Don’t use getItemLayout

Same reason as meliorence/react-native-snap-carousel@ce3bf06

Author: infostreams

src/libraries/ViewPager/index.js

@@ -237,6 +237,13 @@ export default class ViewPager extends PureComponent {
     }
 
     getItemLayout (data, index) {
+        // this method is called 'getItemLayout', but it is not actually used
+        // as the 'getItemLayout' function for the FlatList. We use it within
+        // the code on this page though. The reason for this is that working
+        // with 'getItemLayout' for FlatList is buggy. You might end up with
+        // unrendered / missing content. Therefore we work around it, as
+        // described here
+        // https://github.com/facebook/react-native/issues/15734#issuecomment-330616697
         return {
             length: this.state.width + this.props.pageMargin,
             offset: (this.state.width + this.props.pageMargin) * index,
@@ -310,8 +317,12 @@ export default class ViewPager extends PureComponent {
                   data={pageDataArray}
                   renderItem={this.renderRow}
                   onLayout={this.onLayout}
-                  getItemLayout={this.getItemLayout}
-                  initialScrollIndex={(this.props.initialPage || undefined)}
+
+                  // use contentOffset instead of initialScrollIndex so that we don't have
+                  // to use the buggy 'getItemLayout' prop. See
+                  // https://github.com/facebook/react-native/issues/15734#issuecomment-330616697 and
+                  // https://github.com/facebook/react-native/issues/14945#issuecomment-354651271
+                  contentOffset = {{x: this.getScrollOffsetOfPage(parseInt(this.props.initialPage)), y:0}}
               />
             </View>
         );