QT application programming: QGraphicsView draws pictures as background

1. Environmental introduction

QT version: 5.12.6
Operating system: win10 64 bit

2. Function introduction

Use the specified picture as the background of the QGraphicsView view window, and eliminate the afterimage produced when the primitive is dragged.
Idea: Derive the QGraphicsScene class and override the drawBackground function.

Three, source code

3.1 my_graphicsScene.cpp

#include "my_graphicsScene.h"
my_graphicsScene::my_graphicsScene(QWidget *parent): QGraphicsScene(parent)
{
    
    

}

my_graphicsScene::~my_graphicsScene()
{
    
    

}

void my_graphicsScene::drawBackground(QPainter *painter, const QRectF &rect)
{
    
    
    if(views().count()==0)return;

    // 计算视窗的大小,消除图元拖动时出现的残影
    QGraphicsView* pView=views().first();
    QRect contentRect=pView->viewport()->contentsRect();
    QRectF sceneRect =pView->mapToScene(contentRect).boundingRect();

    //绘制指定图片作为背景
    QPixmap pixmap("D:/linux-share-dir/test/www/123.jpg");
    painter->drawPixmap(sceneRect,pixmap,QRect());
}

3.2 my_graphicsScene.h

#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPainter>
#include <QPaintEvent>
#include <QMouseEvent>
#include <QPainterPath>
#include <QPixmap>
#include <QDebug>

class my_graphicsScene : public QGraphicsScene
{
    
    
    Q_OBJECT
public:
    my_graphicsScene(QWidget *parent = nullptr);
    ~my_graphicsScene();

protected:
 void drawBackground(QPainter *painter, const QRectF &rect);
};
#endif // GRAPHICSSCENE_H

3.3 widget.cpp

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

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    
    
    ui->setupUi(this);
    scene= new my_graphicsScene;  //设置画板
    ui->graphicsView->setScene(scene); //设置场景

    //设置图元更新模式,当图元发生改变时,更新整个视窗
    ui->graphicsView->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    //重写QGraphicsItem 可以设置文本的背景颜色

    //富文本
    textItem=new QGraphicsTextItem;
    textItem->setPlainText("QGraphicsTextItem测试.");
    textItem->setFont(QFont("华文琥珀",12));
    textItem->setDefaultTextColor(Qt::red); //设置字体颜色
    textItem->setFlags(QGraphicsItem::ItemIsMovable|QGraphicsItem::ItemIsSelectable);
    scene->addItem(textItem);
}

/*
工程: graphicsView
日期: 2020-12-20
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 析构函数
*/
Widget::~Widget()
{
    
    
    delete ui;
}

/*
工程: graphicsView
日期: 2020-12-20
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 编辑状态
*/
void Widget::on_pushButton_clicked()
{
    
    
    //设置编辑状态
    textItem->setTextInteractionFlags(Qt::TextEditorInteraction);
}

/*
工程: graphicsView
日期: 2020-12-20
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 取消编辑状态
*/
void Widget::on_pushButton_2_clicked()
{
    
    
     textItem->setTextInteractionFlags(Qt::NoTextInteraction); //取消编辑状态

     //将QGraphicsTextItem上的文本实时显示在plainTextEdit上.
     ui->plainTextEdit->clear();
     ui->plainTextEdit->insertPlainText(textItem->toPlainText());
}

/*
工程: graphicsView
日期: 2020-12-20
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 选择字体
*/
void Widget::on_pushButton_setfont_clicked()
{
    
    
    bool ok;
    QFont font=QFontDialog::getFont(&ok, QFont("幼圆", 12),this,tr("字体选择"));
    if(ok) //用户点击的了确定按钮
    {
    
    
        textItem->setFont(font); //设置当前字体
    }
}

/*
工程: graphicsView
日期: 2020-12-20
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 选择颜色
*/
void Widget::on_pushButton_setcolor_clicked()
{
    
    
    QColor clr=QColorDialog::getColor(Qt::white,this,tr("颜色选择"));
    if(clr.isValid())
    {
    
    
        textItem->setDefaultTextColor(clr);
    }
}