What are list-states

List-state is the collection counterpart of state.

List-state adds extra operators which make it easier to apply updates on multiple items instead of just a single item.

Recall that feeds are stateless and are only read-only data output to the View, and are not responding to changes, whereas states are stateful, and update the Model and its entities upon changes made in the View using two-way data-binding, or on demand via Commands.

So an IState<T> is a stateful feed of a single item of T, whereas an IListState<T> is a stateful feed of multiple items of T.

How to create a list-state

The static ListState class provides factory methods for creating IListState<T> objects, here they are:

Empty

Creates an empty list-state:

IListState<string> MyStrings = ListState<string>.Empty(this);

Value

Creates a list-state with an initial synchronous value:

private readonly IImmutableList<string> _favorites =
    new string[]
    {
        "Terry Fox",
        "David Suzuki",
        "Margaret Atwood"
    }
    .ToImmutableArray();

public IListState<string> Favorites => ListState.Value(this, () => _favorites);

Async

Creates a list-state from an async method:

public ValueTask<IImmutableList<string>> GetStrings(CancellationToken ct) => new(_favorites);


public IListState<string> Favorites => ListState.Async(this, GetStrings);

AsyncEnumerable

public async IAsyncEnumerable<IImmutableList<string>> GetStrings([EnumeratorCancellation] CancellationToken ct)
{
    yield return _favorites;
}

public IListState<string> Favorites => ListState.AsyncEnumerable(this, GetStrings);

FromFeed

public IListFeed<string> FavoritesFeed => ...
public IListState<string> FavoritesState => ListState.FromFeed(this, FavoritesFeed);

Operators

In the following examples, we'll refer to MyStrings which is an IListState<string>, to demonstrate how to use the various operators IListState<T> provides to update its state with modified data.

Add

The AddAsync method adds an item to the end of the List State:

await MyStrings.AddAsync("Gord Downie", cancellationToken);

Insert

The InsertAsync method inserts an item to the beginning of the List State:

await MyStrings.InsertAsync("Margaret Atwood", cancellationToken);

Update

There are various ways to update values in the list-state:

The Update method has an updater parameter like the State does. This parameter is a Func<IImmutableList<T>, IImmutableList<T>>, which when called passes in the existing collection, allows you to apply your modifications to it, and then returns it.

For example:

public async ValueTask TrimAll(CancellationToken ct = default)
{
    await MyStrings.Update(
        updater: existing =>
            existing
            .Select(item =>
                item.Trim())
            .ToImmutableList(),
        ct: ct);
}

Another overload is UpdateAsync, which allows you to apply an update on items that match a criteria. The match predicate is checked for each item. It the item matches the criteria, the updater is invoked which returns a new instance of the item with the update applied:

public async ValueTask TrimLongNames(CancellationToken ct = default)
{
    await MyStrings.UpdateAsync(
        match: item => item?.Length > 10,
        updater: item => item.Trim(),
        ct: ct);
}

Remove

The RemoveAllAsync method uses a predicate to determine which items are to be removed:

await MyStrings.RemoveAllAsync(
    match: item => item.Contains("Ő"),
    ct: cancellationToken);

ForEach

This operator can be called from an IListState<T> to execute an asynchronous action when the data changes. The action is invoked once for the entire set of data, rather than for individual items:

await MyStrings.ForEach(async(list, ct) => await PerformAction(items, ct));

...

private async ValueTask PerformAction(IImmutableList<string> items, CancellationToken ct)
{
    ...
}

Selection operators

Like list-feed, list-state provides out-the-box support for Selection. This feature enables flagging single or multiple items in the State as 'selected'.

Selection works seamlessly and automatically with the ListView and other selection controls. In case you need to select an item manually for example in response to a button pressed or when finding a searched item, you can use the following methods that enable manual changing of the Selection state of items in the list-state:

TrySelectAsync

The TrySelectAsync method attempts to find the first occurrence of the item or items passed in as an argument and flag it as 'selected'.

This method comes in two flavors, one that accepts a single item to be selected, while the other one takes multiple.

It returns a boolean value indicating if the desired selection item was found and has been selected.

Single item selection

IListState<string> Names => ...

private ValueTask SelectCharlie(CancellationToken ct)
{
    bool selected = await Names.TrySelectAsync("charlie", ct);
}

Multi-item selection

IListState<string> Names => ...

private ValueTask SelectCharlieAndJoe(CancellationToken ct)
{
    ImmutableList<string> charlieAndJoe = ImmutableList.Create("charlie", "joe");
    bool selected = await Names.TrySelectAsync(charlieAndJoe, ct);
}

ClearSelection

The ClearSelection method clears the current selection and flags all items as 'not selected':

await MyStrings.ClearSelection(cancellationToken);

Subscribing to the selection

You can create a Feed that reflects the currently selected item or items (when using multi-selection) of a Feed. This is explained in detail in the Selection page.