Create derrived collection of ViewModels with DynamicData which updates existing item instead of creating a new one on source item change

Question

For example, I have an observable of some collections which indicates statuses of objects (I get it periodically through REST API).

class User
{
    int Id { get; }
    string Name { get; }
    string Status { get; }
}

IObservable<User> source;

I want to create a DynamicCache object and update it each time the source gives me a new result. So I wrote:

var models = new SourceCache<User,int>(user => user.Id);
models.Connect()
      .Transform(u => new UserViewModel() {...})
      ...
      .Bind(out viewModels)
      .Subscribe();

source.Subscribe(ul => models.EditDiff(ul, (a, b) => a.Status == b.Status));

But now every time a user changes its status, .Transform(...) method creates a new instance of UserViewModel, which isn't the desired behaviour.

Can I somehow determine a rule of updating existing ViewModel's properties (in the derived collection) when source item with the same Id is changing instead of creating a new one every time?

Answer 1

The answer is you need to create a custom operator. I have posted a gist here TransformWithInlineUpdate which you can copy into your solution. Example usage is:

var users = new SourceCache<User, int>(user => user.Id);

var transformed =  users.Connect()
    .TransformWithInlineUpdate(u => new UserViewModel(u), (previousViewModel, updatedUser) =>
        {
            previousViewModel.User = updatedUser;
        });

For completeness of the answer, here is the code:

    public static IObservable<IChangeSet<TDestination, TKey>> TransformWithInlineUpdate<TObject, TKey, TDestination>(this IObservable<IChangeSet<TObject, TKey>> source,
        Func<TObject, TDestination> transformFactory,
        Action<TDestination, TObject> updateAction = null)
    {
        return source.Scan((ChangeAwareCache<TDestination, TKey>)null, (cache, changes) =>
            {
                //The change aware cache captures a history of all changes so downstream operators can replay the changes
                if (cache == null)
                    cache = new ChangeAwareCache<TDestination, TKey>(changes.Count);

                foreach (var change in changes)
                {
                    switch (change.Reason)
                    {
                        case ChangeReason.Add:
                            cache.AddOrUpdate(transformFactory(change.Current), change.Key);
                            break;
                        case ChangeReason.Update:
                        {
                            if (updateAction == null) continue;

                            var previous = cache.Lookup(change.Key)
                                .ValueOrThrow(()=> new MissingKeyException($"{change.Key} is not found."));
                            //callback when an update has been received
                            updateAction(previous, change.Current);

                            //send a refresh as this will force downstream operators to filter, sort, group etc
                            cache.Refresh(change.Key);
                        }
                            break;
                        case ChangeReason.Remove:
                            cache.Remove(change.Key);
                            break;
                        case ChangeReason.Refresh:
                            cache.Refresh(change.Key);
                            break;
                        case ChangeReason.Moved:
                            //Do nothing !
                            break;
                    }
                }
                return cache;
                
            }).Select(cache => cache.CaptureChanges()); //invoke capture changes to return the changeset
    }

Update: this has been included in the library.