Update measurements for Footer in VirtualizedList when the footer's onLayout is called

Summary:
When any list cell that may contain nested VirtualizedLists is laid out, it's possible that the offset of any child lists relative to the outermost list has changed. We need to tell these children to re-measure themselves relative to that outermost list, so that their viewability calculations are correct.

We already do this for regular list cells -- we just need to extend that logic to any child lists that may live in the ListFooterComponent.

Changelog: [General] [Fixed] - Fix viewability calculations for nested VirtualizedLists inside of a parent list's FooterComponent

Differential Revision: D20310961

fbshipit-source-id: 4bfcfb95c87329f2ee337d5499e5c7162ba692e8

Author: logandaniels

Libraries/Lists/VirtualizedList.js

@@ -1133,7 +1133,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       );
       cells.push(
         <VirtualizedCellWrapper
-          cellKey={this._getCellKey() + '-footer'}
+          cellKey={this._getFooterCellKey()}
           key="$footer">
           <View
             onLayout={this._onLayoutFooter}
@@ -1362,15 +1362,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       this._frames[cellKey].inLayout = true;
     }
 
-    const childListKeys = this._cellKeysToChildListKeys.get(cellKey);
-    if (childListKeys) {
-      for (let childKey of childListKeys) {
-        const childList = this._nestedChildLists.get(childKey);
-        childList &&
-          childList.ref &&
-          childList.ref.measureLayoutRelativeToContainingList();
-      }
-    }
+    this._triggerRemeasureForChildListsInCell(cellKey);
 
     this._computeBlankness();
     this._updateViewableItems(this.props.data);
@@ -1383,6 +1375,18 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     }
   };
 
+  _triggerRemeasureForChildListsInCell(cellKey: string): void {
+    const childListKeys = this._cellKeysToChildListKeys.get(cellKey);
+    if (childListKeys) {
+      for (let childKey of childListKeys) {
+        const childList = this._nestedChildLists.get(childKey);
+        childList &&
+          childList.ref &&
+          childList.ref.measureLayoutRelativeToContainingList();
+      }
+    }
+  }
+
   measureLayoutRelativeToContainingList(): void {
     // TODO (T35574538): findNodeHandle sometimes crashes with "Unable to find
     // node on an unmounted component" during scrolling
@@ -1443,7 +1447,12 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     this.props.onLayout && this.props.onLayout(e);
   };
 
+  _getFooterCellKey(): string {
+    return this._getCellKey() + '-footer';
+  }
+
   _onLayoutFooter = e => {
+    this._triggerRemeasureForChildListsInCell(this._getFooterCellKey());
     this._footerLength = this._selectLength(e.nativeEvent.layout);
   };