gui::blinkPalette()
{	
	static QColor oldBaseColor;
	static QColor oldHighlightColor;
		
	if (++blinkCount > 6){
		blinkCount = 0;
		return;
	}		
	
	QWidget* blinkable = comboBox;
	QPalette palette = blinkable->palette();
	
	if (blinkCount % 2){
		oldBaseColor = blinkable->palette().color(QPalette::Base);		
		palette.setColor(QPalette::Base, Qt::yellow);
		
		oldHighlightColor = blinkable->palette().color(QPalette::Highlight);
		palette.setColor(QPalette::Highlight, Qt::yellow);
	}else{
		palette.setColor(QPalette::Base, oldBaseColor);
		palette.setColor(QPalette::Highlight, oldHighlightColor);
	}
	blinkable->setPalette(palette);
	blinkable->update();

	QTimer::singleShot(200, this, SLOT(blinkPalette()));

	cout << "blink palette! " << endl;
}
//

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The above example has three test cases.
1) blinking the comboBox's style sheet. To do so, make sure blinkMe() calls blinkStyle() and that blinkStyle() sets the blinkable-widget's style sheet. This is the default.

2) blinking the application's style sheet. Call blinkStyle() and make sure the call to qApp is made instead of to the blinkable-widget.

3) blinking the comboBox's palette. To do so, call blinkPalette() instead of blinkStyle().

Expected results:
All three cases should cause the comboBox's background to blink yellow three times before turning white again.

Actual results:
Only the last two cases behave as expected. The first case causes the comboBox to blink yellow only once before turning white again, regardless of what the secondary color is set to.

[jpn]

Hmm, maybe a problem with parsing/applying the style sheet? Passing the same style sheet to the combo box than is passed to the application works also.

Edit: Oh, and by the way, please attach large files instead of pasting them as code blocks. Copy-pasting from the code block is inconvenient because of all the extra line number and line breaks.

[TheRonin]

Hmm, maybe a problem with parsing/applying the style sheet? Passing the same style sheet to the combo box than is passed to the application works also.

But it's strange that it won't work properly for the comboBox since setting the pushButton as the blinkable object gives the expected behavior. Is there any obvious reason why applying the stylesheet directly to the comboBox won't work? Did the example behave as expected for you or did you experience the same behavior? Is this a bug perhaps?

Edit: Oh, and by the way, please attach large files instead of pasting them as code blocks. Copy-pasting from the code block is inconvenient because of all the extra line number and line breaks.

Ok, thanks for the heads-up, i'll attach files in the future.

[jpn]

But it's strange that it won't work properly for the comboBox since setting the pushButton as the blinkable object gives the expected behavior. Is there any obvious reason why applying the stylesheet directly to the comboBox won't work? Did the example behave as expected for you or did you experience the same behavior? Is this a bug perhaps?

Yeah, I experienced the same and I'd call it a bug. Anyway, what I meant with my previous post was that for me it actually works directly for the combobox in case the style sheet is passed in a same way than you pass it to the application eg. including the selector:

// blinkable->setStyleSheet("background-color: yellow");
blinkable->setStyleSheet("QComboBox{background-color: yellow}"); // <-- works

// blinkable->setStyleSheet("background-color: yellow");
blinkable->setStyleSheet("QComboBox{background-color: yellow}"); // <-- works

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[TheRonin]

You're right, that does work. I'll do it that way for now then. Thanks!