Qt文档阅读笔记-QGraphicsItem::paint中QStyleOptionGraphicsItem *option的进一步认识

官方解析

painter : 此参数用于绘图;
option : 提供了item的风格，比如item的状态，曝光度以及详细的信息；
widget : 想画到哪个widget上，如果要画在缓存区上，这个参数一般填0；

博主例子

运行截图如下：

源码如下：

mygraphicsitem.h

#ifndef MYGRAPHICSITEM_H
#define MYGRAPHICSITEM_H

#include 

class MyGraphicsItem : public QGraphicsItem
{
public:
    MyGraphicsItem(QGraphicsItem *parent = 0);

protected:
    QRectF boundingRect() const Q_DECL_OVERRIDE;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) Q_DECL_OVERRIDE;

private:
    int m_width;
    int m_height;
};

#endif // MYGRAPHICSITEM_H

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include 

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private:
    Ui::Widget *ui;
};

#endif // WIDGET_H

main.cpp

#include "widget.h"
#include 

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}

mygraphicsitem.cpp

#include "mygraphicsitem.h"
#include 
#include 

MyGraphicsItem::MyGraphicsItem(QGraphicsItem *parent)
    :QGraphicsItem(parent)
{
    m_width = 100;
    m_height = 100;
}

QRectF MyGraphicsItem::boundingRect() const
{
    qreal penWidth = 1;
    return QRectF(0-penWidth/2, 0-penWidth/2, m_width+penWidth, m_height+penWidth);
}

void MyGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED(widget)
    painter->save();
    if(option->state & QStyle::State_Selected){

        painter->setPen(QPen(Qt::blue, 5));
    }
    else{

        painter->setPen(QPen(Qt::red,5));
    }
    painter->drawRect(0, 0, m_width, m_height);
    painter->restore();
}

widget.cpp

#include "widget.h"
#include "ui_widget.h"
#include "mygraphicsitem.h"

#include 

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);

    ui->graphicsView->setScene(new QGraphicsScene);
    MyGraphicsItem *item = new MyGraphicsItem;
    ui->graphicsView->scene()->addItem(item);

    item->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsSelectable);
}

Widget::~Widget()
{
    delete ui;
}