Fix crash with SectionList.scrollToLocation and private property munging

Summary:
Changelog:
[General][Fixed] - Remove illegal private property access in VirtualizedSectionList.scrollToLocation

`VirtualizedSectionList.scrollToLocation` internally uses `VirtualizedList`'s `_getFrameMetricsApprox` method. This method is private by convention (since it's `_`-prefixed), but under certain build setups this is also enforced at runtime, so using `scrollToLocation` can throw an error.

Here, we rename this internal method to `__getFrameMetricsApprox` (adding another leading underscore) which opts it out of being treated as private, while still communicating that it's not part of the public API. We also delete the Flow error suppression that masked this issue.

For reference: This convention for private methods (including the double-underscore opt out) has its roots in Facebook's pre-Babel [JSTransform](https://github.com/facebookarchive/jstransform/blob/master/visitors/es6-class-visitors.js) compiler and is implemented in Flow as [`munge_underscores=true`](https://flow.org/en/docs/config/options/#toc-munge-underscores-boolean).

Reviewed By: yungsters

Differential Revision: D33982339

fbshipit-source-id: 498563c59d42549c94fe90d363677d6d3ea35d2d

Author: motiz88

Libraries/Lists/VirtualizedList.js

@@ -384,7 +384,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   scrollToEnd(params?: ?{animated?: ?boolean, ...}) {
     const animated = params ? params.animated : true;
     const veryLast = this.props.getItemCount(this.props.data) - 1;
-    const frame = this._getFrameMetricsApprox(veryLast);
+    const frame = this.__getFrameMetricsApprox(veryLast);
     const offset = Math.max(
       0,
       frame.offset +
@@ -458,7 +458,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       });
       return;
     }
-    const frame = this._getFrameMetricsApprox(index);
+    const frame = this.__getFrameMetricsApprox(index);
     const offset =
       Math.max(
         0,
@@ -950,8 +950,8 @@ class VirtualizedList extends React.PureComponent<Props, State> {
           // See if there are any sticky headers in the virtualized space that we need to render.
           for (let ii = firstAfterInitial - 1; ii > lastInitialIndex; ii--) {
             if (stickyIndicesFromProps.has(ii + stickyOffset)) {
-              const initBlock = this._getFrameMetricsApprox(lastInitialIndex);
-              const stickyBlock = this._getFrameMetricsApprox(ii);
+              const initBlock = this.__getFrameMetricsApprox(lastInitialIndex);
+              const stickyBlock = this.__getFrameMetricsApprox(ii);
               const leadSpace =
                 stickyBlock.offset -
                 initBlock.offset -
@@ -968,7 +968,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
                 inversionStyle,
               );
               const trailSpace =
-                this._getFrameMetricsApprox(first).offset -
+                this.__getFrameMetricsApprox(first).offset -
                 (stickyBlock.offset + stickyBlock.length);
               cells.push(
                 <View key="$sticky_trail" style={{[spacerKey]: trailSpace}} />,
@@ -979,9 +979,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
           }
         }
         if (!insertedStickySpacer) {
-          const initBlock = this._getFrameMetricsApprox(lastInitialIndex);
+          const initBlock = this.__getFrameMetricsApprox(lastInitialIndex);
           const firstSpace =
-            this._getFrameMetricsApprox(first).offset -
+            this.__getFrameMetricsApprox(first).offset -
             (initBlock.offset + initBlock.length);
           cells.push(
             <View key="$lead_spacer" style={{[spacerKey]: firstSpace}} />,
@@ -1005,14 +1005,14 @@ class VirtualizedList extends React.PureComponent<Props, State> {
         this._hasWarned.keys = true;
       }
       if (!isVirtualizationDisabled && last < itemCount - 1) {
-        const lastFrame = this._getFrameMetricsApprox(last);
+        const lastFrame = this.__getFrameMetricsApprox(last);
         // Without getItemLayout, we limit our tail spacer to the _highestMeasuredFrameIndex to
         // prevent the user for hyperscrolling into un-measured area because otherwise content will
         // likely jump around as it renders in above the viewport.
         const end = this.props.getItemLayout
           ? itemCount - 1
           : Math.min(itemCount - 1, this._highestMeasuredFrameIndex);
-        const endFrame = this._getFrameMetricsApprox(end);
+        const endFrame = this.__getFrameMetricsApprox(end);
         const tailSpacerLength =
           endFrame.offset +
           endFrame.length -
@@ -1419,16 +1419,16 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     const framesInLayout = [];
     const itemCount = this.props.getItemCount(this.props.data);
     for (let ii = 0; ii < itemCount; ii++) {
-      const frame = this._getFrameMetricsApprox(ii);
+      const frame = this.__getFrameMetricsApprox(ii);
       /* $FlowFixMe[prop-missing] (>=0.68.0 site=react_native_fb) This comment
        * suppresses an error found when Flow v0.68 was deployed. To see the
        * error delete this comment and run Flow. */
       if (frame.inLayout) {
         framesInLayout.push(frame);
       }
     }
-    const windowTop = this._getFrameMetricsApprox(this.state.first).offset;
-    const frameLast = this._getFrameMetricsApprox(this.state.last);
+    const windowTop = this.__getFrameMetricsApprox(this.state.first).offset;
+    const frameLast = this.__getFrameMetricsApprox(this.state.last);
     const windowLen = frameLast.offset + frameLast.length - windowTop;
     const visTop = this._scrollMetrics.offset;
     const visLen = this._scrollMetrics.visibleLength;
@@ -1642,15 +1642,15 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     // Mark as high priority if we're close to the start of the first item
     // But only if there are items before the first rendered item
     if (first > 0) {
-      const distTop = offset - this._getFrameMetricsApprox(first).offset;
+      const distTop = offset - this.__getFrameMetricsApprox(first).offset;
       hiPri =
         hiPri || distTop < 0 || (velocity < -2 && distTop < scrollingThreshold);
     }
     // Mark as high priority if we're close to the end of the last item
     // But only if there are items after the last rendered item
     if (last < itemCount - 1) {
       const distBottom =
-        this._getFrameMetricsApprox(last).offset - (offset + visibleLength);
+        this.__getFrameMetricsApprox(last).offset - (offset + visibleLength);
       hiPri =
         hiPri ||
         distBottom < 0 ||
@@ -1752,7 +1752,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
               maxToRenderPerBatchOrDefault(this.props.maxToRenderPerBatch),
               windowSizeOrDefault(this.props.windowSize),
               state,
-              this._getFrameMetricsApprox,
+              this.__getFrameMetricsApprox,
               this._scrollMetrics,
             );
           }
@@ -1815,13 +1815,11 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     return {index, item, key: this._keyExtractor(item, index), isViewable};
   };
 
-  _getFrameMetricsApprox = (
-    index: number,
-  ): {
+  __getFrameMetricsApprox: (index: number) => {
     length: number,
     offset: number,
     ...
-  } => {
+  } = index => {
     const frame = this._getFrameMetrics(index);
     if (frame && frame.index === index) {
       // check for invalid frames due to row re-ordering

Libraries/Lists/VirtualizedSectionList.js

@@ -137,8 +137,7 @@ class VirtualizedSectionList<
       return;
     }
     if (params.itemIndex > 0 && this.props.stickySectionHeadersEnabled) {
-      // $FlowFixMe[prop-missing] Cannot access private property
-      const frame = this._listRef._getFrameMetricsApprox(
+      const frame = this._listRef.__getFrameMetricsApprox(
         index - params.itemIndex,
       );
       viewOffset += frame.length;