C++, emit signal from custom eventfilter failed

xkungfu

I have a question about qt signal/slot and custom eventfilter.
usually, the default eventfilter is looks like "xxx->installEventFilter(this)".
I create a new class about custom eventfilter. It works well.
but, when I want emit a signal from it, it failed, signal/slot not work.

bellow is some main codes about this:

PlotsView.cpp

void PlotsView::createPlot()
{
         std::map<std::string, std::string> param_map =
         {
             {"tickerstr", 1234}
         };

         QHBoxLayout *candleplotlay = new QHBoxLayout();

         PlotCreatorDF  customcandle;
         QCustomPlot *candleplot;
         candleplot = customcandle.setupCandlestick(param_map);

         QObject::connect(&customcandle, SIGNAL(atesty(int)), this, SLOT(atestyy(int)));

         plotcontainerlay->addWidget(candleplot);

}


void PlotsView::atestyy(int x)
{
    qDebug() << "[CandleNewSecurity_List>>atestyy] x: " << x;
}

PlotCreatorDF.h


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

    public slots:
        void atestz(int x);

    signals:
        void atesty(int x);

};

PlotCreatorDF.cpp

    #include "PlotCreatorDF.h"
    #include "qcustomplot.h"
    #include "Plot/ploteventfilter.h"
    
    PlotCreatorDF::PlotCreatorDF(QWidget *parent) : QWidget(parent)
    {
    }
    
    PlotCreatorDF::~PlotCreatorDF()
    {
        qDebug()<<__LINE__<<__FUNCTION__;
    }
    
    
    QCustomPlot* PlotCreatorDF::setupCandlestick(std::map<std::string, std::string> param_map)
    {
    
        QCustomPlot *daily_plot = new QCustomPlot();
        daily_plot->setObjectName("test");
    
        //:
        PlotEventFilter *customfilter = new PlotEventFilter();
        daily_plot->installEventFilter(customfilter);
    
    
        connect(customfilter, SIGNAL(atestx(int)), this, SIGNAL(atesty(int)));
        connect(customfilter, SIGNAL(atestx(int)), this, SLOT(atestz(int)));
    
        return daily_plot;
    
    }
    
    void PlotCreatorDF::atestz(int x)
    {
        qDebug() << "[PlotCreatorDF>>atestz] x: " << "x";  //
    }

customeventfilter.h

    #ifndef CUSTOMENTFILTER_H
    #define CUSTOMENTFILTER_H
    #include <QWidget>
    
    namespace Ui {
        class CustomEventFilter;
    }
    
    class CustomEventFilter : public QWidget
    {
        Q_OBJECT
        public:
            explicit CustomEventFilter(QWidget *parent = nullptr);
            ~CustomEventFilter();
    
            bool eventFilter(QObject *obj, QEvent *qevt);
        signals:
            void atestx(int x);
    
        private:
    };
    
    #endif // PLOTEVENTFILTER_H

customeventfilter.cpp

    #include "customeventfilter.h"
    CustomEventFilter::CustomEventFilter(QWidget *parent) : QWidget(parent){}

    CustomEventFilter::~CustomEventFilter()
    {
        qDebug()<<__LINE__<<__FUNCTION__;
    }
    bool CustomEventFilter::eventFilter(QObject *obj, QEvent *qevt){
    
        if (qevt->type() == QEvent::MouseButtonPress) { //MouseButtonPress
            qDebug() << "[CustomEventFilter>>eventFilter] mouse clicked ... ";
            emit atestx(123);
        }
        return false;
    }

The connection seems failed to create the connection, and the slot function atestz not work.

Can anyone help me, why and how, thanks!

I have to say all the connections in the code not work.

It seems that any signal can't be sent from "CustomEventFilter".

JonB

@xkungfu
Your problem is not a custom event filter and emitting of signals. That will work. Your code is faulty.

You need to understand object lifetimes and local variables. Your PlotCreatorDF customcandle is a local variable. You use that as the this in your connect()s But as soon as method PlotsView::createPlot() exits the PlotCreatorDF customcandle is destroyed. Its slots cannot be called, so you don't see the signal (correctly emitted) any longer. This is where your problem lies.

Qt is clever enough to see when customcandle goes out of scope and gets destroyed. It disconnects any signals connected to its slots, so your connect()s get removed. This is why the slots don't get called but you don't get an error.

Change PlotCreatorDF customcandle over to PlotCreatorDF *customcandle = new PlotCreatorDF() and your code will work/signals fired/slots called. Of course you then have to think about when it should now get destroyed, to avoid a leak.

You already do this in PlotEventFilter *customfilter = new PlotEventFilter();, and likely that leaks too?

JonB

@xkungfu said in C++, emit signal from custom eventfilter failed:

{
    PlotCreatorDF  customcandle;
    ...
    QObject::connect(&customcandle, SIGNAL(atesty(int)), this, SLOT(atestyy(int)));
}

If this is supposed to work (fire signal to hit slot) for whatever you are asking about, it won't. customcandle goes out of scope at } so it will never emit any signals after that. Is this relevant to what you see?

Since no PlotCreatorDF instance persists, the this in PlotCreatorDF::setupCandlestick()

        connect(customfilter, SIGNAL(atestx(int)), this, SIGNAL(atesty(int)));
        connect(customfilter, SIGNAL(atestx(int)), this, SLOT(atestz(int)));

means these slots get removed from being connected.

xkungfu

I can't understand which } effect the connect. sorry, I am trying to understanding what you said.
I think you found the problem correctly. do you mean the "return daily_plot" throw the connections away?
so, how can I emit a signal from my custom eventfileter?

JonB

@xkungfu
Your problem is not a custom event filter and emitting of signals. That will work. Your code is faulty.

You need to understand object lifetimes and local variables. Your PlotCreatorDF customcandle is a local variable. You use that as the this in your connect()s But as soon as method PlotsView::createPlot() exits the PlotCreatorDF customcandle is destroyed. Its slots cannot be called, so you don't see the signal (correctly emitted) any longer. This is where your problem lies.

Qt is clever enough to see when customcandle goes out of scope and gets destroyed. It disconnects any signals connected to its slots, so your connect()s get removed. This is why the slots don't get called but you don't get an error.

Change PlotCreatorDF customcandle over to PlotCreatorDF *customcandle = new PlotCreatorDF() and your code will work/signals fired/slots called. Of course you then have to think about when it should now get destroyed, to avoid a leak.

You already do this in PlotEventFilter *customfilter = new PlotEventFilter();, and likely that leaks too?

xkungfu

@JonB
thank you very much!
without you answer not only I can't understand lifetimes , but also I would never to think about memory.