some error with mousepressevent() - qt

Question

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QMouseEvent>
#include <QDebug>
#include "my_qlabel.h"
#include<QTimer>
int px;
int py;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
tmrTimer = new QTimer(this);
connect(tmrTimer,SIGNAL(timeout()),this,SLOT(showthepositionofmouse()));
tmrTimer->start(20);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::showthepositionofmouse()
{
ui->plainTextEdit->appendPlainText(QString(" x , y = ")+QString::number(px)+QString::number(py));
}
void my_qlabel::mousePressEvent(QMouseEvent *event)
{
if (event->button()==Qt::RightButton){
px = event->x();
py = event->y();
}
}
i Want to display the position of Mouse clicked
i use ui->plainTextEdit->appendPlainText(QString(" x , y = ")+QString::number(px)+QString::number(py)); to display this position. although i click mouse,it only show x,y = 0 0. why that?

Answer 1

The py and px that are accessed in my_qlabel::mousePressEvent, are member variables of my_qlabel and are not visible to MainWindow::showthepositionofmouse.
If you want them to be visible you should send them using a signal and slot.
http://qt-project.org/doc/qt-4.8/signalsandslots.html
Here is a way to implement it:
In my_qlabel.h have something like this:
class my_qlabel : public QLabel
{
Q_OBJECT
signals:
void right_click_xy(int x, int y);
// ...constructor and other functions
public slots:
void mousePressEvent(QMouseEvent *event)
{
int px, py;
if (event->button()==Qt::RightButton){
px = event->x();
py = event->y();
emit right_click_xy(px, py);
}
}
}
In mainwindow.h have something like this:
class MainWindow : public QMainWindow
{
Q_OBJECT
// ... constructor and other functions
public slots:
void on_display_click_xy(int px, int py)
{
qDebug() << "Received xy over signal slot:" << px << py;
ui->plainTextEdit->appendPlainText(QString(" x , y = ")+QString::number(px)+QString::number(py));
}
}
And inside your MainWindow constructor put the following:
QObject::connect(ui->my_qlabel_instance, SIGNAL(right_click_xy(int,int)),
this, SLOT(on_display_click_xy(int, int)));
Another alternate way of doing this, but is isn't as kosher, would be to drill down into your ui object and access px and py that way.
qDebug() << ui->my_qlabel_instance->py;
But this is not as elegant, and isn't signaled the same way.
Another idea to look into is to use a QPoint object instead of two ints.
Hope that helps.

Answer 2

The whole idea is bad, you must process this event within my_qlabel class via signals and slots, not outside of it. I would suggest something like this (emit signal with coordinates of mouse click):
Header my_qlabel.h:
#ifndef MY_QLABEL_H
#define MY_QLABEL_H
#include <QLabel>
#include <QPoint>
#include <QEvent>
class my_qlabel : public QLabel
{
Q_OBJECT
public:
my_qlabel( const QString & text="", QWidget * parent = 0 );
signals:
void clicked(QPoint pos);
protected:
void mouseReleaseEvent ( QMouseEvent * event );
};
#endif // MY_QLABEL
Source my_qlabel.cpp:
#include "my_qlabel.h"
#include <QMouseEvent>
my_qlabel::my_qlabel( const QString & text, QWidget * parent )
:QLabel(parent)
{
setText(text);
}
void my_qlabel::mouseReleaseEvent ( QMouseEvent * event )
{
emit clicked(event->pos());
}
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include "my_qlabel.h"
#include <QtGui/QWidget>
class MainWindow : public QWidget
{
Q_OBJECT
public:
MainWindow(QWidget *parent = 0);
~MainWindow();
public slots:
void showthepositionofmouse(QPoint pos);
};
#endif // MAINWINDOW_H
and mainwindow.cpp:
#include "mainwindow.h"
#include <QDebug>
#include <QPoint>
#include <QHBoxLayout>
MainWindow::MainWindow(QWidget *parent) :
QWidget(parent)
{
my_qlabel* label=new my_qlabel("Text of my label",this);
label->setAlignment(Qt::AlignCenter);
QGridLayout *lay=new QGridLayout(this);
this->setLayout(lay);
lay->addWidget(label,0,0,1,1);
connect(label,SIGNAL(clicked(QPoint)),this,SLOT(showthepositionofmouse(QPoint)));
}
MainWindow::~MainWindow()
{
}
void MainWindow::showthepositionofmouse(QPoint pos)
{
qDebug()<< "Clicked, position="<<pos;
}