Well it seems that I've found a solution, so I'll share it here for sake of anyone who has the same problem and googles this thread.
First of all, I've found that this is actually a bug in Qt registered back in 2011 and still open:
https://bugreports.qt.io/browse/QTBUG-16592
I've added my vote to it, and then decided to try out using QTableView instead of QListView - and, surpise, I managed to make it work, or so it seems.
Unlike QListView, QTableView only resizes rows upon explicit request, by calling resizeRowToContents(rowNum). So the trick is to call it in a just-in-time fashion for rows that become visible in the viewport.
Here's what I did:
Inherit from QTableView (let's call it MyTableView)
Replace QListView with MyTableView and initialize it like this in the constructor. This assigns custom item delegate, hides table headers and applies "by row" selection mode:
MyTableView::MyTableView(QWidget* parent) : QTableView(parent)
{
setSelectionBehavior(QAbstractItemView::SelectRows);
horizontalHeader()->setStretchLastSection(true);
horizontalHeader()->hide();
verticalHeader()->hide();
setItemDelegateForColumn(0, new CustomDelegate(&table)); // for custom-drawn items
}
In MyTableView, add a QItemSelection private field and a public function that calculates real heights of rows, but only those that are currently visible:
QItemSelection _itemsWithKnownHeight; // private member of MyTableView
void MyTableView::updateVisibleRowHeights()
{
const QRect viewportRect = table.viewport()->rect();
QModelIndex topRowIndex = table.indexAt(QPoint(viewportRect.x() + 5, viewportRect.y() + 5));
QModelIndex bottomRowIndex = table.indexAt(QPoint(viewportRect.x() + 5, viewportRect.y() + viewportRect.height() - 5));
qDebug() << "top row: " << topRowIndex.row() << ", bottom row: " << bottomRowIndex.row();
for (auto i = topRowIndex.row() ; i < bottomRowIndex.row() + 1; ++i)
{
auto index = model()->index(i, 0);
if (!_itemsWithKnownHeights.contains(index))
{
resizeRowToContents(i);
_itemsWithKnownHeights.select(index, index);
qDebug() << "Marked row #" << i << " as resized";
}
}
}
Note: if item heights depend on control's width, you need to override resizeEvent(), clear _itemsWithKnownHeights, and call updateVisibleRowsHeight() again.
Call updateVisibleRowHeights() after assigning a model to MyTableView instance, so that initial view is correct:
table.setModel(&myModel);
table.updateVisibleRowHeights();
In fact it should be done in some MyTableView's method that reacts to model changes, but I'll leave it as an exercise.
Now all that's left is to have something call updateRowHeights whenever table's vertical scroll position changes. So we need to add the following to MyTableView's constructor:
connect(verticalScrollBar(), &QScrollBar::valueChanged, [this](int) {
updateRowHeights();
});
Done - it works really fast even with model of 100,000 items! And startup is instantenious!
A basic proof-of-concept example of this technique (using pure QTableView instead of subclass) can be found here:
https://github.com/ajenter/qt_hugelistview/tree/tableview-experiment
Warning: this technique is not battle proven yet and may contain some yet unknown issues. Use at own risk!
Hope this helps!
