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Thread: eventFilter anywhere in the program.

  1. #1
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    Default eventFilter anywhere in the program.

    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:

    Qt Code:
    1. second.cpp
    2. #include "second.h"
    3. #include <QtWidgets>
    4.  
    5. second::second(QWidget *parent) : QWidget(parent)
    6. {
    7. qApp->installEventFilter(parent);
    8. }
    9.  
    10. second::~second(){
    11.  
    12. }
    13.  
    14. bool second::eventFilter(QObject *watched, QEvent *event){
    15. //it's not working!
    16. }
    To copy to clipboard, switch view to plain text mode 

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

    Qt Code:
    1. #include "first.h"
    2. #include "second.h"
    3.  
    4. first::first(QWidget *parent) : QMainWindow(parent)
    5. {
    6. second *tempSecond = new second(this);
    7. }
    8.  
    9. first::~first()
    10. {
    11.  
    12. }
    13.  
    14. bool first::eventFilter(QObject *watched, QEvent *event){
    15. //it's working!
    16. }
    To copy to clipboard, switch view to plain text mode 

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

  2. #2
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    Default Re: eventFilter anywhere in the program.

    Qt Code:
    1. //qApp->installEventFilter(parent);
    2. qApp->installEventFilter(this);
    To copy to clipboard, switch view to plain text mode 
    When you know how to do it then you may do it wrong.
    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.

  3. #3
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    Default Re: eventFilter anywhere in the program.

    Hello. Truly, I had to do as you say:
    Qt Code:
    1. qApp->installEventFilter(this);
    To copy to clipboard, switch view to plain text mode 

    And implementation of eventFilter:
    Qt Code:
    1. bool second::eventFilter(QObject *watched, QEvent *event){
    2. if (event->type() == QEvent::MouseButtonRelease) {
    3.  
    4. cc++;
    5. qDebug() << cc;
    6.  
    7. return false;
    8. }else{
    9. return false;
    10. }
    11. }
    To copy to clipboard, switch view to plain text mode 

    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?
    Last edited by Oleg21; 12th October 2017 at 19:41.

  4. #4
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    Default Re: eventFilter anywhere in the program.

    Quote Originally Posted by Oleg21 View Post
    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.
    When you know how to do it then you may do it wrong.
    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.

  5. #5
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    Default Re: eventFilter anywhere in the program.

    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
    Qt Code:
    1. #ifndef FIRST_H
    2. #define FIRST_H
    3.  
    4. #include <QMainWindow>
    5.  
    6. class first : public QMainWindow
    7. {
    8. Q_OBJECT
    9.  
    10. public:
    11. first(QWidget *parent = 0);
    12. ~first();
    13. };
    14.  
    15. #endif // FIRST_H
    To copy to clipboard, switch view to plain text mode 
    second.h
    Qt Code:
    1. #ifndef SECOND_H
    2. #define SECOND_H
    3.  
    4. #include <QWidget>
    5.  
    6. class second : public QWidget
    7. {
    8. Q_OBJECT
    9. public:
    10. explicit second(QWidget *parent = nullptr);
    11. ~second();
    12.  
    13. int cc = 0;
    14.  
    15. virtual bool eventFilter(QObject *watched, QEvent *event);
    16. signals:
    17.  
    18. public slots:
    19. };
    20.  
    21. #endif // SECOND_H
    To copy to clipboard, switch view to plain text mode 
    Sources:
    main.cpp
    Qt Code:
    1. #include "first.h"
    2. #include <QApplication>
    3.  
    4. int main(int argc, char *argv[])
    5. {
    6. QApplication a(argc, argv);
    7. first w;
    8. w.show();
    9.  
    10. return a.exec();
    11. }
    To copy to clipboard, switch view to plain text mode 
    first.cpp
    Qt Code:
    1. #include "first.h"
    2. #include "second.h"
    3. #include <QtWidgets>
    4.  
    5. first::first(QWidget *parent) : QMainWindow(parent)
    6. {
    7. second *second1 = new second(this);
    8. }
    9.  
    10. first::~first()
    11. {
    12.  
    13. }
    To copy to clipboard, switch view to plain text mode 
    second.cpp
    Qt Code:
    1. #include "second.h"
    2. #include <QtWidgets>
    3.  
    4. second::second(QWidget *parent) : QWidget(parent)
    5. {
    6. qApp->installEventFilter(this);
    7. }
    8.  
    9. second::~second(){
    10.  
    11. }
    12.  
    13. bool second::eventFilter(QObject *watched, QEvent *event){
    14. if (event->type() == QEvent::MouseButtonRelease) {
    15.  
    16. cc++;
    17. qDebug() << cc;
    18.  
    19. return false;
    20. }else{
    21. return false;
    22. }
    23. }
    To copy to clipboard, switch view to plain text mode 

  6. #6
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    Default Re: eventFilter anywhere in the program.

    print out the address of the 'watched' object, see if both events belong to the same object.
    ==========================signature=============== ==================
    S.O.L.I.D principles (use them!):
    https://en.wikipedia.org/wiki/SOLID_...iented_design)

    Do you write clean code? - if you are TDD'ing then maybe, if not, your not writing clean code.
    Watch these videos, and read his books they are must if you want to write clean code:
    https://sites.google.com/site/unclebobconsultingllc/

    Recommended off-line reading Qt:
    The Foundations of Qt Development (ISBN: 1-59059-831-8)

  7. #7
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    Default Re: eventFilter anywhere in the program.

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

    Qt Code:
    1. cc++;
    2. qDebug() << cc;
    To copy to clipboard, switch view to plain text mode 
    I added the following code:
    Qt Code:
    1. qDebug() << watched->metaObject()->className();
    To copy to clipboard, switch view to plain text mode 

    Received a result:
    Forum_question_2_1.png

    Therefore, in eventFilter, I added the following kind of verification:
    Qt Code:
    1. QString tempStr = "QWidgetWindow";
    2. if (watched->metaObject()->className() == tempStr){
    3. // code here
    4. }
    To copy to clipboard, switch view to plain text mode 

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

  8. #8
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    Default Re: eventFilter anywhere in the program.

    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?)
    ==========================signature=============== ==================
    S.O.L.I.D principles (use them!):
    https://en.wikipedia.org/wiki/SOLID_...iented_design)

    Do you write clean code? - if you are TDD'ing then maybe, if not, your not writing clean code.
    Watch these videos, and read his books they are must if you want to write clean code:
    https://sites.google.com/site/unclebobconsultingllc/

    Recommended off-line reading Qt:
    The Foundations of Qt Development (ISBN: 1-59059-831-8)

  9. #9
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    Default Re: eventFilter anywhere in the program.

    Quote Originally Posted by high_flyer View Post
    (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:
    Qt Code:
    1. QApplication a(argc, argv);
    2. first w;
    3. w.setObjectName("Lollipop");
    4. w.show();
    To copy to clipboard, switch view to plain text mode 
    In the eventFilter:
    Qt Code:
    1. bool second::eventFilter(QObject *watched, QEvent *event){
    2. QString tempStr = "LollipopWindow";
    3. if(watched->objectName() == tempStr){
    4. //Code here
    5. }
    6. }
    To copy to clipboard, switch view to plain text mode 
    However, it should be remembered that to the ObjectName is added string "Window". Why this happens, I do not know.

    Quote Originally Posted by high_flyer View Post
    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:
    Qt Code:
    1. qDebug() << watched->metaObject()->className();
    To copy to clipboard, switch view to plain text mode 
    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?

  10. #10
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    Default Re: eventFilter anywhere in the program.

    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.
    ==========================signature=============== ==================
    S.O.L.I.D principles (use them!):
    https://en.wikipedia.org/wiki/SOLID_...iented_design)

    Do you write clean code? - if you are TDD'ing then maybe, if not, your not writing clean code.
    Watch these videos, and read his books they are must if you want to write clean code:
    https://sites.google.com/site/unclebobconsultingllc/

    Recommended off-line reading Qt:
    The Foundations of Qt Development (ISBN: 1-59059-831-8)

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