如何为Qt视图中的文字实现彩虹渐变效果

class ColorDelegate : public QStyledItemDelegate {
public:
 void paint(QPainter *painter, const QStyleOptionViewItem &option,
    const QModelIndex &index) const override;

 QSize sizeHint(const QStyleOptionViewItem &option,
     const QModelIndex &index) const override;
};

QSize ColorDelegate::sizeHint(const QStyleOptionViewItem &option,
        const QModelIndex &index) const {
 auto text = index.data().toString();
 auto width = option.fontMetrics.width(text) + 20;
 return QSize{width, option.fontMetrics.height()};
}

void ColorDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
       const QModelIndex &index) const {
 auto text = index.data().toString();
 auto myOpt = option;
 myOpt.displayAlignment = Qt::AlignCenter;
 auto sWidth = myOpt.fontMetrics.width(text);
 auto sHeight = myOpt.fontMetrics.height();
 // 计算文字所在的范围，因为是居中对齐，所以需要计算相应的起始位置
 auto sRect = QRectF(myOpt.rect.x() + (myOpt.rect.width() - sWidth) / 2.0,
      myOpt.rect.y() + ((myOpt.rect.height() - sHeight) / 2.0), sWidth, sHeight);

 // 将渐变填充的范围设置成文字所在的范围
 QLinearGradient l(sRect.x(), sRect.y(), sRect.x() + sRect.width(),
      sRect.y() + sRect.height());

 // 设置彩虹色渐变效果，彩虹由赤橙黄绿青蓝紫的颜色组成
 // 因此我们除去起始点为红色，每隔1/6就设置一种颜色
 l.setColorAt(0, Qt::red);
 l.setColorAt(1.0 / 6, QColor(255, 97, 0));
 l.setColorAt(2.0 / 6, QColor(255, 255, 0));
 l.setColorAt(3.0 / 6, Qt::green);
 l.setColorAt(4.0 / 6, Qt::cyan);
 l.setColorAt(5.0 / 6, Qt::blue);
 l.setColorAt(1, QColor(255, 0, 255));

 // 这里并不使用painter，只需要QStyle即可实现效果
 // QPalette::Text为文本显示效果的role
 auto role = QPalette::Text;
 if (option.state & QStyle::State_Selected) {
  // 当前item被选中时绘制高亮的选中框
  role = QPalette::HighlightedText;
  painter->fillRect(option.rect, option.palette.highlight());
 }
 myOpt.palette.setBrush(role, QBrush(l));
 // 注意最后一个参数role，只有设置了它才能让QStyle正确地绘制自定义的文本显示效果
 QApplication::style()->drawItemText(painter, myOpt.rect, myOpt.displayAlignment,
          myOpt.palette, true, text, role);
}

#include 
#include 
#include 
#include 

#include "ColorDelegate.h"

int main(int argc, char **argv) {
 QApplication app(argc, argv);

 auto win = new QWidget{};
 auto model = new QStandardItemModel{win};
 model->appendRow(new QStandardItem{"C++"});
 model->appendRow(new QStandardItem{"GoLang"});
 model->appendRow(new QStandardItem{"JavaScript"});
 model->appendRow(new QStandardItem{"Object C"});
 model->appendRow(new QStandardItem{"Rust"});
 model->appendRow(new QStandardItem{"这是一串非常非常长的被作为测试用例的样本字符串。"});
 model->appendRow(new QStandardItem{"██████████████████"});
 auto view = new QListView{};
 view->setModel(model);
 view->setItemDelegate(new ColorDelegate{}); // 设置自定义delegate

 auto layout = new QVBoxLayout{};
 layout->addWidget(view);
 win->setLayout(layout);
 win->show();

 app.exec();
}