Rework scroll management

Reviewed By: devknoll

Differential Revision: D3138464

fb-gh-sync-id: fd1bb7f623c689836ae52ea8bf384d8efccddfea
fbshipit-source-id: fd1bb7f623c689836ae52ea8bf384d8efccddfea

Libraries/Experimental/WindowedListView.js

@@ -154,6 +154,11 @@ type Props = {
    * This determines how frequently events such as scroll and layout can trigger a re-render.
    */
   recomputeRowsBatchingPeriod: number;
+  /**
+   * Called when rows will be mounted/unmounted. Mounted rows always form a contiguous block so it is expressed as a
+   * range of start plus count.
+   */
+  onMountedRowsWillChange: (firstIdx: number, count: number) => void;
 };
 const defaultProps = {
   enableDangerousRecycling: false,
@@ -368,6 +373,7 @@ class WindowedListView extends React.Component {
       }
     }
     if (this.state.firstRow !== firstRow || this.state.lastRow !== lastRow) {
+      this.props.onMountedRowsWillChange && this.props.onMountedRowsWillChange(firstRow, lastRow - firstRow + 1);
       console.log('WLV: row render range changed:', {firstRow, lastRow});
     }
     this.setState({firstRow, lastRow});