title: Get the height of SafeAreaView from within BottomTabBar
body: <p>I am attempting to use <a href="https://github.com/Andr3wHur5t/react-native-keyboard-spacer" rel="noreferrer"><code>react-native-keyboard-spacer</code></a> in conjunction with <a href="https://github.com/react-navigation/react-navigation" rel="noreferrer"><code>react-navigation</code></a>.</p>
  
  <p>I am currently setting the <code>topSpacing</code> of the keyboard spacer to be <code>-49</code> which is the height of the tab bar from <code>react-navigation</code>, but the tab bar is within a <a href="https://facebook.github.io/react-native/docs/safeareaview" rel="noreferrer"><code>SafeAreaView</code></a> which <em>magically</em> adds padding to move content into an area that doesn't interfere with native UI.</p>
  
  <p>This means that when viewing the app on an iPhone X, or other similar devices, the tab bar becomes taller than <code>50</code>.</p>
  
  <p>What would be the best way to get the height of the <code>SafeAreaView</code>?</p>