既然没有yield关键字,有没有办法在VB.NET中实现类似Caliburn的co例程

yield in vb.net 问题就在这里。

Caliburn 最近。很多很棒的东西,包括 co-routines

small MVVM framework .

一切看起来都很好,除了使用VB的co例程。由于使用了vb10,我们可以尝试 Bill McCarthy's suggestion :

Public Function Lines(ByVal rdr as TextReader) As IEnumerable(Of String)
     Return New GenericIterator(Of String) 
          (Function(ByRef nextItem As String) As Boolean
              nextItem = rdr.ReadLine
              Return nextItem IsNot Nothing
           End Function)
End Function

我只是不明白下面这种稍微复杂一点的协同例程方法(取自Rob的游戏库)是如何用VB编写的:

public IEnumerable<IResult> ExecuteSearch()
{
    var search = new SearchGames
    {
        SearchText = SearchText
    }.AsResult();

    yield return Show.Busy();
    yield return search;

    var resultCount = search.Response.Count();

    if (resultCount == 0)
        SearchResults = _noResults.WithTitle(SearchText);
    else if (resultCount == 1 && search.Response.First().Title == SearchText)
    {
        var getGame = new GetGame
        {
            Id = search.Response.First().Id
        }.AsResult();

        yield return getGame;
        yield return Show.Screen<ExploreGameViewModel>()
            .Configured(x => x.WithGame(getGame.Response));
    }
    else SearchResults = _results.With(search.Response);

    yield return Show.NotBusy();
}

你知道如何实现这一点吗?或者你对在VB中使用Caliburn co例程有什么想法吗?

编辑:

Marco 给我指出了一个正确的方向。在查看了Rob游戏库的Reflector-Visual Basic代码后,我成功地修改了Bill McCarthy的GenericIterator,使其成为穷人的状态机:

Private _state As Integer = -1

Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
    _state += 1
    Return _func(_Current, _state)
End Function

我们可以这样使用它:

Public Function ExecuteSearch() As IEnumerable(Of String)
    ' If we need some variable shared across states, define it here
    Dim someSharedStuff As String = String.Empty

    ' Notice the second lambda function parameter below - state
    Return New GenericIterator(Of IResult) 
        (Function(ByRef nextItem As IResult, state As Integer) As Boolean
            Select Case state
                Case 0
                    someSharedStuff = "First state"
                    nextItem = Show.Busy
                    Return True
                Case 1
                    nextItem = Show.SomeLoadingScreen
                    ' Do some additional processing here...
                    Return True
                Case 2
                    ' Do something with someSharedStuff variable...
                    Console.WriteLine(someSharedStuff)
                    nextItem = PerforSomemWebServiceCall()
                    Return True
                '...
                Case 6
                    nextItem = Show.NotBusy
                    Return False
            End Select

            Return False
         End Function)

End Function

它肯定没有C版本那么优雅,但看起来是可行的。我们看看这个有没有问题。如果有人有更好的主意,我洗耳恭听。