Fix TabController RTL support

Author: ethanshar

src/components/tabController/TabBar.js

@@ -231,16 +231,17 @@ class TabBar extends PureComponent {
     }
   };
 
-  onItemLayout = ({width, x}, itemIndex) => {
+  onItemLayout = ({width}, itemIndex) => {
     this._itemsWidths[itemIndex] = width;
-    this._itemsOffsets[itemIndex] = x;
     if (!_.includes(this._itemsWidths, null)) {
       this.setItemsLayouts();
     }
   };
 
   setItemsLayouts = () => {
     const {selectedIndex} = this.context;
+    // It's important to calculate itemOffsets for RTL support
+    this._itemsOffsets = _.times(this._itemsWidths.length, (i) => _.chain(this._itemsWidths).take(i).sum().value());
     const itemsOffsets = _.map(this._itemsOffsets, (offset) => offset + INDICATOR_INSET);
     const itemsWidths = _.map(this._itemsWidths, (width) => width - INDICATOR_INSET * 2);
     this.contentWidth = _.sum(this._itemsWidths);

src/components/tabController/TabBarItem.js

@@ -127,7 +127,7 @@ export default class TabBarItem extends PureComponent {
       this.itemWidth = width;
       this.itemRef.current.setNativeProps({style: {width, paddingHorizontal: null, flex: null}});
       if (onLayout) {
-        onLayout({width, x}, index);
+        onLayout({width}, index);
       }
     }
   };