eventFilter anywhere in the program.

[Oleg21]

Hi everyone. The following situation arose: I have a project, it contains two classes (first.cpp and second.cpp).
first.cpp is a constant and compulsory project class, and the second.cpp is an optional class and is connected as needed.
So, in the second.cpp class, I have a desire to implement eventFilter at the program level. For this I write the following code:

second.cpp
#include "second.h"
#include <QtWidgets>
 
second::second(QWidget *parent) : QWidget(parent)
{
    qApp->installEventFilter(parent);
}
 
second::~second(){
 
}
 
bool second::eventFilter(QObject *watched, QEvent *event){
    //it's not working!
}

second.cpp
#include "second.h"
#include <QtWidgets>

second::second(QWidget *parent) : QWidget(parent)
{
    qApp->installEventFilter(parent);
}

second::~second(){

}

bool second::eventFilter(QObject *watched, QEvent *event){
    //it's not working!
}

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Unfortunately, eventFilter in this class does not work.
In order for everything to work, I need to implement eventFilter in class first.cpp:

#include "first.h"
#include "second.h"
 
first::first(QWidget *parent) : QMainWindow(parent)
{
    second *tempSecond = new second(this);
}
 
first::~first()
{
 
}
 
bool first::eventFilter(QObject *watched, QEvent *event){
    //it's working!
}

#include "first.h"
#include "second.h"

first::first(QWidget *parent) : QMainWindow(parent)
{
    second *tempSecond = new second(this);
}

first::~first()
{

}

bool first::eventFilter(QObject *watched, QEvent *event){
    //it's working!
}

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However, this approach contradicts my opinion about the possibility of disabling the class second.cpp.
Tell me, how to solve this problem?

[Santosh Reddy]

//qApp->installEventFilter(parent);
qApp->installEventFilter(this);

//qApp->installEventFilter(parent);
qApp->installEventFilter(this);

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[Oleg21]

Hello. Truly, I had to do as you say:

qApp->installEventFilter(this);

qApp->installEventFilter(this);

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And implementation of eventFilter:

bool second::eventFilter(QObject *watched, QEvent *event){
    if (event->type() == QEvent::MouseButtonRelease) {
 
        cc++;
        qDebug() << cc;
 
        return false;
    }else{
        return false;
    }
}

bool second::eventFilter(QObject *watched, QEvent *event){
    if (event->type() == QEvent::MouseButtonRelease) {

        cc++;
        qDebug() << cc;

        return false;
    }else{
        return false;
    }
}

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However, there is a small problem here: eventFilter function gets MouseButtonRelease event twice.
Should this be so? If so, maybe you know why this happens?

[Santosh Reddy]

Hello. Truly, I had to do as you say:
However, there is a small problem here: eventFilter function gets MouseButtonRelease event twice.
Should this be so? If so, maybe you know why this happens?

You have installed the event filter on two objects, or may be installed event filer on same object twice.

[Oleg21]

Hmm, weird. I install event filter only in one place of code and only for one object.
Be kind, take a look at the code of my project, and tell me where I was mistaken?

Headers:
first.h

#ifndef FIRST_H
#define FIRST_H
 
#include <QMainWindow>
 
class first : public QMainWindow
{
    Q_OBJECT
 
public:
    first(QWidget *parent = 0);
    ~first();
};
 
#endif // FIRST_H

#ifndef FIRST_H
#define FIRST_H

#include <QMainWindow>

class first : public QMainWindow
{
    Q_OBJECT

public:
    first(QWidget *parent = 0);
    ~first();
};

#endif // FIRST_H

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second.h

#ifndef SECOND_H
#define SECOND_H
 
#include <QWidget>
 
class second : public QWidget
{
    Q_OBJECT
public:
    explicit second(QWidget *parent = nullptr);
    ~second();
 
    int cc = 0;
 
    virtual bool eventFilter(QObject *watched, QEvent *event);
signals:
 
public slots:
};
 
#endif // SECOND_H

#ifndef SECOND_H
#define SECOND_H

#include <QWidget>

class second : public QWidget
{
    Q_OBJECT
public:
    explicit second(QWidget *parent = nullptr);
    ~second();

    int cc = 0;

    virtual bool eventFilter(QObject *watched, QEvent *event);
signals:

public slots:
};

#endif // SECOND_H

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Sources:
main.cpp

#include "first.h"
#include <QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    first w;
    w.show();
 
    return a.exec();
}

#include "first.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    first w;
    w.show();

    return a.exec();
}

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first.cpp

#include "first.h"
#include "second.h"
#include <QtWidgets>
 
first::first(QWidget *parent) : QMainWindow(parent)
{
    second *second1 = new second(this);
}
 
first::~first()
{
 
}

#include "first.h"
#include "second.h"
#include <QtWidgets>

first::first(QWidget *parent) : QMainWindow(parent)
{
    second *second1 = new second(this);
}

first::~first()
{

}

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second.cpp

#include "second.h"
#include <QtWidgets>
 
second::second(QWidget *parent) : QWidget(parent)
{
    qApp->installEventFilter(this);
}
 
second::~second(){
 
}
 
bool second::eventFilter(QObject *watched, QEvent *event){
    if (event->type() == QEvent::MouseButtonRelease) {
 
        cc++;
        qDebug() << cc;
 
        return false;
    }else{
        return false;
    }
}

#include "second.h"
#include <QtWidgets>

second::second(QWidget *parent) : QWidget(parent)
{
    qApp->installEventFilter(this);
}

second::~second(){

}

bool second::eventFilter(QObject *watched, QEvent *event){
    if (event->type() == QEvent::MouseButtonRelease) {

        cc++;
        qDebug() << cc;

        return false;
    }else{
        return false;
    }
}

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[high_flyer]

print out the address of the 'watched' object, see if both events belong to the same object.

[Oleg21]

Hello. I already did it. But I have determined that events belong to different objects.
After this code:

cc++;
qDebug() << cc;

cc++;
qDebug() << cc;

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I added the following code:

qDebug() << watched->metaObject()->className();

qDebug() << watched->metaObject()->className();

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Received a result:
Forum_question_2_1.png

Therefore, in eventFilter, I added the following kind of verification:

QString tempStr = "QWidgetWindow";
if (watched->metaObject()->className() == tempStr){
// code here
}

QString tempStr = "QWidgetWindow";
if (watched->metaObject()->className() == tempStr){
// code here
}

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In this case, the issue is resolved. But I do not know if it is correct to do so?

[high_flyer]

But I do not know if it is correct to do so?

Depends what your definition of "correct" is.
I would suggest however to not install the event filter on qApp, but on the actual object which of whom you want to filter events, in this case you 'first' object.
This will save you the need to "filter" your objects based on class name which is not optimal (what if you have several objects of the same class?)

[Oleg21]

(what if you have several objects of the same class?)

For each object, I can say a unique ObjectName which allows me to know from which object I get the events.
For example:

QApplication a(argc, argv);
first w;
w.setObjectName("Lollipop");
w.show();

QApplication a(argc, argv);
first w;
w.setObjectName("Lollipop");
w.show();

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In the eventFilter:

bool second::eventFilter(QObject *watched, QEvent *event){
  QString tempStr = "LollipopWindow";
  if(watched->objectName() == tempStr){
    //Code here
  }
}

bool second::eventFilter(QObject *watched, QEvent *event){
  QString tempStr = "LollipopWindow";
  if(watched->objectName() == tempStr){
    //Code here
  }
}

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However, it should be remembered that to the ObjectName is added string "Window". Why this happens, I do not know.

I would suggest however to not install the event filter on qApp, but on the actual object which of whom you want to filter events, in this case you 'first' object.

The following code:

qDebug() << watched->metaObject()->className();

qDebug() << watched->metaObject()->className();

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returns the following string: "QWidgetWindow", after him "first", after - "second". Why on the "first" should I set the event filter?
Perhaps "QWidgetWindow" is the very first class? And that it should be set to filter events?
But, I do not find a class with that name.
Forum_question_2_2.png
What is this class, "QWidgetWindow"? How to find it? Where can I find out about it?

[high_flyer]

For each object, I can say a unique ObjectName which allows me to know from which object I get the events.

Sure you *can* do it, but this is hardly good practice, it makes the code very rigid and you are ignoring the fact that when you have the pointer 'watched' it already points to a known (to you) object, so why bother with setting and querying strings when you can simply compare addresses (when you insist to filter on the application level)?
It would more generic, and less rigid and robust to changes you might have later in your code.

I could not understand the text below the qDebug() snippet, sorry.

[Oleg21]

I could not understand the text below the qDebug() snippet, sorry.

I apologize for the wrong text. I wanted to say that qDebug () displays the following values:
Forum_question_2_3.png
I am interested in the QWidgetWindow value, which is marked in orange.
To which class is this name? Can I install event filter for an object of this class?

Again I apologize for the incomprehensible text.