Fix virtualized cell keys for list headers and footers

Summary:
The change enabling virtualization in nested lists contained a hidden assumption that nested lists would only appear within the *cells* of a parent list.

If a list header or footer component contains a `VirtualizedList`, that child list won't be wrapped in a `CellRenderer` component and therefore won't have access to `virtualizedCellRenderer` through its context. This causes an error when the child list tries to access the `cellKey` property on an undefined object.

This change wraps the header/footer views in a `VirtualizedCellWrapper` component which supplies that context properly.

Reviewed By: sahrens

Differential Revision: D6603342

fbshipit-source-id: 4d2d82f04947048a16ec9968121d8ecc8c95655a

Author: logandaniels

Libraries/Lists/VirtualizedList.js

@@ -432,7 +432,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   };
 
   static contextTypes = {
-    virtualizedListCellRenderer: PropTypes.shape({
+    virtualizedCell: PropTypes.shape({
       cellKey: PropTypes.string,
     }),
     virtualizedList: PropTypes.shape({
@@ -466,6 +466,13 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     };
   }
 
+  _getCellKey(): string {
+    return (
+      (this.context.virtualizedCell && this.context.virtualizedCell.cellKey) ||
+      'rootList'
+    );
+  }
+
   _getScrollMetrics = () => {
     return this._scrollMetrics;
   };
@@ -559,10 +566,8 @@ class VirtualizedList extends React.PureComponent<Props, State> {
 
     if (this._isNestedWithSameOrientation()) {
       const storedState = this.context.virtualizedList.registerAsNestedChild({
-        cellKey: this.context.virtualizedListCellRenderer.cellKey,
-        key:
-          this.props.listKey ||
-          this.context.virtualizedListCellRenderer.cellKey,
+        cellKey: this._getCellKey(),
+        key: this.props.listKey || this._getCellKey(),
         ref: this,
       });
       if (storedState) {
@@ -575,8 +580,6 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     this.state = initialState;
   }
 
-  componentWillMount() {}
-
   componentDidMount() {
     if (this.props.initialScrollIndex) {
       this._initialScrollIndexTimeout = setTimeout(
@@ -593,9 +596,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   componentWillUnmount() {
     if (this._isNestedWithSameOrientation()) {
       this.context.virtualizedList.unregisterAsNestedChild({
-        key:
-          this.props.listKey ||
-          this.context.virtualizedListCellRenderer.cellKey,
+        key: this.props.listKey || this._getCellKey(),
         state: {
           first: this.state.first,
           last: this.state.last,
@@ -743,12 +744,13 @@ class VirtualizedList extends React.PureComponent<Props, State> {
         <ListHeaderComponent />
       );
       cells.push(
-        <View
-          key="$header"
-          onLayout={this._onLayoutHeader}
-          style={inversionStyle}>
-          {element}
-        </View>,
+        <VirtualizedCellWrapper
+          cellKey={this._getCellKey() + '-header'}
+          key="$header">
+          <View onLayout={this._onLayoutHeader} style={inversionStyle}>
+            {element}
+          </View>
+        </VirtualizedCellWrapper>,
       );
     }
     const itemCount = this.props.getItemCount(data);
@@ -867,12 +869,13 @@ class VirtualizedList extends React.PureComponent<Props, State> {
         <ListFooterComponent />
       );
       cells.push(
-        <View
-          key="$footer"
-          onLayout={this._onLayoutFooter}
-          style={inversionStyle}>
-          {element}
-        </View>,
+        <VirtualizedCellWrapper
+          cellKey={this._getCellKey() + '-footer'}
+          key="$footer">
+          <View onLayout={this._onLayoutFooter} style={inversionStyle}>
+            {element}
+          </View>
+        </VirtualizedCellWrapper>,
       );
     }
     const scrollProps = {
@@ -1522,14 +1525,14 @@ class CellRenderer extends React.Component<
   };
 
   static childContextTypes = {
-    virtualizedListCellRenderer: PropTypes.shape({
+    virtualizedCell: PropTypes.shape({
       cellKey: PropTypes.string,
     }),
   };
 
   getChildContext() {
     return {
-      virtualizedListCellRenderer: {
+      virtualizedCell: {
         cellKey: this.props.cellKey,
       },
     };
@@ -1621,6 +1624,29 @@ class CellRenderer extends React.Component<
   }
 }
 
+class VirtualizedCellWrapper extends React.Component<{
+  cellKey: string,
+  children: React.Node,
+}> {
+  static childContextTypes = {
+    virtualizedCell: PropTypes.shape({
+      cellKey: PropTypes.string,
+    }),
+  };
+
+  getChildContext() {
+    return {
+      virtualizedCell: {
+        cellKey: this.props.cellKey,
+      },
+    };
+  }
+
+  render() {
+    return this.props.children;
+  }
+}
+
 const styles = StyleSheet.create({
   verticallyInverted: {
     transform: [{scaleY: -1}],