title: rxjs observe array push
body: <p>I want to monitor when an object is pushed onto an array using observables. I want to start with an empty array and when a push occurs I want the observable to detect and process it and then wait till the next push. This would be very similar to "fromEvent" where the observable waits for the event. The code below immediately calls completed() because the array is empty, how do I make it wait for a push?</p>
  
  <pre><code>    var testArray = [];
  
      test(){
        var o = {timestamp: new Date()}
        testArray.push(o)
      }
  
      var o = Observable
          .from(testArray)
          .concatMap( x =&gt; {
            return x;
      });
  
      o.subscribe( 
        x =&gt; { console.log("onNext x=",x.timestamp) },
        e =&gt; console.log('onError:', e),
        () =&gt; {console.log('onCompleted');} );
  </code></pre>
  
  <p>Note: The input mechanism does not have to be an array. Any type of message queue object will work for me.</p>