I wanted to try to render QProgressBar widgets inside cells of a table and I wanted the progress bars to change color if the progress value was, for example, greater than 50 (to represent a warning). To test it I created a very small project using a QTableView, a QStandardItemModel and a QProgressBar delegate to render a single table cell.

I tried different ways and none is working, the progress bar color is always green. Below the latest version of my code subclassing QProxyStyle.

Qt Code: 
  1. #include <QApplication>
  2. #include <QStyledItemDelegate>
  3. #include <QPainter>
  4. #include <QTableView>
  5. #include <QStandardItemModel>
  6. #include <QHeaderView>
  7. #include <QPushButton>
  8. #include <QVBoxLayout>
  9. #include <QProxyStyle>
  10.  
  11. class ProgressBarStyle : public QProxyStyle
  12. {
  13. public:
  14.     ProgressBarStyle(QStyle *style = nullptr) : QProxyStyle(style) {
  15.         qDebug() << "ProgressBarStyle constructor called";
  16.     }
  17.  
  18.     void drawControl(ControlElement element, const QStyleOption *option, QPainter *painter, const QWidget *widget) const override
  19.     {
  20.         if (element == CE_ProgressBar)
  21.         {
  22.             const QStyleOptionProgressBar *progressBarOption = qstyleoption_cast<const QStyleOptionProgressBar *>(option);
  23.             if (progressBarOption && progressBarOption->progress > 50)
  24.             {
  25.                 QStyleOptionProgressBar progressBarOptionCopy(*progressBarOption);
  26.                 progressBarOptionCopy.palette.setColor(QPalette::Highlight, Qt::red);
  27.  
  28.                 QProxyStyle::drawControl(element, &progressBarOptionCopy, painter, widget);
  29.                 return;
  30.             }
  31.         }
  32.  
  33.         QProxyStyle::drawControl(element, option, painter, widget);
  34.     }
  35. };
  36.  
  37. class ProgressBarDelegate : public QStyledItemDelegate
  38. {
  39. public:
  40.     ProgressBarDelegate(QObject *parent = nullptr) : QStyledItemDelegate(parent) {}
  41.  
  42.     void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const override
  43.     {
  44.         int progress = index.data().toInt();
  45.         QStyleOptionProgressBar progressBarOption;
  46.         progressBarOption.rect = option.rect;
  47.         progressBarOption.minimum = 0;
  48.         progressBarOption.maximum = 100;
  49.         progressBarOption.progress = progress;
  50.         progressBarOption.text = QString::number(progress) + "%";
  51.         progressBarOption.textVisible = true;
  52.  
  53.         QApplication::style()->drawControl(QStyle::CE_ProgressBar, &progressBarOption, painter);
  54.     }
  55. };
  56.  
  57. int main(int argc, char *argv[])
  58. {
  59.     QApplication app(argc, argv);
  60.     QWidget window;
  61.     QVBoxLayout layout(&window);
  62.     QTableView tableView;
  63.     layout.addWidget(&tableView);
  64.     QPushButton button("Update Progress");
  65.     layout.addWidget(&button);
  66.     QStandardItemModel model(5, 3); // 5 rows and 3 columns
  67.     tableView.setModel(&model);
  68.  
  69.     // Set up data
  70.     for (int row = 0; row < model.rowCount(); ++row) {
  71.         for (int col = 0; col < model.columnCount(); ++col) {
  72.             model.setData(model.index(row, col), (row + col) * 2);
  73.         }
  74.     }
  75.  
  76.     // Set up delegate
  77.     ProgressBarDelegate delegate;
  78.     tableView.setItemDelegate(&delegate);
  79.  
  80.     // Set up style
  81.     ProgressBarStyle *style = new ProgressBarStyle;
  82.     tableView.setStyle(style);
  83.     app.setStyle(style);
  84.  
  85.     // Connect button to update progress
  86.     QObject::connect(&button, &QPushButton::clicked, [&model, &tableView]() {
  87.         for (int row = 0; row < model.rowCount(); ++row) {
  88.             for (int col = 0; col < model.columnCount(); ++col) {
  89.                 int progress = model.data(model.index(row, col)).toInt();
  90.                 progress = (progress + 10) % 110;
  91.                 model.setData(model.index(row, col), progress);
  92.             }
  93.         }
  94.         tableView.viewport()->update();
  95.     });
  96.  
  97.     tableView.horizontalHeader()->setSectionResizeMode(QHeaderView::Stretch);
  98.     window.show();
  99.     return app.exec();
  100. }
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Any advise please? I am using Qt 6.5.0