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POFirstOrLast IObservable Extension
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  1. POFirstOrLast IObservable Extension
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    copied!<p>I want to run through an <code>IObservable&lt;T&gt;</code> looking for an element that matches a predicate, and if not found, return the last element of the <code>IObservable&lt;T&gt;</code>. I don't want to have to store the entire contents of the <code>IObservable&lt;T&gt;</code>, and I don't want to loop through the <code>IObservable</code> twice, so I've set up an extension method</p> <pre><code>public static class ObservableExtensions { public static IObservable&lt;T&gt; FirstOrLastAsync&lt;T&gt;(this IObservable&lt;T&gt; source, Func&lt;T, bool&gt; pred) { return Observable.Create&lt;T&gt;(o =&gt; { var hot = source.Publish(); var store = new AsyncSubject&lt;T&gt;(); var d1 = hot.Subscribe(store); var d2 = hot.FirstAsync(x =&gt; pred(x)).Amb(store).Subscribe(o); var d3 = hot.Connect(); return new CompositeDisposable(d1, d2, d3); }); } public static T FirstOrLast&lt;T&gt;(this IObservable&lt;T&gt; source, Func&lt;T, bool&gt; pred) { return source.FirstOrLastAsync(pred).Wait(); } } </code></pre> <p>The Async method creates a hot observable from a potentially cold one passed in. It subscribes an <code>AsyncSubject&lt;T&gt;</code> to remember the last element, and an <code>IObservable&lt;T&gt;</code> that looks for the element. It then takes the first element from either of those <code>IObservable&lt;T&gt;</code>s, which ever returns a value first via <code>.Amb</code> (<code>AsyncSubject&lt;T&gt;</code> doesn't return a value until it gets an <code>.OnCompleted</code> message).</p> <p>My questions are the following: </p> <ul> <li>Can this be written better or more concisely using different Observable methods? </li> <li>Do all of those disposables need to be included in the CompositeDisposable? </li> <li>When the hot observable is completed without finding a matching element, is there a race condition between FirstAsync throwing an exception, and the AsyncSubject propagating its value? </li> <li>If so, do I need to change the line to:</li> </ul> <p><code>var d2 = hot.Where(x =&gt; pred(x)).Take(1).Amb(store).Subscribe(o);</code></p> <p>I'm pretty new to RX, and this is my first extension on IObservable.</p> <p><em>EDIT</em></p> <p>I ended up going with</p> <pre><code>public static class ObservableExtensions { public static IObservable&lt;T&gt; FirstOrLastAsync&lt;T&gt;(this IObservable&lt;T&gt; source, Func&lt;T, bool&gt; pred) { var hot = source.Publish().RefCount(); return hot.TakeLast(1).Amb(hot.Where(pred).Take(1).Concat(Observable.Never&lt;T&gt;())); } public static T FirstOrLast&lt;T&gt;(this IObservable&lt;T&gt; source, Func&lt;T, bool&gt; pred) { return source.FirstOrLastAsync(pred).First(); } } </code></pre>
 

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