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//MyView.h
#ifndef MYVIEW_H
#define MYVIEW_H
#include <qcanvas.h>

class QPoint;
class MyView : public QCanvasView
{
  Q_OBJECT
  public:
    MyView(QCanvas* canvas, QWidget* parent = 0, const char* name = 0);
  signals:
    void transmitCoords(QPoint&);
  protected:
    void contentsMousePressEvent(QMouseEvent* event);
  private:
    QCanvasItem* item;
};
#endif // MYVIEW_H

//MyView.cpp
#include <qcanvas.h>
#include <qpoint.h>
#include "MyLine.h"
#include "MyView.h"

void receiveCoords(QPoint&);

MyView::MyView(QCanvas* canvas, QWidget* parent, const char* name)
  :QCanvasView(canvas, parent, name)
{
  item = 0;
}

void MyView::contentsMousePressEvent(QMouseEvent* event)
{
  item = new MyLine(canvas());
  QPoint position = event->pos();
  connect(this, SIGNAL(transmitCoords(QPoint&)), (QObject*)item, SLOT(receiveCoords(QPoint&)));
  emit transmitCoords(position);
}

//MyLine.h
#ifndef MYLINE_H
#define MYLINE_H
#include <qcanvas.h>

class MyLine : public QCanvasLine
{
  public:
    MyLine(QCanvas* canvas);
  public slots:
    void receiveCoords(QPoint&);
};

#endif // MYLINE_H

//MyLine.cpp
#include <qcanvas.h>
#include <qpoint.h>
#include "MyLine.h"

MyLine::MyLine(QCanvas* canvas)
  :QCanvasLine(canvas)
{}

void MyLine::receiveCoords(QPoint& position)
{
  //Некоторая обработка
}

//main.cpp
#include <qapplication.h>
#include <qcanvas.h>
#include "MyView.h"

int main(int argc, char** argv)
{
  QApplication app(argc, argv);
  QCanvas* canvas = new QCanvas(400, 400);
  MyView* diagram = new MyView(canvas);
  app.setMainWidget(diagram);
  diagram->show();
  return app.exec();
}