QGraphicsItem how to change the boundary colors of the rectangale

[Qt Enthusiast]

how to get pointer of Qpainter and QPen
objects in QGraphicsitem

[mrjj]

@Qt-Enthusiast
In paintevent you have painter.
it has both.

[Qt Enthusiast]

so you mean after the slot
can I need to call i assume you have myItem::paintEvent ? is called automatcially after the slot

[mrjj]

@Qt-Enthusiast

• can I need to call i assume you have myItem::paintEvent ? is called automatcially after the slot

Not sure what u say here :)

[A Former User]

Hi! Here is a minimal example. Note that we need to derive our custom item not from QGraphicsItem but from QGraphicsObject so that we can use signals and slots with it.

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QGraphicsScene>
#include "myitem.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QGraphicsScene *scene = new QGraphicsScene;
    MyItem *myItem = new MyItem();
    scene->addItem( myItem );
    ui->graphicsView->setScene(scene);
    // ui->pushButton_2 is the button to toggle the color of your rectangle
    connect(ui->pushButton_2, &QPushButton::clicked, myItem, &MyItem::toggleColor);
}

myitem.h

#ifndef MYITEM_H
#define MYITEM_H

#include <QGraphicsObject> 

// it uses signals & slots, so inheriting from QGraphicsItem wouldn't be enough
class MyItem : public QGraphicsObject
{
    Q_OBJECT
public:
    MyItem(QGraphicsItem *parent = Q_NULLPTR);
    QRectF boundingRect() const override;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;

public slots:
    void toggleColor();

private:
    const qreal m_penWidth = 10;
    bool m_color = false; // color of the rectangle; true = lime, false = purple
};

#endif // MYITEM_H

myitem.cpp

#include "myitem.h"

#include <QPen>
#include <QPainter>


MyItem::MyItem(QGraphicsItem *parent)
    : QGraphicsObject(parent)
{
}

QRectF MyItem::boundingRect() const
{
    return QRectF(-20 - m_penWidth / 2, -20 - m_penWidth / 2, 40 + m_penWidth, 40 + m_penWidth);
}

void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED(option)
    Q_UNUSED(widget)
    QPen pen( m_color ? QColor("lime") : QColor("purple") );
    pen.setWidth(m_penWidth);
    painter->setPen(pen);
    painter->drawRect(-20, -20, 40, 40);
}

void MyItem::toggleColor()
{
    m_color = !m_color;
    update();
}

Hope it helps!

[Qt Enthusiast]

One more question , how to get filled color of colored QGraphicsitem filled rectangle

[CorD SaC]

@Wieland said in QGraphicsItem how to change the boundary colors of the rectangale:

ere is a minimal example. Note that we need to derive our custom item not from QGraphicsItem but from QGraphicsObject so that we can use signals and slots with it.
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QGraphicsScene>
#include "myitem.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),

Could you please explain why this error occur. ```
error: 'myitem' does not name a type
myitem *myitem = new myitem;

[mrjj]

@CorD-SaC

The myitem.h is a custom file

if you do not have it, it means that
it dont know what myitem is for a class.

• 'myitem' does not name a type
  =
  What the heck is "myitem" , compiler says

[CorD SaC]

@mrjj

problem is solved,I change the code, myitem *mi = new myitem(); instead of
//myitem *myitem = new myitem(); I don't know what was the reason,however after change the code like this,problem solved.

[mrjj]

Oh the type (of the class) was used as variable also

like
int *int = new int()

that is not allowed :)

[CorD SaC]

@mrjj

Ooops,thanks :)