QWidget keyPressEvent override

Question

I'm trying for half an eternity now overriding QWidgets keyPressEvent function in QT but it just won't work. I've to say i am new to CPP, but I know ObjC and standard C.

My problem looks like this:

class QSGameBoard : public QWidget {
Q_OBJECT

public:
  QSGameBoard(QWidget *p, int w, int h, QGraphicsScene *s);

signals:
  void keyCaught(QKeyEvent *e);

protected:
  virtual void  keyPressEvent(QKeyEvent *event);
};

QSGameBoard is my QWidget subclass and i need to override the keyPressEvent and fire a SIGNAL on each event to notify some registered objects.

My overridden keyPressEvent in QSGameBoard.cpp looks like this:

void QSGameBoard::keyPressEvent(QKeyEvent *event) {
  printf("\nkey event in board: %i", event->key());
  //emit keyCaught(event);
}

When i change QSGameBoard:: to QWidget:: it receives the events, but i cant emit the signal because the compiler complains about the scope. And if i write it like this the function doesn't get called at all.

What's the problem here?

Solution 1

EDIT: As pointed out by other users, the method I outlined originally is not the proper way to resolve this. Answer by Vasco Rinaldo

Use Set the FocusPolicy to Qt::ClickFocus to get the keybordfocus by mouse klick. setFocusPolicy(Qt::ClickFocus);

The previous (albeit imperfect) solution I gave is given below:

Looks like your widget is not getting "focus". Override your mouse press event:

void QSGameBoard::mousePressEvent ( QMouseEvent * event ){
    printf("\nMouse in board");
    setFocus();
}

Here's the source code for a working example:

QSGameBoard.h

#ifndef _QSGAMEBOARD_H
#define _QSGAMEBOARD_H

#include <QWidget>
#include <QGraphicsScene>

class QSGameBoard : public QWidget {
Q_OBJECT

public:
  QSGameBoard(QWidget *p, int w, int h, QGraphicsScene *s);

signals:
  void keyCaught(QKeyEvent *e);

protected:
  virtual void  keyPressEvent(QKeyEvent *event);
  void  mousePressEvent ( QMouseEvent * event );
};


#endif  /* _QSGAMEBOARD_H */

QSGameBoard.cpp

#include <QKeyEvent>
#include <QLabel>
#include <QtGui/qgridlayout.h>
#include <QGridLayout>

#include "QSGameBoard.h"


QSGameBoard::QSGameBoard(QWidget* p, int w, int h, QGraphicsScene* s) :
    QWidget(p){

    QLabel* o = new QLabel(tr("Test Test Test"));
    QGridLayout* g  = new QGridLayout(this);
    g->addWidget(o);
}

void QSGameBoard::keyPressEvent(QKeyEvent* event){
    printf("\nkey event in board: %i", event->key());
}

void QSGameBoard::mousePressEvent ( QMouseEvent * event ){
    printf("\nMouse in board");
    setFocus();
}

main.cpp

#include <QtGui/QApplication>
#include <QtGui/qmainwindow.h>

#include "QSGameBoard.h"

int main(int argc, char *argv[]) {
    // initialize resources, if needed
    // Q_INIT_RESOURCE(resfile);

    QApplication app(argc, argv);

    QMainWindow oM;
    QGraphicsScene o;
    QSGameBoard a(&oM, 1, 2, &o);
    oM.setCentralWidget(&a);
    a.show();
    oM.show();

    // create and show your widgets here

    return app.exec();
}

Solution 2

You don't have to reimplement mousePressEvent yourself just to call setFocus. Qt planed it already.

Set the FocusPolicy to Qt::ClickFocus to get the keybordfocus by mouse klick.

setFocusPolicy(Qt::ClickFocus);

As said in the manual:

This property holds the way the widget accepts keyboard focus.

The policy is Qt::TabFocus if the widget accepts keyboard focus by tabbing, Qt::ClickFocus if the widget accepts focus by clicking, Qt::StrongFocus if it accepts both, and Qt::NoFocus (the default) if it does not accept focus at all.

You must enable keyboard focus for a widget if it processes keyboard events. This is normally done from the widget's constructor. For instance, the QLineEdit constructor calls setFocusPolicy(Qt::StrongFocus).

If the widget has a focus proxy, then the focus policy will be propagated to it.

Solution 3

Set the FocusPolicy to Qt::ClickFocus to get the keybordfocus by mouse klick. setFocusPolicy(Qt::ClickFocus);