title: How to create a stream that emit a value only when another stream emit a value?
body: <p>I'm learning rxjs.
  Assume I have an array of value:</p>
  
  <pre><code>[12, 23, 34, 56, 78, 99]
  </code></pre>
  
  <p>I want to create a stream <code>A$</code> of the above values, but it don't emit values one after another <strong>immediately</strong>, instead it should wait to emit value only when another stream <code>B$</code> emits a value.</p>