Fix virtualization logic with horizontal RTL lists (facebook#38529)

Summary:
Pull Request resolved: facebook#38529

VirtualizedList internally represents metrics along the scrolling axis using `offset` (x, y), and `length` (width, height). This one dimensional representation allows the list to be generic to being horizontal or vertical.

Right now offset conversion directly takes the `x` or `y` axis value, but  this is not correct when we are using a horizontal FlatList in RTL.

This change converts most VirtualizedList code handling  `offset,length` coordinates consistently flow from start to end, including in horizontal RTL and in inverted FlatList.

This is paired with a fix to Android native code in D47627115. With these together, basic Horizontal FlatList scenarios should work correctly when laid out in RTL.

Changelog:
[General][Fixed] - Fix virtualization logic with horizontal RTL lists

Reviewed By: vincentriemer

Differential Revision: D46586420

fbshipit-source-id: 56a29abc4bdf3b9505b510bcc207fd750c3c08a7

packages/virtualized-lists/Lists/ListMetricsAggregator.js

@@ -8,6 +8,7 @@
  * @format
  */
 
+import type {Layout} from 'react-native/Libraries/Types/CoreEventTypes';
 import type {Props as VirtualizedListProps} from './VirtualizedListProps';
 import {keyExtractor as defaultKeyExtractor} from './VirtualizeUtils';
 
@@ -23,7 +24,8 @@ export type CellMetrics = {
    */
   length: number,
   /**
-   * Offset to the cell along the scrolling axis
+   * Distance between this cell and the start of the list along the scrolling
+   * axis
    */
   offset: number,
   /**
@@ -32,6 +34,13 @@ export type CellMetrics = {
   isMounted: boolean,
 };
 
+// TODO: `inverted` can be incorporated here if it is moved to an order
+// based implementation instead of transform.
+export type ListOrientation = {
+  horizontal: boolean,
+  rtl: boolean,
+};
+
 /**
  * Subset of VirtualizedList props needed to calculate cell metrics
  */
@@ -44,52 +53,85 @@ export type CellMetricProps = {
   ...
 };
 
+type UnresolvedCellMetrics = {
+  index: number,
+  layout: Layout,
+  isMounted: boolean,
+
+  // The length of list content at the time of layout is needed to correctly
+  // resolve flow relative offset in RTL. We are lazily notified of this after
+  // the layout of the cell, unless the cell relayout does not cause a length
+  // change. To keep stability, we use content length at time of query, or
+  // unmount if never queried.
+  listContentLength?: ?number,
+};
+
 /**
  * Provides an interface to query information about the metrics of a list and its cells.
  */
 export default class ListMetricsAggregator {
   _averageCellLength = 0;
-  _frames: {[string]: CellMetrics} = {};
+  _cellMetrics: {[string]: UnresolvedCellMetrics} = {};
+  _contentLength: ?number;
   _highestMeasuredCellIndex = 0;
-  _totalCellLength = 0;
-  _totalCellsMeasured = 0;
+  _measuredCellsLength = 0;
+  _measuredCellsCount = 0;
+  _orientation: ListOrientation = {
+    horizontal: false,
+    rtl: false,
+  };
 
   /**
    * Notify the ListMetricsAggregator that a cell has been laid out.
    *
    * @returns whether the cell layout has changed since last notification
    */
-  notifyCellLayout(
+  notifyCellLayout({
+    cellIndex,
+    cellKey,
+    orientation,
+    layout,
+  }: {
+    cellIndex: number,
     cellKey: string,
-    index: number,
-    length: number,
-    offset: number,
-  ): boolean {
-    const next: CellMetrics = {
-      offset,
-      length,
-      index,
+    orientation: ListOrientation,
+    layout: Layout,
+  }): boolean {
+    this._invalidateIfOrientationChanged(orientation);
+
+    const next: UnresolvedCellMetrics = {
+      index: cellIndex,
+      layout: layout,
       isMounted: true,
     };
-    const curr = this._frames[cellKey];
+    const curr = this._cellMetrics[cellKey];
+
     if (
       !curr ||
-      next.offset !== curr.offset ||
-      next.length !== curr.length ||
-      index !== curr.index
+      this._selectOffset(next.layout) !== this._selectOffset(curr.layout) ||
+      this._selectLength(next.layout) !== this._selectLength(curr.layout) ||
+      (curr.listContentLength != null &&
+        curr.listContentLength !== this._contentLength)
     ) {
-      this._totalCellLength += next.length - (curr ? curr.length : 0);
-      this._totalCellsMeasured += curr ? 0 : 1;
+      if (curr) {
+        const dLength =
+          this._selectLength(next.layout) - this._selectLength(curr.layout);
+        this._measuredCellsLength += dLength;
+      } else {
+        this._measuredCellsLength += this._selectLength(next.layout);
+        this._measuredCellsCount += 1;
+      }
+
       this._averageCellLength =
-        this._totalCellLength / this._totalCellsMeasured;
-      this._frames[cellKey] = next;
+        this._measuredCellsLength / this._measuredCellsCount;
+      this._cellMetrics[cellKey] = next;
       this._highestMeasuredCellIndex = Math.max(
         this._highestMeasuredCellIndex,
-        index,
+        cellIndex,
       );
       return true;
     } else {
-      this._frames[cellKey].isMounted = true;
+      this._cellMetrics[cellKey].isMounted = true;
       return false;
     }
   }
@@ -98,12 +140,31 @@ export default class ListMetricsAggregator {
    * Notify ListMetricsAggregator that a cell has been unmounted.
    */
   notifyCellUnmounted(cellKey: string): void {
-    const curr = this._frames[cellKey];
+    const curr = this._cellMetrics[cellKey];
     if (curr) {
-      this._frames[cellKey] = {...curr, isMounted: false};
+      this._cellMetrics[cellKey] = {
+        ...curr,
+        isMounted: false,
+        listContentLength: curr.listContentLength ?? this._contentLength,
+      };
     }
   }
 
+  /**
+   * Notify ListMetricsAggregator that the lists content container has been laid out.
+   */
+  notifyListContentLayout({
+    orientation,
+    layout,
+  }: {
+    orientation: ListOrientation,
+    layout: $ReadOnly<{width: number, height: number}>,
+  }): void {
+    this._invalidateIfOrientationChanged(orientation);
+    const newLength = this._selectLength(layout);
+    this._contentLength = newLength;
+  }
+
   /**
    * Return the average length of the cells which have been measured
    */
@@ -150,17 +211,20 @@ export default class ListMetricsAggregator {
     const {data, getItem, getItemCount, getItemLayout} = props;
     invariant(
       index >= 0 && index < getItemCount(data),
-      'Tried to get frame for out of range index ' + index,
+      'Tried to get metrics for out of range cell index ' + index,
     );
     const keyExtractor = props.keyExtractor ?? defaultKeyExtractor;
-    const frame = this._frames[keyExtractor(getItem(data, index), index)];
-    if (!frame || frame.index !== index) {
-      if (getItemLayout) {
-        const {length, offset} = getItemLayout(data, index);
-        return {index, length, offset, isMounted: true};
-      }
+    const frame = this._cellMetrics[keyExtractor(getItem(data, index), index)];
+    if (frame && frame.index === index) {
+      return this._resolveCellMetrics(frame);
     }
-    return frame;
+
+    if (getItemLayout) {
+      const {length, offset} = getItemLayout(data, index);
+      return {index, length, offset, isMounted: true};
+    }
+
+    return null;
   }
 
   /**
@@ -176,4 +240,93 @@ export default class ListMetricsAggregator {
       return frameMetrics.offset + remainder * frameMetrics.length;
     }
   }
+
+  /**
+   * Returns the length of all ScrollView content along the scrolling axis.
+   */
+  getContentLength(): number {
+    return this._contentLength ?? 0;
+  }
+
+  /**
+   * Whether a content length has been observed
+   */
+  hasContentLength(): boolean {
+    return this._contentLength != null;
+  }
+
+  /**
+   * Converts a cartesian offset along the x or y axis to a flow-relative
+   * offset, (e.g. starting from the left in LTR, but right in RTL).
+   */
+  flowRelativeOffset(layout: Layout, referenceContentLength?: ?number): number {
+    const {horizontal, rtl} = this._orientation;
+
+    if (horizontal && rtl) {
+      const contentLength = referenceContentLength ?? this._contentLength;
+      invariant(
+        contentLength != null,
+        'ListMetricsAggregator must be notified of list content layout before resolving offsets',
+      );
+      return contentLength - this._selectOffset(layout);
+    } else {
+      return this._selectOffset(layout);
+    }
+  }
+
+  /**
+   * Converts a flow-relative offset to a cartesian offset
+   */
+  cartesianOffset(flowRelativeOffset: number): number {
+    const {horizontal, rtl} = this._orientation;
+
+    if (horizontal && rtl) {
+      invariant(
+        this._contentLength != null,
+        'ListMetricsAggregator must be notified of list content layout before resolving offsets',
+      );
+      return this._contentLength - flowRelativeOffset;
+    } else {
+      return flowRelativeOffset;
+    }
+  }
+
+  _invalidateIfOrientationChanged(orientation: ListOrientation): void {
+    if (orientation.rtl !== this._orientation.rtl) {
+      this._cellMetrics = {};
+    }
+
+    if (orientation.horizontal !== this._orientation.horizontal) {
+      this._averageCellLength = 0;
+      this._contentLength = 0;
+      this._highestMeasuredCellIndex = 0;
+      this._measuredCellsLength = 0;
+      this._measuredCellsCount = 0;
+    }
+
+    this._orientation = orientation;
+  }
+
+  _selectLength({
+    width,
+    height,
+  }: $ReadOnly<{width: number, height: number, ...}>): number {
+    return this._orientation.horizontal ? width : height;
+  }
+
+  _selectOffset({x, y}: $ReadOnly<{x: number, y: number, ...}>): number {
+    return this._orientation.horizontal ? x : y;
+  }
+
+  _resolveCellMetrics(metrics: UnresolvedCellMetrics): CellMetrics {
+    metrics.listContentLength ??= this._contentLength;
+    const {index, layout, isMounted, listContentLength} = metrics;
+
+    return {
+      index,
+      length: this._selectLength(layout),
+      isMounted,
+      offset: this.flowRelativeOffset(layout, listContentLength),
+    };
+  }
 }