Allow reverse-order lists

Summary:
We have a special case for treating horizontal RTL lists differently. This change adds similar functionality for capturing the correct virtualized list cell metrics when a list items children are rendered in reverse order, e.g., with `flexDirection: 'column-reverse'`.

## Changelog

[General][Fixed] Fixed FlatList to support reverse ordered items

Differential Revision: D64575365

packages/virtualized-lists/Lists/ListMetricsAggregator.js

@@ -38,6 +38,7 @@ export type CellMetrics = {
 // based implementation instead of transform.
 export type ListOrientation = {
   horizontal: boolean,
+  reversed: boolean,
   rtl: boolean,
 };
 
@@ -66,6 +67,7 @@ export default class ListMetricsAggregator {
   _orientation: ListOrientation = {
     horizontal: false,
     rtl: false,
+    reversed: false,
   };
 
   /**
@@ -265,9 +267,9 @@ export default class ListMetricsAggregator {
    * right in RTL) from a layout box.
    */
   flowRelativeOffset(layout: Layout, referenceContentLength?: ?number): number {
-    const {horizontal, rtl} = this._orientation;
+    const {horizontal, reversed, rtl} = this._orientation;
 
-    if (horizontal && rtl) {
+    if ((horizontal && rtl) || reversed) {
       const contentLength = referenceContentLength ?? this._contentLength;
       invariant(
         contentLength != null,
@@ -286,9 +288,9 @@ export default class ListMetricsAggregator {
    * Converts a flow-relative offset to a cartesian offset
    */
   cartesianOffset(flowRelativeOffset: number): number {
-    const {horizontal, rtl} = this._orientation;
+    const {horizontal, reversed, rtl} = this._orientation;
 
-    if (horizontal && rtl) {
+    if ((horizontal && rtl) || reversed) {
       invariant(
         this._contentLength != null,
         'ListMetricsAggregator must be notified of list content layout before resolving offsets',
@@ -311,6 +313,14 @@ export default class ListMetricsAggregator {
       this._measuredCellsCount = 0;
     }
 
+    if (orientation.reversed !== this._orientation.reversed) {
+      this._cellMetrics.clear();
+      this._averageCellLength = 0;
+      this._highestMeasuredCellIndex = 0;
+      this._measuredCellsLength = 0;
+      this._measuredCellsCount = 0;
+    }
+
     this._orientation = orientation;
   }
 

packages/virtualized-lists/Lists/VirtualizedList.js

@@ -252,10 +252,14 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
       return;
     }
 
-    const {horizontal, rtl} = this._orientation();
-    if (horizontal && rtl && !this._listMetrics.hasContentLength()) {
+    const {horizontal, reversed, rtl} = this._orientation();
+    if (
+      ((horizontal && rtl) || reversed) &&
+      !this._listMetrics.hasContentLength()
+    ) {
+      const mode = horizontal && rtl ? 'RTL' : 'reversed lists';
       console.warn(
-        'scrollToOffset may not be called in RTL before content is laid out',
+        `scrollToOffset may not be called in ${mode} before content is laid out`,
       );
       return;
     }
@@ -267,8 +271,8 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
   }
 
   _scrollToParamsFromOffset(offset: number): {x?: number, y?: number} {
-    const {horizontal, rtl} = this._orientation();
-    if (horizontal && rtl) {
+    const {horizontal, reversed, rtl} = this._orientation();
+    if ((horizontal && rtl) || reversed) {
       // Add the visible length of the scrollview so that the offset is right-aligned
       const cartOffset = this._listMetrics.cartesianOffset(
         offset + this._scrollMetrics.visibleLength,
@@ -1489,8 +1493,17 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
   }
 
   _orientation(): ListOrientation {
+    const horizontal = horizontalOrDefault(this.props.horizontal);
+    const contentFlexDirection = StyleSheet.flatten(
+      this.props.contentContainerStyle,
+    )?.flexDirection;
+    const reversed =
+      (horizontal && contentFlexDirection === 'row-reverse') ||
+      contentFlexDirection === 'column-reverse';
+
     return {
-      horizontal: horizontalOrDefault(this.props.horizontal),
+      horizontal,
+      reversed,
       rtl: I18nManager.isRTL,
     };
   }
@@ -1731,8 +1744,8 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
 
   _offsetFromScrollEvent(e: ScrollEvent): number {
     const {contentOffset, contentSize, layoutMeasurement} = e.nativeEvent;
-    const {horizontal, rtl} = this._orientation();
-    if (horizontal && rtl) {
+    const {horizontal, reversed, rtl} = this._orientation();
+    if ((horizontal && rtl) || reversed) {
       return (
         this._selectLength(contentSize) -
         (this._selectOffset(contentOffset) +