title: How to tell when a view will appear with ReactNavigation
body: <p>I'm using ReactNavigation and am wondering how I can tell when a view will appear (so that I can trigger a data refresh).  I see this example using a different navigator <a href="https://stackoverflow.com/questions/29619673/navigatorios-is-there-a-viewdidappear-or-viewwillappear-equivalent">NavigatorIOS - Is there a viewDidAppear or viewWillAppear equivalent?</a> but not sure how it would work with <a href="https://reactnavigation.org/" rel="noreferrer">https://reactnavigation.org/</a>.</p>
  
  <p>I see the console.log on the navigation dispatches - but is there some event handler that I can hook into on the screen/component level to know if that component/screen is in the foreground?  Should I hook into the <code>getStateForAction</code> method somehow?  <a href="https://reactnavigation.org/docs/routers/api" rel="noreferrer">https://reactnavigation.org/docs/routers/api</a> .  I don't want to really create custom route handlers per se, just to detect if the view is entering foreground/will appear, and I'm guessing I can use the navigation event to do that somehow.</p>