无法在 QComboBox 上安装 EventFilter

Question

I'm trying to install eventFilter on QComboBox but the function isn't getting triggered

Header:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    bool eventFilter(QObject *obj, QEvent *event);

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

Source:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    ui->comboBox->installEventFilter(ui->comboBox);
}

MainWindow::~MainWindow()
{
    delete ui;
}

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
    QComboBox* combo = qobject_cast<QComboBox*>(obj);
    if(combo)
    {
        if(event->type() == QEvent::Wheel)
        {
            if(combo->focusPolicy() == Qt::WheelFocus)
            {
                event->accept();
                return false;
            }
            else
            {
                event->ignore();
                return true;
            }
        }
        else if(event->type() == QEvent::FocusIn)
        {
            combo->setFocusPolicy(Qt::WheelFocus);
        }
        else if(event->type() == QEvent::FocusOut)
        {
            combo->setFocusPolicy(Qt::StrongFocus);
        }
    }
    return QObject::eventFilter(obj, event);
}

UI:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <widget class="QComboBox" name="comboBox">
    <property name="geometry">
     <rect>
      <x>220</x>
      <y>240</y>
      <width>241</width>
      <height>22</height>
     </rect>
    </property>
    <item>
     <property name="text">
      <string>A</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>B</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>C</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>D</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>E</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>F</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>G</string>
     </property>
    </item>
    <item>
     <property name="text">
      <string>H</string>
     </property>
    </item>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

Answer 1

You need to pass this operator to installEventFilter

ui->comboBox->installEventFilter(this);

and use virtual and override keyword for eventFilter function

   virtual bool eventFilter(...) override;