List placeholder

equeim

So, I wanted to implemented a placeholder text for a list (QListView) which would be displayed if the list has no items.

Currently I'm doing it using QStackedLayout but it requires quite a bit of boilerplate. You need to create a wrapper QWidget that will hold QStackedLayout, and also another wrapper QWidget for placeholder text so it can be centered:

        auto stackedLayoutWrapper = new QWidget(parent);
        auto stackedLayout = new QStackedLayout(stackedLayoutWrapper);

        stackedLayout->addWidget(listView);

        auto placeholderWrapper = new QWidget();
        auto placeholderLayout = new QVBoxLayout(placeholderWrapper);
        auto placeholder = new QLabel("No items");
        placeholderLayout->addItem(placeholder, 0, Qt::AlignCenter);
        stackedLayout->addWidget(placeholderWrapper);

And then I discovered another neat way to do it, using QListView::viewport():

        auto placeholderLayout = new QVBoxLayout(listView->viewport());
        auto placeholder = new QLabel("No items");
        placeholderLayout->addWidget(placeholder, 0, Qt::AlignCenter);

This works right now, but question is, how safe is it set a layout on the viewport() like this?

JonB

@equeim
Would QStackedWidget instead of QStackedLayout reduce how many things you have to create for the first approach?

If your simpler viewport() code works I cannot see why you should not use it.

equeim

@JonB Right, I forgot that QStackedWidgets exists too.

My problem with viewport() is that it feels like an implementation detail of QListView, so I'm afraid to break in some edge cases or on different platforms.

Chris Kawa

@equeim viewport() is a documented API, so it's not an implementation detail, but you're doing a lot of work that is not really needed.
You can simply subclass the listview and draw the text there, e.g.

void MyListView::paintEvent(QPaintEvent* evt)
{
    QListView::paintEvent(evt);

    if (!model() || model()->rowCount() == 0)
    {
        QPainter p(viewport());
        p.drawText(rect(), Qt::AlignCenter, "No items");
    }
}