title: How to get size of a View before render it for react-native FlatList getItemLayout
body: <p>I need set <code>initialScrollIndex</code> on FlatList to 1, so second row will be visible on initial loading. It's depend on <code>getItemLayout</code>.
  My FlatList cells contains text of different length, which leads to different height of each cell, which leads i can not return fixed value in <code>getItemLayout</code>.
  My strategy is:</p>
  
  <ol>
  <li>On load, iterate through dataset for list and calculate height of each row  </li>
  <li>Cache calculated data into array or dictionary  </li>
  <li>Return cached data inside getItemLayout depend on index</li>
  </ol>
  
  <p>The problem is - i can not find the way how to calculate the height of an view before it will be mounted into DOM hierarchy. I read a lot about onLayout prop, but it looks like <code>View</code> should be rendered first to get it to work.</p>
  
  <p>So, what do you use to achieve scrolling to specified row in react-native when list cell is not the same for each row?</p>