Request: Header and Footer offsets for ListView

[drew-512]

Hi friends,

With the power and worldspace-ness of ListViews, one wants a semi-transparent header or footer floating over (above) a ListView. For this, one would need to specify header and footer offsets that specify blank space along the layout axis. Basically, the equivalent would be adding faux DataSource elements at 0 and N-1 that map to blank cell of a specified length along the layout axis.

This is currently possible by using faux list items but that makes life painful on our side because that has to get baked into the DataSource plus a custom prefab and bindings (makes for terrible code to maintain). Forgive me if I'm wrong, but I suspect it's a small and clean add that on the Nova side: a header and footer length added to the total virtual list length plus a single offset value with a "dirty propagation" pattern identical to changing any other UIBlock2D layout property.

I know y'all have a ton on your plate, but if you could squeeze this lil guy in that would be huge. Happy to talk bounty if that's available. This and a OnRectTransformChanged() analogue would encourage me up to put a repo live that contains the UIBlock-friendly blur as well as some other utils that I was planning to gift to Nova and the community. I was planning to do it either way, I'm just swamped myself and would love to feel like low hanging fruit is being picked where possible -- I know how tough and tricky the main features are to design and develop! Meanwhile, since we have to make URP utils for UIBlock over here, then why not share and let everyone make use of that.

[adgray1]

Just so we understand correctly, is the header/footer being scrollable within the ListView a requirement of what you're describing? Or could you position a fixed header/footer outside the ListView, e.g. as a sibling of the ListView?

If it must be scrollable, one option to avoid baking header/footer elements into your actual List<T> datasource, would be wrapping a List<T> in some other class which implements the IList<T> interface

E.g.

/// <summary>
/// Wrapper around a List but with an additional element for a Header/Footer.
/// </summary>
/// <typeparam name="TData">The type of data in the List.</typeparam>
public class HeaderFooterList<TData> : IList<TData>
{
    public TData Header;
    public TData Footer;
    public List<TData> Data;

    /// <summary>
    /// Wrapper around <see cref="Data"/>. 
    /// </summary>
    /// <param name="index"></param>
    /// <returns>
    /// If <c><paramref name="index"/> == 0</c>, returns <see cref="Header"/>.
    /// If <c><paramref name="index"/> == <see cref="Count"/> - 1</c>, returns <see cref="Footer"/>.
    /// Otherwise returns <c><paramref name="index"/> - 1</c> into <see cref="Data"/>.
    /// </returns>
    public TData this[int index]
    {
        get
        {
            return index == 0 ? Header :
                   index == Count - 1 ? Footer :
                   Data[index - 1];
        }
        set
        {
            if (index == 0)
            {
                Header = value;
            }
            else if (index == Count - 1)
            {
                Footer = value;
            }
            else
            {
                Data[index - 1] = value;
            }
        }
    }

    /// <summary>
    /// Returns size of <see cref="Data"/> + 2, for <see cref="Header"/> and <see cref="Footer"/>.
    /// </summary>
    public int Count => Data.Count + 2;
   
   // implement rest of IList<TData> interface as desired 
}

And then you can bind that to the ListView, instead of the raw List<T>. ListView intentionally supports IList<T>, as opposed to List<T>, to be more flexible to user requirements. Just an option to consider if it works for your project!

[drew-512]

So to recap, internally, two length values would just sum into the total virtual aggregate distance and all items are offset downward by the header length.

[adgray1]

Got it. Appreciate the clarification!

[drew-512]

In some frameworks, they use the phrase "section" headers and footers bc a section is defined by having its own header and footer. That's overkill and overly complicating imo. My request is two scalars (header and footer lengths that default to 0) or sockets for a header and footer. The latter one I assumed to be too much of an ask, is more work for y'all, and is only marginally more useful since only having the former makes the latter easily possible from the Nova user side.

[drew-512]

Thanks again for the consideration Adam. Generous of you to follow up so closely.

[drew-512]

Fun fact on the section header/footer overkill I mentioned...

In the earliest versions of iOS the Address Book (Contacts) app needed a single list view to display a list of all contacts (list elements), but you also needed spacing and label cells for each new letter of the alphabet. Beyond that, section support is almost never used!