Load QGraphicsScene From QDataStream

Qt allows you to read binary data from files into QDataStream. In this sample serialized QGprahicsItem is created on the QGraphicsScene by reading related data from a text file.
Sample project started as a QT Gui Application and contains main.cpp, LoadSceneFromQDataStream.h, LoadSceneFromQDataStream.cpp, MyGraphicsItem.h and MyGraphicsItem.cpp files.
Project Directory Structure in Qt Creator IDE :


LoadSceneFromQDataStream.pro is the project configuration file and contains:

QT       += core gui
TARGET = LoadSceneFromQDataStream
TEMPLATE = app
SOURCES += main.cpp\
        LoadSceneFromQDataStream.cpp \
    MyGraphicsItem.cpp
HEADERS  += LoadSceneFromQDataStream.h \
    MyGraphicsItem.h

main.cpp file contains the custom QDialog instance and shows it.

#include <QApplication>
#include "LoadSceneFromQDataStream.h"
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Dialog w;
    w.show();
    return a.exec();
}

LoadSceneFromQDataStream.h file contains custom QDialog which displays content of binary file in a QGraphicsScene instance.

#ifndef LOADSCENEFROMQDATASTREAM_H
#define LOADSCENEFROMQDATASTREAM_H
#include <QDialog>

class QGraphicsScene;
class QGraphicsView;
class QVBoxLayout;

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

public slots:
    void sl_loadSceneFromFile();

private:
    QGraphicsScene* scene;
    QGraphicsView* view;
    QVBoxLayout* layout;
    QPushButton* btnLoad;
};

#endif // LOADSCENEFROMQDATASTREAM_H

LoadSceneFromQDataStream.cpp file contains implementation details for the custom QDialog.

#include "LoadSceneFromQDataStream.h"
#include "MyGraphicsItem.h"

#include <QVBoxLayout>
#include <QPushButton>
#include <QGraphicsView>
#include <QFile>
#include <QMessageBox>
#include <QDir>

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    scene = new QGraphicsScene(this);
    view = new QGraphicsView(scene,this);
    layout = new QVBoxLayout(this);
    layout->addWidget(view);
    btnLoad = new QPushButton("Load Scene From File");
    connect(btnLoad, SIGNAL(clicked()), this, SLOT(sl_loadSceneFromFile()));
    layout->addWidget(btnLoad);
    setLayout(layout);
    resize(270,200);
}

Dialog::~Dialog()
{
    delete btnLoad;
    delete layout;
    delete scene;
    delete view;
}

void Dialog::sl_loadSceneFromFile()
{
    QString fileName(QDir::currentPath().append("/sceneData.txt"));
    QFile file(fileName);
    if (!file.open(QIODevice::ReadOnly))
    {
            return;
    }
    QDataStream in(&file);
    int itemListSize;
    in >> itemListSize;
    for( int i = 0; i< itemListSize; i++ )
    {
        qreal xCoord = 0;
        qreal yCoord = 0;
        in >> xCoord;
        in >> yCoord;
        MyGraphicsItem* item = new MyGraphicsItem();
        item->setPos(xCoord,yCoord);
        scene->addItem(item);
    }
    QMessageBox::warning(this,"Success","Loaded Scene Items From File");
    close();
}

sl_loadSceneFromFile() opens sceneData.txt file and reads its content into QDataStream instance.
sceneData.txt file is located under LoadSceneFromQDataStream/LoadSceneFromQDataStream folder of the project. sceneData.txt file is created by the previous sample application.

First serialized data to the sceneData.txt file is the number of QGraphicsItems on the QGraphicsScene. It is read into the variable “itemListSize” from sceneData.txt file. Then for each QGraphicsItem on the QGraphicsScene, xCoordinate and yCoordinate of the QGraphicsItem is stored in the file in binary format. By reading x and y coordinate of the QGraphicsItem from sceneData.txt file, item’s position is set. After setting the pos of the QGraphicsItem, it is inserted into the list of QGraphicsItems on the QGraphicsScene by calling the addItem function of QGraphicsScene.

Regenerated QGraphicsItem instance on the QGraphicsScene is a custom/derived QGprahicsItem and is defined in MyGraphicsItem.h file.

#ifndef MYGRAPHICSITEM_H
#define MYGRAPHICSITEM_H
#include <QGraphicsItem>
class MyGraphicsItem : public QGraphicsItem
{
public:
    MyGraphicsItem(QGraphicsItem *parent = 0, QGraphicsScene *scene = 0);
    ~MyGraphicsItem();

protected:
    QRectF boundingRect() const;

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

#endif // MYGRAPHICSITEM_H

Custom QGraphicsItem contains constructor, destructor, boundingRect() and paint() methods.

MyGraphicsItem.cpp file contains implementation details for the custom QGraphicsItem class.

#include "MyGraphicsItem.h"
#include <QPainter>
MyGraphicsItem::MyGraphicsItem(QGraphicsItem* parent, QGraphicsScene* scene) : QGraphicsItem(parent,scene)
{
}

MyGraphicsItem::~MyGraphicsItem(){}

QRectF MyGraphicsItem::boundingRect() const
{
    qreal penWidth = 1;
    return QRectF(-10 - penWidth / 2, -10 - penWidth / 2,
                  20 + penWidth, 20 + penWidth);
}

void MyGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
           QWidget *widget)
{
    Q_UNUSED(widget)
    Q_UNUSED(option)

    QPen pen;
    pen.setWidthF(2);
    pen.setStyle(Qt::DashLine);
    painter->setPen(pen);
    painter->setBrush(QBrush(QColor(50,120,80)));
    painter->drawRoundedRect(boundingRect(), 25, 25, Qt::RelativeSize);
}

When you run the application, a dialog with a "load scene from file" button is displayed.By clicking the qpushbutton, application searches for the sceneData.txt file and loads files content into the QGraphicsScene.