使用动态数据创建 ViewModel 的派生集合，更新现有项目而不是在源项目更改时创建新项目

Question

例如，我有一些指示对象状态的集合的可观察对象（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 给我一个新结果时更新它。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(...) 方法都会创建一个 UserViewModel 的新实例，这不是所需的行为。

当具有相同 ID 的源项目发生变化时，我能否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的？

Answer 1

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

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

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

为了回答的完整性，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
    }

使用动态数据创建 ViewModel 的派生集合，更新现有项目而不是在源项目更改时创建新项目

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

c#

dynamic-data

system.reactive

reactiveui

reactive