How do I change the state of a QProgressBar?

Question

I have a custom dialog that imitates and embellishes QProgressDialog with some additional application-specific information and enables pausing/resuming the worker thread running in the background. Ultimately what I'm looking for is the ability to change the appearance of the QProgressBar to reflect the paused state as described in this question.

Oddly enough QWinTaskbarProgress seems to support this exact functionality as of Qt 5.2, but a plain QProgressBar does not, in Qt4 or Qt5. I'm okay with fiddling around in the Qt source code, but I've dug around in the Qt source code and I can't figure out where the actual state of the form control is queried/provided, so I can't figure out what I need to change. Maybe it's not there because this is strictly a Windows thing, but maybe not?

Using CSS to override the StyleSheet as recommended in the docs (and here) results in a very ugly progress bar, completely different in appearance from the stock Windows 7 progress bar.

Normal:

Stylesheet:

I don't want to use this option.

Answer 1

The simple way is setting QGraphicsColorizeEffect to the progress bar.

Like this:

QProgressBar* progress = new QProgressBar;
progress->setGraphicsEffect(new QGraphicsColorizeEffect);

Result on Win7:

Umm...the result looks like fine, but we can make it better, only change the chunk colour.

Here is the final result:

Reimplement QGraphicsEffect::draw to specific & customize the colourize effect area:

class Colorize : public QGraphicsEffect {
public:
    explicit Colorize(QObject *parent = Q_NULLPTR) :
        QGraphicsEffect(parent),
        strength(1),
        color(Qt::red),
        effectRect()
    { }
    quint32 strength;
    QColor color;
    QRectF effectRect;

protected:
    void draw(QPainter* painter) {
        QPoint offset;
        const QPixmap pixmap = sourcePixmap(Qt::LogicalCoordinates, &offset);
        draw(painter, offset, pixmap, QRect());
    }

    void draw(QPainter *painter, const QPointF &dest, const QPixmap &src, const QRectF &srcRect) const
    {
        if (src.isNull())
            return;

        QImage srcImage;
        QImage destImage;

        if (srcRect.isNull()) {
            srcImage = src.toImage();
            srcImage = srcImage.convertToFormat(srcImage.hasAlphaChannel() ? QImage::Format_ARGB32_Premultiplied : QImage::Format_RGB32);
            destImage = QImage(srcImage.size(), srcImage.format());
        } else {
            QRect rect = srcRect.toAlignedRect().intersected(src.rect());

            srcImage = src.copy(rect).toImage();
            srcImage = srcImage.convertToFormat(srcImage.hasAlphaChannel() ? QImage::Format_ARGB32_Premultiplied : QImage::Format_RGB32);
            destImage = QImage(rect.size(), srcImage.format());
        }
        destImage.setDevicePixelRatio(src.devicePixelRatioF());

        // do colorizing
        QPainter destPainter(&destImage);
        grayscale(srcImage, destImage, srcImage.rect());
        destPainter.setCompositionMode(QPainter::CompositionMode_Screen);
        destPainter.fillRect(effectRect, color);
        destPainter.end();

        // alpha blending srcImage and destImage
        if(0 < strength && strength < 1){
            QImage buffer = srcImage;
            QPainter bufPainter(&buffer);
            bufPainter.setOpacity(strength);
            bufPainter.drawImage(0, 0, destImage);
            bufPainter.end();
            destImage = buffer;
        }

        if (srcImage.hasAlphaChannel())
            destImage.setAlphaChannel(srcImage.alphaChannel());

        painter->drawImage(dest, destImage);
    }
}; 

Calculate the grove rect of the progress bar:

QRectF getGrooveRect() const {
    StyleOptionProgressBar option;
    option.initFrom(this); // this ⇒ progress bar
    return style()->subElementRect(QStyle::SE_ProgressBarGroove, &option, this);
}

...

class StyleOptionProgressBar : public QStyleOptionProgressBar {
public:
    using QStyleOptionProgressBar::QStyleOptionProgressBar;

    void initFrom(const ColorizeProgressBar* w) {
        init(w);
        minimum = w->minimum();
        maximum = w->maximum();
        progress = w->value();
        text = w->text();
        textAlignment = w->alignment();
        textVisible = w->isTextVisible();
        orientation = w->orientation();
        invertedAppearance = w->invertedAppearance();
    }
};

Complete source on Github.

BTW, Referred Qt source code