Dynamically changing QLabel background colour

[T4ng10r]

In GroupBox I've placed few QLabels. Now - depending on which of this labels I click - I want to change background colour of one of those QLabel.
I've tried

QPalette	palette = label->palette();
if (somethink)
 palette.setColor(label->backgroundRole(),QColor(Qt::blue));
else
 palette.setColor(label->backgroundRole(),QPalette::Window);
label->setText(someText);
label->setPalette(palette);
label->setAutoFillBackground(true);
label->update();

QPalette	palette = label->palette();
if (somethink)
 palette.setColor(label->backgroundRole(),QColor(Qt::blue));
else
 palette.setColor(label->backgroundRole(),QPalette::Window);
label->setText(someText);
label->setPalette(palette);
label->setAutoFillBackground(true);
label->update();

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In the result - if i click on QLabel which should change background of others - no effect. When label->setAutoFillBackground(true); is set - widget turns him self into black block. When i click on widgets which should change background - then they are changing it to a proper one.

Then I've tried Style sheets. In places where I was changing palette - now I've placed

label->setStyleSheet("QLabel { background-color: blue }");

label->setStyleSheet("QLabel { background-color: blue }");

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and where I reset

label->setStyleSheet("");

label->setStyleSheet("");

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No difference.
PS. I'm coding under Windows XP. My application is using different styles like QPlastique, QWindows, QMotif - under each of them I see no effect from above code.

[Claymore]

label->setStyleSheet("QLabel { background-color: blue }");

label->setStyleSheet("QLabel { background-color: blue }");

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Have you tried this?

label->setStyleSheet("background-color: blue");

label->setStyleSheet("background-color: blue");

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Also, you could create a QSS file where you can specify how the button behaves when you hover and click on it.

[No-Nonsense]

label->setStyleSheet("QLabel { background-color: blue }");

label->setStyleSheet("QLabel { background-color: blue }");

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and where I reset

label->setStyleSheet("");

label->setStyleSheet("");

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Try:

"QLabel { background: blue }"

"QLabel { background: blue }"

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-Jens

[smacchia]

You can use style sheets as the other folks suggest. But if you want to use palettes, I think what is wrong is where you have:

if (somethink)
    palette.setColor(label->backgroundRole(),QColor(Qt::blue));
else
    palette.setColor(label->backgroundRole(),QPalette::Window);

if (somethink)
    palette.setColor(label->backgroundRole(),QColor(Qt::blue));
else
    palette.setColor(label->backgroundRole(),QPalette::Window);

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Notice in the else branch, you have QPalette::Window which is a role, not a color. So in that line you have 2 roles. Frankly, I'm surprised it even compiles. You need to specify a color, i.e.,

palette.setColor(label->backgroundRole(), QColor(Qt::<somecolor>));

palette.setColor(label->backgroundRole(), QColor(Qt::<somecolor>));

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or

palette.setColor(
    Qt::Active, 
    label->backgroundRole(), 
    QColor(Qt::<somecolor>));

palette.setColor(
    Qt::Active, 
    label->backgroundRole(), 
    QColor(Qt::<somecolor>));

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where Qt::Active is a color group.

Carefully review http://doc.trolltech.com/4.2/qpalette.html

It should contain all the info on how to use palettes.

HTH,
Susan

[wysota]

Notice in the else branch, you have QPalette::Window which is a role, not a color. So in that line you have 2 roles. Frankly, I'm surprised it even compiles.

It compiles because QColor is castable from int (and QPalette::Window is an int) - notice that Qt::red is an int as well and you could write:

palette.setColor(label->backgroundRole(), Qt::red);

palette.setColor(label->backgroundRole(), Qt::red);

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You don't need to go through the QColor constructor.

[smacchia]

Ah you're correct, forgot about that. But that could still be why there is no color change. And while it the code compiles, is logically incorrect.

I've been using palettes for this very thing successfully (but I must admit, I've been using it for everything *but* labels). The app I am working on has MANY custom colors for every concievable role/state, so I've had to use the palette extensively.

[wysota]

And while it the code compiles, is logically incorrect.

Yes.

I've been using palettes for this very thing successfully (but I must admit, I've been using it for everything *but* labels). The app I am working on has MANY custom colors for every concievable role/state, so I've had to use the palette extensively.

Labels are tricky. This works for all styles I could test (excluding aqua and windowsxp):

#include <QLabel>
#include <QApplication>
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QLabel label;
  label.setText("Testing");
  label.setStyleSheet("background-color: red;");
  label.setAutoFillBackground(true);
  label.show();
  return app.exec();
}

#include <QLabel>
#include <QApplication>

int main(int argc, char **argv){
  QApplication app(argc, argv);
  QLabel label;
  label.setText("Testing");
  label.setStyleSheet("background-color: red;");
  label.setAutoFillBackground(true);
  label.show();
  return app.exec();
}

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This works as well:

#include <QLabel>
#include <QApplication>
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QLabel label;
  label.setText("Testing");
  QPalette pal = label.palette();
  pal.setColor(label.backgroundRole(), Qt::red);
  label.setPalette(pal);
  label.setAutoFillBackground(true);
  label.show();
  return app.exec();
}

#include <QLabel>
#include <QApplication>

int main(int argc, char **argv){
  QApplication app(argc, argv);
  QLabel label;
  label.setText("Testing");
  QPalette pal = label.palette();
  pal.setColor(label.backgroundRole(), Qt::red);
  label.setPalette(pal);
  label.setAutoFillBackground(true);
  label.show();
  return app.exec();
}

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[smacchia]

Good stuff - thanks. I'll be storing this in my arsenal

I would bet it's

label.setAutoFillBackground(true);

label.setAutoFillBackground(true);

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that is the key.

[T4ng10r]

Hmm, now I see my mistakes. And, yes, without label.setAutoFillBackground(true/false); it didn't worked.
Thank you all for help.

[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?