使用 DynamicData 创建 ViewModel 的派生集合，该集合更新现有项目，而不是在源项目更改时创建新项目

Question

例如，我有一些可观察的集合，它指示对象的状态（我通过 REST API 定期获取它）。

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

IObservable<User> source;

我想创建一个

DynamicCache

对象并在每次

source

给出新结果时更新它。所以我写道：

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));

但现在每次用户更改其状态时，

.Transform(...)

方法都会创建

UserViewModel

的新实例，这不是所需的行为。

当具有相同 Id 的源项发生更改时，我是否可以以某种方式确定更新现有 ViewModel 属性（在派生集合中）的规则，而不是每次都创建一个新属性？

Answer 1

答案是您需要创建一个自定义运算符。我在这里发布了一个要点 TransformWithInlineUpdate 您可以将其复制到您的解决方案中。用法示例是：

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

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

为了答案的完整性，这里是代码：

    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
    }

更新：这已包含在库中。