Qt: What to emit when inserting items into my model to hint to the view to recheck canFetchMore?

Question

I have a custom model (extends QAbstractTableModel) where row data is added pragmatically. For performance reasons, I am using the fetch functionality (canFetchMore, fetchMore) to avoid UI lag when items are not visible.

When a new row is inserted, I do not want to trigger QAbstractItemModel::rowsInserted. But if those rows would be visible in the view, I'd like them to appear automatically. They do appear if the user interacts with the view, e.g. selecting the last item (the view calls 'canFetchMore' and will call fetchMore only if those items would be visible).

How do I signal to the view that more rows are available to display, but need to be fetched?

Answer 1

Solution for Qt 5.6.1 - untested for other versions!

After digging around in the sources of QAbstractItemView, I found the simplest way to trigger fetchMore is to start the QAbstractItemViewPrivate's internal and undocumented fetchMoreTimer.

This is heavily implementation dependent and may change in future Qt versions!

Subclass your QAbstractItemView derivate (e.g. QListView, ...) to get access to one of the protected functions starting the timer:

class CFetchMoreListView : public QListView
{
    Q_OBJECT

public:
    explicit CFetchMoreListView(QWidget *parent = Q_NULLPTR)
        : QListView(parent)
    { }

    inline void CheckFetchMore()
    {
        int mode = 0;
        switch(mode)
        {
        case 0: // works - but I didn't check how much unneccessary updating it performs
            updateGeometries();
            break;

        case 1: // updates the view allright, but also loads items not currently in view
            verticalScrollbarValueChanged(verticalScrollBar()->maximum());
            break;

        case 2: // needs at least one item already inserted
            if(currentIndex().isValid())
            {
                QModelIndex const curr = currentIndex();
                currentChanged(curr, curr);
            }
            break;

        case 3: // leads to flicker
            setVisible(false);
            rowsInserted(QModelIndex(), 0, 0);
            setVisible(true);
            break;
        }
    }
};

Now, after adding items to your model, you can call view->CheckFetchMore();

Edit

It might be possible to override rowsInserted(...) and only call the base implementation if the newly added rows would be visible.
But that seems kludgy as well.

void QListView::rowsInserted(const QModelIndex &parent, int start, int end)
{
    Q_D(QListView);
    // ### be smarter about inserted items
    d->clear();
    d->doDelayedItemsLayout();
    QAbstractItemView::rowsInserted(parent, start, end);
}

(I love how the comment in the Qt code pinpoints your problem...)