Drawing Line at current position using QGraphicsLineItem - qt

Question

I want to draw a line using QGraphicsLineItem. What exactly I want is that on clicking at GraphicsView, after second click Line must be drawn. I am confused with the syntax of QGraphicsLineItem and also how to use it. I am new to Qt. Please help me out to solve this problem.

Answer 1

You can use this code snippet.
*h
#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H
#include <QGraphicsScene>
#include <QStack>
#include <QPoint>
#include <QMouseEvent>
class GraphicsScene : public QGraphicsScene
{
Q_OBJECT
public:
explicit GraphicsScene(QObject *parent = 0);
signals:
protected:
void mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent);
public slots:
private:
QStack<QPoint> stack;
};
#endif // GRAPHICSSCENE_H
*.cpp
#include "graphicsscene.h"
#include <QDebug>
#include <QGraphicsSceneMouseEvent>
GraphicsScene::GraphicsScene(QObject *parent) :
QGraphicsScene(parent)
{
}
void GraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
qDebug() << "in";
if (mouseEvent->button() == Qt::LeftButton)
{
QPoint pos = mouseEvent->scenePos().toPoint();
if(stack.isEmpty())
stack.append(pos);
else if(stack.count() == 1)
{
stack.append(pos);
addLine(QLine(stack.pop(),stack.pop()),QPen(Qt::green));
}
}
}
Usage:
GraphicsScene *scene = new GraphicsScene(this);
ui->graphicsView->setScene(scene);
ui->graphicsView->show();
Edit: more beautiful solution which works as you need.
void GraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
qDebug() << "in";
if (mouseEvent->button() == Qt::LeftButton)
{
QPoint pos = mouseEvent->scenePos().toPoint();
if(stack.isEmpty())
stack.append(pos);
else
addLine(QLine(pos,stack.pop()),QPen(Qt::green));
}
}

Answer 2

You can derive the graphics view/scene and override the mousePressEvent
Below is example using derived QGraphicsScene and overridden mousePressEvent
Class Definition :
class MyScene : public QGraphicsScene
Data Members :
QList<QPointF> m_clickPositions;
int m_mode;
Code :
void MyScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
if(false == sceneRect().contains(event->scenePos()))
{
QGraphicsScene::mousePressEvent(event);
}
else if(Qt::LeftButton == event->button() && m_mode == ConstructMode)
{
m_clickPositions.append(event->scenePos());
if(m_clickPositions.size() == 2)
{
QLineF lineF(m_clickPositions[0], m_clickPositions[1]);
QGraphicsLineItem* item = this->addLine(lineF);
m_clickPositions.clear();
m_mode = ScrollMode;
}
}
}
I had used something similar in my project and extracted the code. Hope this helps.
Please comment is this is not working.
Edit ::
ConstructMode and Scroll mode are used in the above program so that I can distinguish whether I want to Draw/Construct or just scroll the scene. You can remove them and the declaration of m_mode if not required by you.
If you want to use the modes you can define some public constants and add a method setMode(). Please see the code below.
MyScene.h or some Constant file if you have one
#define ConstructMode 100
#define ScrollMode 101
And add the following function
void MyScene::setMode(int mode)
{
m_mode = mode;
}
After this if you want to enter the construction mode you will need to call myScene->setMode(ConstructMode) everytime, as after the item is constructed the mode is reset to ScrollMode.