Why? A problem about Q_OBJECT and setStyleSheet().

[JaderoChan]

A tiny code example follow:

//main.cpp
#include "mainwidget.h"
#include <QtWidgets/QApplication>

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    MainWidget w;
    w.show();
    return a.exec();
}

// mainwidget.h
#pragma once

#include <qwidget.h>

#define USE_MASK_WIDGET_BY_CLASS

class MaskWidget : public QWidget
{
    Q_OBJECT

public:
    MaskWidget(QWidget* parent = nullptr) :
        QWidget(parent)
    {
        setStyleSheet("background-color:#88000000;");
        if (parentWidget())
        {
            QRect rect = parentWidget()->geometry();
            rect.setX(0);
            rect.setY(0);
            setGeometry(rect);
        }

        show();
    }

    ~MaskWidget() {}
};

class MainWidget : public QWidget
{
    Q_OBJECT

public:
    MainWidget(QWidget* parent = nullptr);
    ~MainWidget();

protected slots:
    void onButtonClicked();
};

// mainwidget.cpp
#include "mainwidget.h"

#include <qpushbutton.h>

static QWidget* getMaskWidget(QWidget* parent = nullptr)
{
    auto wgt = new QWidget(parent);

    wgt->setStyleSheet("background-color:#88000000;");
    if (wgt->parentWidget())
    {
        QRect rect = wgt->parentWidget()->geometry();
        rect.setX(0);
        rect.setY(0);
        wgt->setGeometry(rect);
    }

    return wgt;
}

MainWidget::MainWidget(QWidget* parent) :
    QWidget(parent)
{
    setFixedSize(600, 400);

    auto getBtn = new QPushButton("Set mask widget", this);
    connect(getBtn, &QPushButton::clicked, this, &MainWidget::onButtonClicked);
}

MainWidget::~MainWidget()
{}

void MainWidget::onButtonClicked()
{
#ifdef USE_MASK_WIDGET_BY_CLASS
    auto wgt = new MaskWidget(this);
#else
    auto wgt = getMaskWidget(this);
#endif
    wgt->show();
}

I want to implement a simple mask widget.
You can try above code, you‘ll observe the mask not effect when define the USE_MASK_WIDGET_BY_CLASS macro. And i observe the key of this problem is the Q_OBJECT macro, while you define it at MaskWidget class , the mask will not effect, and has effect if you not define it.

So, why? Will the Q_OBJECT affect the setStyleSheet() fucntion?

If you can clear my mind, thanks to much.

[SGaist]

Hi and welcome to devnet,

You have to provide an implementation of paintEvent and use the Q_OBJECT macro when styling a custom widget like you do.

You have the code in the stylesheet reference under QWidget. It's juste above the list of properties part.

[JaderoChan]

@SGaist Thank you, it work. And is all widget that use Q_OBJECT and setStyleSheet custom widget need override the paintEvent?

[SGaist]

I am not sure to understand your phrase correctly.

So the rule is: if you create a QWidget subclass where you want to use style sheets, then it needs both the Q_OBJECT macro and the paintEvent reimplementation.

[JaderoChan]

@SGaist Thank you, I get it.