例如，我有一些指示對(duì)象狀態(tài)的集合的可觀察對(duì)象（我通過(guò) REST API 定期獲取它）。class User{ int Id { get; } string Name { get; } string Status { get; }}IObservable<User> source;我想創(chuàng)建一個(gè)DynamicCache對(duì)象并在每次source給我一個(gè)新結(jié)果時(shí)更新它。所以我寫(xiě)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));但是現(xiàn)在每次用戶更改其狀態(tài)時(shí)，.Transform(...)方法都會(huì)創(chuàng)建一個(gè) 的新實(shí)例UserViewModel，這不是所需的行為。當(dāng)具有相同 Id 的源項(xiàng)發(fā)生更改而不是每次都創(chuàng)建一個(gè)新項(xiàng)時(shí)，我能否以某種方式確定更新現(xiàn)有 ViewModel 屬性（在派生集合中）的規(guī)則？
1 回答
慕容森
TA貢獻(xiàn)1853條經(jīng)驗(yàn) 獲得超18個(gè)贊
答案是您需要?jiǎng)?chuàng)建一個(gè)自定義運(yùn)算符。我在TransformWithInlineUpdate此處發(fā)布了一個(gè)要點(diǎn)，您可以將其復(fù)制到您的解決方案中。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;
});
為了答案的完整性，這里是代碼：
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
}
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