QListView max number of items in view - qt

Question

I need to calculate max number of items in current view of QListView.
I wrote code like this:
void MyListView::resizeEvent(QResizeEvent *event)
{
QListView::resizeEvent(event);
QFontMetrics fm (this->font());
int fontHeight = fm.lineSpacing();
QRect cr = contentsRect();
int windowHeight = cr.bottom() - cr.top();
int maxItemsCount = windowHeight / fontHeight;
qDebug()<<"max items in view: "<< maxItemsCount;
}
but calculated max number of items is is incorrect.
E.g. in case of my window height and font height I get 32 max items in view when in fact current view has 28 items. Perhaps someone can suggest something, how to calculate it properly?

Answer 1

My idea is to use QListView::indexAt() (inherited from QAbstractView) to obtain the row index for
the top-left corner
the bottom-left corner
of the list view viewport and determining the number of visible items by difference of them.
To check this out, I made an MCVE testQListViewNumVisibleItems.cc:
// Qt header:
#include <QtWidgets>
class ListWidget: public QListWidget {
public:
ListWidget(QWidget *pQParent = nullptr): QListWidget(pQParent) { }
virtual ~ListWidget() = default;
ListWidget(const ListWidget&) = delete;
ListWidget& operator=(const ListWidget&) = delete;
int getNumVisibleItems() const
{
const QSize size = viewport()->size();
const QModelIndex qMIdx0 = indexAt(QPoint(0, 0));
const QModelIndex qMIdx1 = indexAt(QPoint(0, size.height() - 1));
//qDebug() << "qMIdx0:" << qMIdx0 << "qMIdx1:" << qMIdx1;
return qMIdx0.isValid()
? (qMIdx1.isValid() ? qMIdx1.row() : count()) - qMIdx0.row()
: 0;
}
};
const int MaxItems = 20;
// main application
int main(int argc, char **argv)
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
// setup GUI
ListWidget qLst;
qLst.resize(200, 200);
qLst.show();
// timer to populate list view
using namespace std::chrono_literals;
QTimer qTimer;
qTimer.setInterval(1000ms);
// install signal handlers
int n = 0;
QObject::connect(&qTimer, &QTimer::timeout,
[&]() {
qLst.addItem(QString("item %0").arg(++n));
qDebug() << "Visible items:" << qLst.getNumVisibleItems();
if (n >= MaxItems) qTimer.stop();
});
// runtime loop
qTimer.start();
return app.exec();
}
and a CMakeLists.txt:
project(QListViewNumVisibleItems)
cmake_minimum_required(VERSION 3.10.0)
set_property(GLOBAL PROPERTY USE_FOLDERS ON)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
find_package(Qt5Widgets CONFIG REQUIRED)
include_directories("${CMAKE_SOURCE_DIR}")
add_executable(testQListViewNumVisibleItems testQListViewNumVisibleItems.cc)
target_link_libraries(testQListViewNumVisibleItems Qt5::Widgets)
built and tested in VS2017 on Windows 10:
After having implemented what came in my mind, I googled a bit to possibly see other approaches. (I admit I should've done before.)
Thereby I found the following possible duplicate:
SO: Simple way to get all visible items in the QListView
The accepted answer doesn't contain much more than the hint for indexAt and a link to a Qt-FAQ article:
How can i get hold of all of the visible items in my QListView?
In order to get hold of the visible items in a QListView http://doc.qt.io/qt-5/qlistview.html, then you can iterate over them using indexAt() http://doc.qt.io/qt-5/qlistview.html#indexAt. You can get hold of the first visible item using indexAt(QPoint(0, 0)), then in order to get the index at the next position then use visualRect() http://doc.qt.io/qt-5/qlistview.html#visualRect to find out what your next call to itemAt() should be. This position would be:
visualRect.y() + visualRect.height() + 1 effectively.
See the following example for an illustration:
#include <QtGui>
QList <QModelIndex>myList;
class ListView : public QListView
{
Q_OBJECT
public:
ListView()
{
QStringListModel *myModel = new QStringListModel(this);
QStringList list;
list << "a" << "b" <<"c" <<"d" <<"e" <<"f" <<"g" <<"h" <<"i" <<"j" <<"k";
myModel->setStringList(list);
setModel(myModel);
QTimer::singleShot(3000, this, SLOT(test1()));
}
public slots:
void test1()
{
QModelIndex firstIndex = indexAt(QPoint(0, 0));
if (firstIndex.isValid()) {
myList << firstIndex;
} while (viewport()->rect().contains(QPoint(0, visualRect(firstIndex).y() + visualRect(firstIndex).height() + 1 ))) {
firstIndex = indexAt(QPoint(0, visualRect(firstIndex).y() + visualRect(firstIndex).height() + 1 ));
myList << firstIndex;
}
qDebug() << myList.count() << "are visible";
}
};
#include "main.moc"
int main(int argc, char** argv)
{
QApplication app(argc, argv);
ListView window;
window.resize(100, 50);
window.show();
return app.exec();
}