Why Inherit QWidget cant set background color If use Q_OBJECT In Qt6.5

Quiccz

If i use Q_OBJECT in InheritWidget, i can see

if i dont use it, i can see

Here is my code, I dont understand why my inherit widget cant set background color.

#include <QWidget>
#include "Mywidget.h"
#include <QApplication>



int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    MyWidget *mywidget = new MyWidget(nullptr);
    
    mywidget->SetSubWidget();
    mywidget->show();
    return app.exec();;
}

#pragma once
#include <QWidget>

class InheritWidget : public QWidget
{
Q_OBJECT
public:
	InheritWidget(QWidget* parent);
};

#include "InheritWidget.h"

InheritWidget::InheritWidget(QWidget* parent)
	:QWidget(parent)
{

}

#pragma once
#include <QWidget>

class MyWidget: public QWidget
{
public:
    MyWidget(QWidget *parent);
    void SetSubWidget();
};

#include "Mywidget.h"
#include "InheritWidget.h"
MyWidget::MyWidget(QWidget* parent)
: QWidget(parent)
{
    this->setGeometry(0,0, 300, 300);
    this->setWindowFlags(Qt::FramelessWindowHint);
    this->setStyleSheet("background-color:black;");
    this->setAutoFillBackground(true);
}

void MyWidget::SetSubWidget()
{
    InheritWidget*subWidget = new InheritWidget(this);
    subWidget->setGeometry(100, 100, 100, 100);
    subWidget->setStyleSheet("background-color:yellow;");
    subWidget->setAutoFillBackground(true);
    subWidget->show();

    QWidget*ssubWidget = new QWidget(this);
    ssubWidget->setGeometry(0, 0, 100, 100);
    ssubWidget->setStyleSheet("background-color:green;");
    ssubWidget->setAutoFillBackground(true);
    ssubWidget->show();
}

Christian Ehrlicher

Because style sheets are propagated to the children (if the children a correct QMetaObjects).

Quiccz

@Christian-Ehrlicher Is there a way to set the background color of a child widget?

Christian Ehrlicher

@Quiccz said in Why Inherit QWidget cant set background color If use Q_OBJECT In Qt6.5:

Is there a way to set the background color of a child widget?

If you use css on the parent then you have to set the background color of the child also with css.

JonB

@Quiccz
Not sure exactly why, and not sure if you are saying this behaviour is to do with specific 6.5 version.

One thing: you are inheriting from QWidget and you use stylesheet with setAutoFillBackground

Warning: Use this property with caution in conjunction with Qt Style Sheets. When a widget has a style sheet with a valid background or a border-image, this property is automatically disabled.

The other thing comes from How to Change the Background Color of QWidget

Note: If you subclass a custom widget from QWidget, then in order to use the StyleSheets you need to provide a paintEvent to the custom widget :

Only @Christian-Ehrlicher knows how/whether this applies here....

Quiccz

@Christian-Ehrlicher Thanks, i use QPalette works

JoeCFD

@Quiccz Or your class inherits QLabel or QFrame and you can simply set background with stylesheet.

JonB

@JoeCFD
But the OP's does no, it derives directly from QWidget. Hence I don't know how they manage without overriding the paintEvent(), per my previous post, because I recently found I did have to do that.

JoeCFD

@JonB https://codebrowser.dev/qt5/qtbase/src/widgets/kernel/qwidget.cpp.html
it is empty
void QWidget::paintEvent(QPaintEvent *)
{
}

JonB

@JoeCFD
I know that. The issue is from the Qt wiki, https://wiki.qt.io/How_to_Change_the_Background_Color_of_QWidget, Using Style Sheet sub-heading:

Note: If you subclass a custom widget from QWidget, then in order to use the StyleSheets you need to provide a paintEvent to the custom widget :

void CustomWidget::paintEvent(QPaintEvent* event)
{
    QStyleOption opt;
    opt.init(this);
    QPainter p(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);

    QWidget::paintEvent(event);
}

And (at least I thought) I recall having to do this, recently. I don't know how come the OP does not need same. There you are.