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在Silverlight中如何使用反应式扩展(Rx)组织这些调用?

system.reactive asynchronous silverlight

Jeremy Likness · 技术社区 · 14 年前

void IService.Query(Action<IEnumerable<Widget>,Exception> callback);
void IService.Load(Action<Widget,Exception> callback);

考虑到这一点,下面是视图模型的草图:

public class ViewModel : BaseViewModel
{
   public ViewModel()
   {
      Widgets = new ObservableCollection<Widget>();

      WidgetService.Query((widgets,exception) =>
      {
          if (exception != null) 
          {
              throw exception;
          }

          Widgets.Clear();

          foreach(var widget in widgets)
          {
             Widgets.Add(widget);
          }

          WidgetService.Load((defaultWidget,ex) =>
          {
             if (ex != null)
             {
                 throw ex;
             }
             if (defaultWidget != null)
             {
                CurrentWidget = defaultWidget;
             }
          }
      });
   }

   public IService WidgetService { get; set; } // assume this is wired up

   public ObservableCollection<Widget> Widgets { get; private set; }

   private Widget _currentWidget; 

   public Widget CurrentWidget 
   {
      get { return _currentWidget; }
      set 
      {
         _currentWidget = value; 
         RaisePropertyChanged(()=>CurrentWidget);
      }
   }
}

我想做的是简化依次调用query和default的工作流程。也许最好的方法是用lambda表达式嵌套,正如我所展示的,但是我认为使用Rx可能有一种更优雅的方法。我不想为了Rx而使用Rx,但是如果它能让我组织上面的逻辑,这样在方法中更容易阅读/维护,我会利用它。理想情况下,类似于:

Observable.Create(
   ()=>firstAction(), 
   ()=>secondAction())
.Subscribe(action=>action(),error=>{ throw error; });

使用power线程库,我可以执行以下操作:

Service.Query(list=>{result=list};
yield return 1;
ProcessList(result);
Service.Query(widget=>{defaultWidget=widget};
yield return 1;
CurrentWidget = defaultWidget;

任何类似的事情对我来说都是有意义的。

所以问题的实质是:我是在尝试将一个方形的peg放入一个圆形的孔中,还是有办法使用Rx重新定义嵌套的异步调用?

2 回复 | 直到 14 年前

desco 14 年前

选择多个

        WidgetService.Query()
            .SelectMany(
                widgets =>
                {
                    Widgets.Clear();
                    foreach (var w in widgets)
                    {
                        Widgets.Add(w);
                    }

                    return WidgetService.Load();
                }
            )
            .Do(
                defaultWidget =>
                {
                    if (defaultWidget != null)
                        Default = defaultWidget;
                }
            )
            .Subscribe(
                _ => { },
                e => { throw e; }
            );

不过,IMO F#Async看起来更清楚(在示例中,我假设服务的方法返回Async>和异步)。请注意,示例没有考虑线程正在修改的数据字段,在实际代码中,您应该注意以下几点:

    let load = async {
            let! widgets = WidgetService.Query()

            Widgets.Clear()
            for w in widgets do
                Widgets.Add(w)

            let! defaultWidget = WidgetService.Load()
            if defaultWidget <> null then
                Default <- defaultWidget

            return ()
        }

    Async.StartWithContinuations(
        load, 
        ignore, // success continuation - ignore result
        raise,  // error continuation - reraise exception
        ignore  // cancellation continuation - ignore
        )

事实上,您在问题中提到的迭代器可以使用以下技术:

    private IEnumerable<IObservable<object>> Intialize()
    {
        var widgetsList = WidgetService.Query().Start();
        yield return widgetsList;

        Widgets.Clear();
        foreach (var w in widgetsList[0])
        {
            Widgets.Add(w);
        }

        var defaultWidgetList = WidgetService.Load().Start();
        yield return defaultWidgetList;

        if (defaultWidgetList[0] != null)
            Default = defaultWidgetList[0];
    }

    Observable
        .Iterate(Intialize)
        .Subscribe(
        _ => { },
        ex => { throw ex; }
        );

Ana Betts 14 年前

你也可以用 ReactiveXaml ,但是由于CurrentWidget和Widget都是可变的,所以不能使其保持干净(有一个类称为 ObservableAsPropertyHelper

public class ViewModel
{
    public ViewModel()
    {
        // These return a Func that wraps an async call in an IObservable<T>
        // that always yields only one item (the result of the call)
        var QueryAsObservable = Observable.FromAsyncCommand<IEnumerable<Widget>>(WebService.BeginQuery, WebService.EndQuery);
        var LoadAsObservable = Observable.FromAsyncCommand<Widget>(WebService.BeginLoad, WebService.EndLoad);

        // Create a new command 
        QueryAndLoad = new ReactiveAsyncCommand();

        // QueryAndLoad fires every time someone calls ICommand.Execute
        // The .Do is the hacky part, for sync calls it's hidden by RegisterAsyncFunction
        var async_results = QueryAndLoad.SelectMany(_ => QueryAsObservable())
                                        .Do(_ => DoTranslate.AsyncCompletedNotification.OnNext(new Unit()));

        // Query up the Widgets 
        async_results.Subscribe(x => x.Run(Widgets.Add));

        // Now execute the Load
        async_results.SelectMany(_ => LoadAsObservable())
                     .Subscribe(x => CurrentWidget = x);

        QueryAndLoad.Execute();
    }

    public ReactiveAsyncCommand QueryAndLoad {get; private set; }

    public ObservableCollection<Widget> Widgets {get; private set; }

    public Widget CurrentWidget {get; set; }
}