Combining latest from an observable of observables

问题:

Suppose I have a set of URIs that I am monitoring for availability. Each URI is either "up" or "down", and new URIs to monitor may be added to the system at any time:

public enum ConnectionStatus {     Up,     Down }  public class WebsiteStatus {     public string Uri     {         get;         set;     }      public ConnectionStatus Status     {         get;         set;     } }  public class Program {     static void Main(string[] args)     {         var statusStream = new Subject<WebsiteStatus>();         Test(statusStream);          Console.WriteLine("Done");         Console.ReadKey();     }      private static void Test(IObservable<WebsiteStatus> statusStream)     {     } } 

Now suppose in Test() I want to reactively ascertain:

• whether all URIs are down (as a bool)
• which URIs are down (as IEnumerable<string>)

So Test would end up creating an observable like IObservable<Tuple<bool, IEnumerable<string>>> where the bool indicates whether all URIs are down and the IEnumerable<string> contains those URIs that are.

How do I go about this? My initial thinking is that I would need to group by the URI, then combine the latest from each group into a list that I could then perform a Select against. However, this did not work out due to the way CombineLatest works.

EDIT: Thanks to Matthew's answer I looked into rxx and found that it implemented a CombineLatest overload in exactly the fashion I would have expected in rx out of the box, except that I needed to change it so that it publishes even when there is only a single source stream being combined (by default it was waiting for a minimum of two source streams). Also, I can't justify pulling in an extra 2MB of binaries for the sake of one method, so I have copy/pasted it into my project. Doing so, I was able to solve as follows:

private static void Test(IObservable<WebsiteStatus> statusStream) {     statusStream         .GroupBy(x => x.Uri)         .CombineLatest()         .Select(             x =>             {                 var down = x.Where(y => y.Status == ConnectionStatus.Down);                 var downCount = down.Count();                 var downUris = down.Select(y => y.Uri).ToList();                  return new                 {                     AllDown = x.Count == downCount,                     DownUris = downUris                 };             })         .Subscribe(x =>         {             Console.WriteLine("    Sources down ({0}): {1}", x.AllDown ? "that's all of them" : "some are still up", x.DownUris.Aggregate("", (y, z) => y += (z + " | ")));         }); } 

回答1:

The neatest way is to use the Rxx extension in this answer. An alternative is below, it just keeps a list of sites that are down/up.

var downStream = statusStream     .Aggregate<WebsiteStatus, IEnumerable<string>>(new string[0], (down, newStatus) =>     {         if (newStatus.IsUp)             return down.Where(uri => uri != newStatus.Uri);         else if (!down.Contains(newStatus.Uri))             return down.Concat(new string[] { newStatus.Uri });         else             return down;     });  var upStream = statusStream     .Aggregate<WebsiteStatus, IEnumerable<string>>(new string[0], (up, newStatus) =>     {         if (!newStatus.IsUp)             return up.Where(uri => uri != newStatus.Uri);         else if (!up.Contains(newStatus.Uri))             return down.Concat(new string[] { newStatus.Uri });         else             return up;     });  var allDown = upStream.Select(up => !up.Any()); 

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