Understanding QWidget's Background/Foreground role

[Binary91]

Hi,

I have some problems understanding the sense of QPalette::ColorRole.

I read the documentation a lot of times, and this is how I understood it:
Every QWidget uses a palette (QPalette) for design settings (BackgroundRole --> background color, ForegroundRole --> foreground color). Also, every QWidget has three states:
1. active (its parent window has focus)
2. inactive (another window has focus)
3. disabled
For each of these states, the QWidget's palette has a design setting (so each QWidget has three background colors, depending on its state).

Am I right so far?

Then, I was wondering about the fact that QPalette::ColorRole has more than one value for background settings:
- QPalette::Window
- QPalette::Base
- QPalette::Button
...

My guess is, that there are pre-defined colors for those "groups". Documentation says, QPalette::Window is used for basically window design, so I think it stands for grey background???
QPalette::Base is used by editable QWidgets (TextEdit/LineEdit, Table-/Listview and so on), and they all have a white background, so I guess this ColorRole stands for white background???

If all I mentioned above is right and I'm on the right way, then I don't understand why I can't do the following to change a QLineEdits background color to grey (like window background):

QLineEdit* edit;
edit->setBackgroundRole(QPalette::Window);

QLineEdit* edit;
edit->setBackgroundRole(QPalette::Window);

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What I also do not understand is why the following works to set a red background:

QPalette palette;
palette.setColor(QPalette::Base, QColor(255,0,0));
QLineEdit* edit;
edit->setPalette(palette);

QPalette palette;
palette.setColor(QPalette::Base, QColor(255,0,0));
QLineEdit* edit;
edit->setPalette(palette);

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, but this doesn't:

QPalette palette;
palette.setColor(QPalette::Window, QColor(255,0,0));
QLineEdit* edit;
edit->setBackgroundRole(QPalette::Window);
edit->setPalette(palette);

QPalette palette;
palette.setColor(QPalette::Window, QColor(255,0,0));
QLineEdit* edit;
edit->setBackgroundRole(QPalette::Window);
edit->setPalette(palette);

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Why does a function of QLineEdit to change its backgroundRole (from standard QPalette::Base to some other) even exist when it does not work??


Well, I think I did not understand the whole thing and I'm totally on the wrong way. So if there is anyone who understands that all and wants to explain that to me, I would be really glad

Thank you in anticipation!

[wysota]

If all I mentioned above is right and I'm on the right way, then I don't understand why I can't do the following to change a QLineEdits background color to grey (like window background):

Some styles (e.g. modern Windows) delegate drawing of standard controls to the system so palette settings are ignored for them.

[Binary91]

Hi and thank you for your reply.

So do you mean that in case of QLineEdits, they ignore the command 'setBackgroundRole(QPalette::Window)' and always use QPalette::Base, so the only way to change background is using QPalette::setColor(QPalette::Base, QColor) ??

[wysota]

So do you mean that in case of QLineEdits, they ignore the command 'setBackgroundRole(QPalette::Window)' and always use QPalette::Base, so the only way to change background is using QPalette::setColor(QPalette::Base, QColor) ??

No, I mean that in case of a Windows XP or newer running the default Qt style for that platform for any widget the amount of aspects the palette can influence is very limited.

[Binary91]

Ok, that sounds like there is no 'general' way to change background/foreground settings for QWidgets when QPalette's influence varies so much, right?
That's too bad...

But were my speculations about the way of using QPalette right (first post)? Did I understand that stuff correctly? In principle, would it work to set the QWidget's background color to the same background color as grey windows by setting backgroundRole to QPalette::Window (if OS would allow it) ? Or did I understand the whole stuff fundamentally wrong?

[wysota]

Ok, that sounds like there is no 'general' way to change background/foreground settings for QWidgets when QPalette's influence varies so much, right?

You can try using stylesheets however that will replace the original look of the widget with a more generic look.

But were my speculations about the way of using QPalette right (first post)? Did I understand that stuff correctly? In principle, would it work to set the QWidget's background color to the same background color as grey windows by setting backgroundRole to QPalette::Window (if OS would allow it) ? Or did I understand the whole stuff fundamentally wrong?

Launch the application with a different style (e.g. by passing -style plastique in Qt4 or -style fusion in Qt5) and see for yourself.

[Binary91]

Ok, I'll try that. Thank's again for supporting :-)

[anda_skoa]

Usually the way to change colors is to modify a widget's palette

QPalette p = widget->palette();
p.setColor(...);
widget->setPaletted(p);

QPalette p = widget->palette();
p.setColor(...);
widget->setPaletted(p);

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Cheers,
_