Made `getItem` and `getItemCount` required props (VirtualizedList)

Summary:
The main reason to use **VirtualizedList** is to set the `getItem` and `getItemCount` props, so having default values for these props makes things error prone.

* In **VirtualizedList**, changed the `getItem` and `getItemCount` props from optional to required, and removed default values.
* Ensured that implementing classes **FlatList** and **SectionVirtualizedList** are always passing these props.
* Updated VirtualizedList-test.js accordingly.

Reviewed By: sahrens

Differential Revision: D4980236

fbshipit-source-id: ad1838931253bc61ff9068c40929f6e9c755b92c

Libraries/Lists/FlatList.js

@@ -178,8 +178,6 @@ type Props<ItemT> = RequiredProps<ItemT> & OptionalProps<ItemT> & VirtualizedLis
 
 const defaultProps = {
   ...VirtualizedList.defaultProps,
-  getItem: undefined,
-  getItemCount: undefined,
   numColumns: 1,
 };
 type DefaultProps = typeof defaultProps;

Libraries/Lists/VirtualizedList.js

@@ -38,6 +38,14 @@ type RequiredProps = {
    * getItem, getItemCount, and keyExtractor to handle any type of index-based data.
    */
   data?: any,
+  /**
+   * A generic accessor for extracting an item from any sort of data blob.
+   */
+  getItem: (data: any, index: number) => ?Item,
+  /**
+   * Determines how many items are in the data blob.
+   */
+  getItemCount: (data: any) => number,
 };
 type OptionalProps = {
   /**
@@ -57,14 +65,6 @@ type OptionalProps = {
    * `data` prop, stick it here and treat it immutably.
    */
   extraData?: any,
-  /**
-   * A generic accessor for extracting an item from any sort of data blob.
-   */
-  getItem: (data: any, index: number) => ?Item,
-  /**
-   * Determines how many items are in the data blob.
-   */
-  getItemCount: (data: any) => number,
   getItemLayout?: (data: any, index: number) =>
     {length: number, offset: number, index: number}, // e.g. height, y
   horizontal?: ?boolean,
@@ -254,8 +254,6 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
 
   static defaultProps = {
     disableVirtualization: false,
-    getItem: (data: any, index: number) => data[index],
-    getItemCount: (data: any) => data ? data.length : 0,
     horizontal: false,
     initialNumToRender: 10,
     keyExtractor: (item: Item, index: number) => {

Libraries/Lists/__tests__/VirtualizedList-test.js

@@ -23,6 +23,8 @@ describe('VirtualizedList', () => {
       <VirtualizedList
         data={[{key: 'i1'}, {key: 'i2'}, {key: 'i3'}]}
         renderItem={({item}) => <item value={item.key} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={(data) => data.length}
       />
     );
     expect(component).toMatchSnapshot();
@@ -33,6 +35,8 @@ describe('VirtualizedList', () => {
       <VirtualizedList
         data={[]}
         renderItem={({item}) => <item value={item.key} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={(data) => data.length}
       />
     );
     expect(component).toMatchSnapshot();
@@ -43,6 +47,8 @@ describe('VirtualizedList', () => {
       <VirtualizedList
         data={undefined}
         renderItem={({item}) => <item value={item.key} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={(data) => 0}
       />
     );
     expect(component).toMatchSnapshot();
@@ -55,6 +61,8 @@ describe('VirtualizedList', () => {
         ListFooterComponent={() => <footer />}
         ListHeaderComponent={() => <header />}
         data={new Array(5).fill().map((_, ii) => ({id: String(ii)}))}
+        getItem={(data, index) => data[index]}
+        getItemCount={(data) => data.length}
         keyExtractor={(item, index) => item.id}
         getItemLayout={({index}) => ({length: 50, offset: index * 50})}
         refreshing={false}
@@ -87,6 +95,8 @@ describe('VirtualizedList', () => {
           infos.push(info);
           return <item title={info.item.key} />;
         }}
+        getItem={(data, index) => data[index]}
+        getItemCount={(data) => data.length}
       />
     );
     expect(component).toMatchSnapshot();