c++ - 如何更改QGraphicsRectItem的颜色

Question

我找不到我做错了的地方。我想在发生特定事件时更改项目(QGraphicsRectItem)的颜色。事实是，似乎一旦调用了覆盖绘画方法，无论如何颜色都不会改变。这是我所做的简单代码:

item.h

class Item : public QGraphicsRectItem
{
public:
    Item(QGraphicsView *graphView);
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *) override;

private:
    QPointF newPos;
    QGraphicsView *graph;
};

item.cpp

Item::Item(QGraphicsView *graphWidget) : graph(graphWidget) { }

void Item::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
{
     painter->setPen(Qt::NoPen);
     painter->setBrush(Qt::black);
     painter->drawEllipse(-7, -7, 20, 20);
}

main.cpp

int main(int argc, char *argv[])
{
     srand(QDateTime::currentDateTime().toMSecsSinceEpoch());
     QApplication a(argc, argv);
     QGraphicsScene scene;
     QGraphicsView view(&scene);

     Item *item = new Item(&view);
     scene.addItem(item);
     item->setPos(0, 0);

     item->setBrush(Qt::red);
     item->update();

     view.show();
     return a.exec();
}

Answer 1

如果我正确理解您的问题，那么问题是在item->setBrush(Qt::red)之后，您的圈子没有被涂成红色。如果是这种情况，则问题是您在Qt::NoPen函数中强行使用了特定的笔(Qt::red)和画笔(paint)，而没有使用Item::pen()和Item::brush()来检索信息。相反，您可以执行以下操作:

Item::Item(QGraphicsView *graphWidget) : graph(graphWidget)
{
  setPen(Qt::NoPen);
  setBrush(Qt::black);
}

void Item::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
{
  Q_UNUSED(option);
  painter->setPen(pen());
  painter->setBrush(brush());
  painter->drawEllipse(-7, -7, 20, 20);
}

这样，您可以在构造函数中定义默认的画笔，但是仍然可以使用 Item::setPen和 Item::setBrush对其进行更改。此外，对于此示例，您最好继承 QAbstractGraphicsShapeItem ，但随后必须实现 Item::boundingRect函数。下面的示例输出一个红色圆圈(这是我怀疑您要执行的操作)，并且还用黑色绘制轮廓(尽管这不是您想要的，但是为了显示笔也会改变):

#include <QApplication>
#include <QDateTime>
#include <QGraphicsRectItem>
#include <QGraphicsScene>
#include <QGraphicsView>

class Item : public QAbstractGraphicsShapeItem
{
public:
  Item(QGraphicsView *graphView);

  void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *) override;

  virtual QRectF boundingRect() const override;

private:
  QPointF newPos;
  QGraphicsView *graph;
};

Item::Item(QGraphicsView *graphWidget) : graph(graphWidget)
{
  setPen(Qt::NoPen);
  setBrush(Qt::black);
}

void Item::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
{
  Q_UNUSED(option);
  painter->setPen(pen());
  painter->setBrush(brush());
  painter->drawEllipse(-7, -7, 20, 20);
}

QRectF Item::boundingRect() const
{
  double pw = pen().widthF() / 2;
  return QRectF(QPointF(-7 - pw, -7 - pw), QSizeF(20 + 2 * pw, 20 + 2 * pw));
}

int main(int argc, char *argv[])
{
  srand(QDateTime::currentDateTime().toMSecsSinceEpoch());
  QApplication a(argc, argv);
  QGraphicsScene scene;
  QGraphicsView view(&scene);

  Item *item = new Item(&view);
  scene.addItem(item);
  item->setPos(0, 0);

  item->setBrush(Qt::red);
  item->setPen(QPen(Qt::black));
  item->update();

  view.show();
  return a.exec();
}