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.