What are list-feeds?

A list-feed (IListFeed) is like a feed which is stateless and keeps no track of changes, but is specialized for handling collections.

Unlike a feed, where each asynchronous operation, returns one single item (or a series of single items in the case of an IAsyncEnumerable), a list-feed returns a collection of items.

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.

• The list-feed is uses the key equality to track multiple versions of the same entity within different messages of the list-feed. Read more about key equality.

How to create a list feed

To create an IListFeed<T>, use the static class ListFeed to call one of the same Async, AsyncEnumerable, and Create methods as in feed. The only difference is that it expects the Task or IAsyncEnumerable to return an IImmutableList<T> instead of T. A list-feed can be created in several ways.

Async

Using the Async factory method, the service returns a list of names on load/refresh - using a pull technique. The list-feed is then created with the async call.

Service code:

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

Model code:

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

AsyncEnumerable

In this way, a list-feed is created with an Async Enumerable method that returns an immutable list of names when available - using push technique.

Service code:  

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

Model code:

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

Pull and push are explained more in the [feeds page](xref:Overview.Mvux.Feeds#creation-of-feeds).

Convert from feed of an item-collection

A list-feed can also be created from a feed when the feed exposes a collection (IFeed<IImmutableCollection<T>>):

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

Support for selection and pagination

MVUX also provides built-in support for selection and pagination.
See more information on Selection or Pagination.

Operators

As mentioned, unlike a Feed<List<T>> operators on a list-feed are directly interacting with the collection's items instead of the list itself.

Where

This operator allows the filtering of the items.

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

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

AsFeed

This operator enables converting an IListFeed<T> to IFeed<IImmutableList<T>>:

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