title: ReactJS: How to signal a child component?
body: <p>For special testing purposes in a ReactJS app, I want to signal a child component like this:</p>
  <ol>
  <li>User clicks a button on parent component A</li>
  <li>A sends some kind of signal to child component B</li>
  <li>B executes some logic in response to the signal</li>
  </ol>
  <p>It's trivial to get B to communicate up to A, but there doesn't seem to be a good way to go the other direction. My first naive attempt was to pass a callback to B in props, but props are read-only, so that didn't work.  So before I go too deep hacking this, is there a clean, &quot;blessed&quot; way to do this?</p>