ListFeed

The IListFeed<T> is feed specialized for handling collections. It allows the declaration of an operator directly on items instead of dealing with the list itself. Unlike a feed, where each asynchronous operation returns one single item (or a series of single items in the case of an IAsyncEnumerable), a ListFeed returns a collection of items. A list feed goes in None if the list does not have any elements.

Note

The ListFeed uses the key equality to track multiple versions of the same entity within different messages of the ListFeed. Read more about key equality.

A couple of points to note about list-feeds:

• Operators are applied to the items within the returned collection, rather than on the entire collection.

• When an empty collection is returned by the service, it's treated as an Empty message. The returned data axis Option will be None, even though the result was not null. This is because when a control of data items is displayed with an empty collection (for instance a ListView), there is no reason to display the FeedView's ValueTemplate with an empty ListView. The "No data records" NoneTemplate makes much more sense in this case. For that reason, both a null result and an empty collection are regarded as None.

Sources: How to create a list feed

To create an IListFeed<T>, use the static class ListFeed to call one of the following methods:

Async: Creates a ListFeed using a method that returns a Task<IImmutableList<T>>.

public ValueTask<IImutableList<string>> GetNames(CancellationToken ct = default);

For example:

public IListFeed<string> Names => ListFeed.Async(service.GetNames);

AsyncEnumerable: Creates a ListFeed using an IAsyncEnumerable<IImmutableList<T>>.

public IAsyncEnumerable<IImutableList<string>> GetNames(
    [EnumeratorCancellation] CancellationToken ct = default);

For example:

public IListFeed<string> Names => ListFeed.AsyncEnumerable(service.GetNames);

Pull and push are explained more in the feeds page.

Create: Provides custom initialization for a ListFeed.

public static IListFeed<T> Create<T>(Func<CancellationToken, IAsyncEnumerable<Message<IImmutableList<T>>>> sourceProvider);

There are also 2 helpers that allow you to convert from/to a feed to/from a list feed.

Operators: How to interact with a list feed

Unlike a Feed<List<T>> operators on a list feed are directly interacting with items instead of the list itself.

PaginatedAsync

This allows the creation of a feed of a paginated list. The pagination can be made by cursor (cf. ListFeed<T>.AsyncPaginatedByCursor), or using a simple page index with ListFeed.AsyncPaginated. Used among the generated view models and a ListView, when the user scroll and reach the end of the list, a PageRequest will be sent to the ListFeed, which will trigger the load of the next page using the delegate that you provided.

public static IListFeed<T> PaginatedAsync<T>(Func<PageRequest, CancellationToken, Task<IImmutableList<T>>> getPage);

For example:

public IListFeed<City> Cities => ListFeed.AsyncPaginated(async (page, ct) => _service.GetCities(pageIndex: page.Index, perPage: 20));

Caution

On the Page struct you have a DesiredSize property. This is the number of items the view is requesting to properly fill its "viewport", BUT there is no garantee that this value remains the same between multi pages, espcially if the user resize the app. As a consequency, it must not be used with Index for a "skip/take" pattern like source.Skip(page.Index * page.DesiredSize).Take(page.DesiredSize). For such patterns, you can either just hard-code your page size (e.g. source.Skip(page.Index * 20).Take(20), either use the page.TotalCount property (e.g. source.Skip(page.TotalCount).Take(page.DesiredSize)).

Where

This operator allows the filtering of items.

Warning

If all items of the collection are filtered out, the resulting feed will go in None state.

public static IListFeed<TSource> Where<TSource>(this IListFeed<TSource> source, Predicate<TSource> predicate);

For example:

public IListFeed<string> LongNames => Names.Where(name => name.Length >= 10);

AsFeed

This does the opposite of AsListFeed and converts a list feed to a feed of list.

public static IFeed<IImmutableCollection<T>> AsFeed<T>(this IListFeed<T> source);

For example:

public void SetUp()
{
    IListFeed<string> stringsListFeed = ...;
    IFeed<IImmutableCollection<string>> stringsFeed = stringsListFeed.AsFeed();
}

AsListFeed

A ListFeed can also be created from a Feed when the Feed exposes a collection (IFeed<IImmutableCollection<T>>):

public static IListFeed<T> AsListFeed<T>(this IFeed<IImmutableCollection<T>> source);

For example:

public IListFeed<WeatherInfo> Forecast => Feed
    .Async(async ct => new []
    {
        await _weatherService.GetWeatherForecast(DateTime.Today.AddDays(1), ct),
        await _weatherService.GetWeatherForecast(DateTime.Today.AddDays(2), ct),
    })
    .Select(list => list.ToImmutableList())
    .AsListFeed();