Qt/C++ - Lesson 064. How to hide items from the QComboBox drop-down list

Qt, QComboBox, QListView

In some cases, it is required to hide some items from the QComboBox drop-down list. For example, the user is given the opportunity to select one of several options in QComboBox, but by default the invalid option should be displayed, which should be absent from the drop-down list. That is, by default, something wrong is selected, which, when validated, will be checked and will give the user a message that one should choose one of the possible options that are available, whereas the wrong variant will be absent from the list.

When you try to find information in search engines on this topic, you first come across options when trying to use a custom delegate inherited from QItemDelegate , or trying to play with flags that you can set to items in QComboBox , but both options do not work.

The first option with QItemDelegate will not work because even if you make sure that a certain element is not rendered, its location will nevertheless be in the list. Just this place will be an empty rectangle.

The second option with flags is not suitable because there is no such flag that would cause QComboBox to hide a certain element in the list, although the closest in value is the Qt::ItemIsEnabled flag, but if it is reset, then the text of the element becomes gray, indicating that Element can not be selected.

You can also consider the option with the data model and drawing the elements, but we again return to QItemDelegate , which again does not work.

What then?

Let's take a look at QComboBox from a different point of view.

What is it in the simplest version? - This is a button by clicking on which drop-down list appears.

What is a drop-down list? - This is a QFrame in which an object is inherited from QAbstractItemView . And to be precise, it's QListView .

In this case, QListView will contain a data model from which the currently selected item is substituted in QComboBox and the same data model serves to display a list of all the elements. And QListView itself can !ATTENTION! hide lines and do not delete them from the data model !!! And in this case, in QComboBox, the element that can be hidden from the drop-down list can be displayed as the current element. WIN!!!

Implementation

For example, we implement the following application. There is a combo box with five elements. There is LineEdit, in which you can write the number of the element that you can hide. And there is a button, by clicking which, we hide the element in the drop-down list. For verification, you can select an item and hide it from the list. Then you make sure that it will be displayed on QComboBox , but it will not be visible in the drop-down list. (Item numbers start with 0)

The application will look like this.

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QComboBox>
#include <QPushButton>
#include <QLineEdit>
#include <QListView>

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private slots:
    void hideRow();

private:
    QComboBox *comboBox;  
    QPushButton *hideButton; // Click on which we will hide the specified element
    QLineEdit *lineEdit;  // LineEdit, in which we will set the element to hide
    QListView *listView; // pointer to the list of elements
};

#endif // WIDGET_H

widget.cpp

#include "widget.h"

#include <QGridLayout>

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    comboBox(new QComboBox(this)),
    hideButton(new QPushButton("Hide", this)),
    lineEdit(new QLineEdit(this))
{
    QGridLayout *gridLayout = new QGridLayout(this);
    setLayout(gridLayout);
    gridLayout->addWidget(comboBox, 0, 0, 1, 2);
    gridLayout->addWidget(hideButton, 1, 0, 1, 1);
    gridLayout->addWidget(lineEdit, 1, 1, 1, 1);

    listView = new QListView(comboBox);
    comboBox->setView(listView);

    comboBox->addItem("0");
    comboBox->addItem("1");
    comboBox->addItem("2");
    comboBox->addItem("3");
    comboBox->addItem("4");

    connect(hideButton, &QPushButton::clicked, this, &Widget::hideRow);
}

Widget::~Widget()
{

}

void Widget::hideRow()
{
    listView->setRowHidden(lineEdit->text().toInt(), true);
}

Archive with project