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installEventFilter a bit differently



  • Hi, I was wondering how to process multiple events of one QObject without a need to subclass it and override some method. I know, there are generally 5 ways how to process events (described here), but in each one, I need to subclass either an object whose messages I want to process, or the object who is going to filter the events before they get to the receiver (eventFilter). Why is there no method like QObject::installEventFilter which takes a functor/function pointer (taking a pointer to a QEvent and returning bool)? Are there any problems that would arise from this type of event-handling? Thanks in advance.

    EDIT:
    I even add the code, so that there's no confusion

    // this would be added into the QObject class
    class QObject
    {
    public:
        void installEventFilter(std::function<bool(QEvent*)> functor);
    };
    
    // -------------------------------
    // USER'S CODE
    // this can be some global or static member function - doesn't really matter
    bool MyEventFilteringFunction(QEvent* ev)
    {
        if (ev->type() == QEvent::Paint)
        {
            // do something
            return true;
        }
    
        return false;
    }
    
    // somewhere in the code
    // no need to subclass and override button, nor it's parent dialog or anything
    QPushButton b;
    b.installEventFilter(&MyEventFilteringFunction);
    

  • Lifetime Qt Champion

    Hi,

    Why:

    • usually because nobody requested nor provided that feature
    • not feasible for technical reasons at that time
    • not possible to integrate it for some reasons

    All in all, you can open a feature request for that to get the discutions started. Even better if you can submit a patch that implements what you are asking for.



  • Thanks for your answer. All I'm asking is - would this be a sane thing to implement, or am I missing something obvious? If the rest of the event-system allowed such functionality, would it be able to e.g. replace the original installEventFilter method? Obviously, my version would need to have 1 additional parameter besides QEvent - that would be QObject representing the receiver of the event. I don't have any specific requirement, this is just an off-topic, I guess.


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