title: rxjs: Subscribe each time value is emitted
body: <p>I want to subscribe to mousemove event each time mouse is clicked down and unsubscribe each time mouse is clicked up.</p>
  
  <pre><code>// start when mousedown
  Observable.fromEvent(element, 'mousedown')
            .subscribe(() =&gt; {
                Observable.fromEvent(element, 'mousemove')
                          // finish when mouseup
                          .takeUntil(Observable.fromEvent(element, 'mouseup'))
                          .subscribe(() =&gt; {/*do something on mousemove*/});
            });
  </code></pre>
  
  <p>Is it possible to optimize the code (to have single "subscribe" method)?</p>