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PORxJS: Recursive list of observables and single observer
 

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    <p>I've been having some trouble with a recursive chain of observables.</p> <p>I am working with RxJS, which is currently in version 1.0.10621, and contains most basic Rx functionality, in conjunction with Rx for jQuery.</p> <p>Let me introduce an example scenario for my problem: I am polling the <a href="https://dev.twitter.com/docs/using-search" rel="noreferrer">Twitter search API</a> (JSON response) for tweets/updates containing a certain keyword. The response also includes a "refresh_url" which one should use to generate follow-up request. The response to that follow-up request will again contain a new refresh_url, etc.</p> <p>Rx.jQuery allows me to make the Twitter search API call an observable event, which produces one onNext and then completes. What I have tried so far is to have the onNext handler remember the refresh_url and use it in the onCompleted handler to produce both a new observable and corresponding observer for the next request. This way, one observable + observer pair follows the other indefinitely.</p> <p>The problem with this approach is:</p> <ol> <li><p>The follow-up observable/observer are already alive when their predecessors have not yet been disposed of.</p></li> <li><p>I have to do lots of nasty bookkeeping to maintain a valid reference to the currently living observer, of which there can actually be two. (One in onCompleted and the other somewhere else in its life-cycle) This reference is, of course, needed to unsubscribe/dispose of the observer. An alternative to the bookkeeping would be to implement a side effect by the means of a "still running?"-boolean, as I have done in my example.</p></li> </ol> <p>Example code:</p> <pre><code> running = true; twitterUrl = "http://search.twitter.com/search.json"; twitterQuery = "?rpp=10&amp;q=" + encodeURIComponent(text); twitterMaxId = 0; //actually twitter ignores its since_id parameter newTweetObserver = function () { return Rx.Observer.create( function (tweet) { if (tweet.id &gt; twitterMaxId) { twitterMaxId = tweet.id; displayTweet(tweet); } } ); } createTwitterObserver = function() { twitterObserver = Rx.Observer.create( function (response) { if (response.textStatus == "success") { var data = response.data; if (data.error == undefined) { twitterQuery = data.refresh_url; var tweetObservable; tweetObservable = Rx.Observable.fromArray(data.results.reverse()); tweetObservable.subscribe(newTweetObserver()); } } }, function(error) { alert(error); }, function () { //create and listen to new observer that includes a delay if (running) { twitterObservable = $.getJSONPAsObservable(twitterUrl, twitterQuery).delay(3000); twitterObservable.subscribe(createTwitterObserver()); } } ); return twitterObserver; } twitterObservable = $.getJSONPAsObservable(twitterUrl, twitterQuery); twitterObservable.subscribe(createTwitterObserver()); </code></pre> <p>Don't be fooled by the double layer of observables/observers from requests to tweets. My example concerns itself mainly with the first layer: requesting data from Twitter. If in solving this problem the second layer (converting responses into tweets) can become one with the first one, that would be fantastic; But i think that's a whole different thing. For now.</p> <p>Erik Meijer pointed out the Expand operator to me (see example below), and suggested <a href="http://headinthebox.posterous.com/" rel="noreferrer">Join patterns</a> as an alternative.</p> <pre><code>var ys = Observable.Expand (new[]{0}.ToObservable() // initial sequence , i =&gt; ( i == 10 ? Observable.Empty&lt;int&gt;() // terminate : new[]{i+1}.ToObservable() // recurse ) ); ys.ToArray().Select(a =&gt; string.Join(",", a)).DumpLive(); </code></pre> <p>This should be copy-pastable into LINQPad. It assumes singleton observables and produces one final observer.</p> <p>So my question is: How can I do the expand trick nicest in RxJS?</p> <p>EDIT:<br/> The expand operator can probably be implemented as shown in <a href="http://social.msdn.microsoft.com/Forums/da-DK/rx/thread/2746e373-bf43-4381-834c-8cc182704ae9" rel="noreferrer">this thread</a>. But one would need <a href="https://developer.mozilla.org/en/New_in_JavaScript_1.7#Generators" rel="noreferrer">generators</a> (and I only have JS &lt; 1.6).<br/> Unfortunately <a href="http://nuget.org/packages/RxJS-Main/2.0.20304-beta" rel="noreferrer">RxJS 2.0.20304-beta</a> does not implement the Extend method.</p>
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    1. COI have come to the conclusion that the solution to this problem is indeed the [Expand operator](http://social.msdn.microsoft.com/Forums/da-DK/rx/thread/2746e373-bf43-4381-834c-8cc182704ae9) which is not yet implemented in RxJS up to version 2.0.20304-beta.
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