When add Q_OBJECT , the styleSheet is useless.

[Gaobo]

Hey, I have a question.
I have a class,

class SettingsItem : public QWidget
{
public:
    explicit SettingsItem(QWidget* parent = nullptr)
        : QWidget(parent)
    {
        setProperty("widgetType", "settingsItem");
    }

    ~SettingsItem() 
    {

    }
};

I add it to a qwidget

    SettingsItem* m_item = new SettingsItem();
    ui->widget_4->layout()->addWidget(m_item);

I set the QAppliaction style sheet like this

    QApplication a(argc, argv);
    QFile qssFile(":/MainWindow/qss");
    QByteArray baQss;
    if (qssFile.open(QIODevice::ReadOnly)) {
        baQss = qssFile.readAll();
    }
    a.setStyleSheet(baQss);

    MainWindow w;
    w.show();
    return a.exec();

the qss file:

QWidget[widgetType=settingsItem]
{
	border: 2px solid red;
	border-radius: 2px;
}

when running, the settingsItem's border is change

but when i add "Q_OBJECT" to the SettingsItem

class SettingsItem : public QWidget
{
    Q_OBJECT
public:
    explicit SettingsItem(QWidget* parent = nullptr)
        : QWidget(parent)
    {
        setProperty("widgetType", "settingsItem");
    }

    ~SettingsItem() 
    {

    }
};

This border is gone.Why?
And why don't i add Q_OBJECT, the styleSheet is in effect.

[Chris Kawa]

It's a little known quirk of the stylesheets. When you subclass QWidget and your class has a meta object you need to implement paintEvent like this:

void SettingsItem::paintEvent(QPaintEvent*)
{
    QStyleOption opt;
    opt.initFrom(this);
    QPainter p(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);
}

It's documented here at the end of the table, but it's easy to miss. You don't have to do that when you're subclassing any other widget like line edit or button, just with plain QWidget.

[Gaobo]

@Chris-Kawa said in When add Q_OBJECT , the styleSheet is useless.:

You don't have to do that when you're subclassing any other widget like line edit or button, just with plain QWidget.

Thank u. But i don't understand this "You don't have to do that when you're subclassing any other widget like line edit or button, just with plain QWidget." .

[Chris Kawa]

@Gaobo I mean if you have this

class Something : public QWidget

you need that custom paintEvent, but if you have for example this

class Something : public QLineEdit //or any other widget type

then you don't.

[Gaobo]

@Chris-Kawa
Oh, thank u. i understand. It's mean, if havn't a meta object, i don't need to custom paintEvent, right?

[Gaobo]

@Chris-Kawa
Oh, I think the reason for that the styleSheet is in effect when i don't add Q_OBJECT is:

1. when i not add Q_OBJECT, the "settingsItem" don't have parent, because it don't have Q_OBJECT, so ,the "settingsItem" background is itself background.
2. when i add Q_OBJECT, and not "setAttribute(Qt::WA_StyledBackground)", it's background is the background of its parent object.
3. the "WA_StyledBackground" is specify that it's background determined by style sheet.