C#5 ReadAsync and Iterators

问题

I am trying to convert the below class to lazily return a file.

public class ObservableFile2 : IObservable<string>
{
    private readonly IObservable<string> subject;

    public ObservableFile2(string fileName)
    {
        subject = Observable.Using<string, StreamReader>
            (
                () => new StreamReader(new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read)),
                streamReader => ObserveLines(streamReader)
            );
    }

    private IObservable<string> ObserveLines(StreamReader streamReader)
    {
        return ReadLines(streamReader).ToObservable();
    }

    private IEnumerable<string> ReadLines(StreamReader streamReader)
    {
        while (!streamReader.EndOfStream)
        {
            yield return streamReader.ReadLine();
        }
    }        

    public IDisposable Subscribe(IObserver<string> observer)
    {
        return subject.Subscribe(observer);
    }
}

I am now trying to convert this to use

StreamReader.ReadLineAsync() 

or even better would be to chunk the data i.e.

await SourceStream.ReadAsync(buffer, 0, chunkSize). 

I seem to not have grasped how to wrap and unwrap Tasks

Assistance would be welcomed.

Thanks

回答1:

I am not an Rx master, so there may be a better way than my answer.

I believe this should be possible using the async-enabled Create:

public static class ObservableFile2
{
  public static IObservable<string> Create(string fileName)
  {
    return Observable.Create<string>(async (subject, token) =>
    {
      try
      {
        using (var streamReader = new StreamReader(new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read))
        {
          while (true)
          {
            token.ThrowIfCancellationRequested();
            var line = await streamReader.ReadLineAsync();
            if (line == null)
            {
              subject.OnCompleted();
              return;
            }
            subject.OnNext(line);
          }
        }
      }
      catch (Exception ex)
      {
        subject.OnError(ex);
      }
    });
  }
}

回答2:

I don't know if the async-features are needed for your solution as you did not mention it - all I can see is that you want to consume the file "lazily" - my guess is that you want to get the lines one at a time and if so this one should do the trick:

public static IEnumerable<string> EnumerateLines(string fileName)
{
    using (
        var streamReader =
            new StreamReader(new FileStream(fileName,
                                            FileMode.Open,
                                            FileAccess.Read,
                                            FileShare.Read)))
    {
        while (true)
        {
            if (streamReader.EndOfStream)
                yield break;

            Console.WriteLine("read another line...");
            yield return streamReader.ReadLine();
        }
    }
}

please note that this depends just on the implementation-details of StreamReader.ReadLine. You could try to make even this non-strict evalutated by using Lazy - but then you get into trouble with disopsing the file-handles as you won't know when the values are really getting consumed (even Haskell has this problem :) ) - my advice: don't try to be to lazy with files ... this can get you easy into trouble

来源：https://stackoverflow.com/questions/13158971/c5-readasync-and-iterators