Make sure VirtualizedList's windowSize is greater than 0

Summary:
Setting `windowSize = 0` doesn't make sense. Let's make sure we catch this problem in the constructor so that it doesn't cause inexplicable list behavior.

Also fixed an invariant in `VirtualizeUtils` that is meant to prohibit non-monotonically-increasing offset arrays. As written, the invariant condition can never actually be violated.

Reviewed By: sahrens

Differential Revision: D6625302

fbshipit-source-id: b2a983cbe7bb5fbe0aed7c5d59e69a8a00672993

Author: logandaniels

Libraries/Lists/VirtualizeUtils.js

@@ -25,17 +25,19 @@ function elementsThatOverlapOffsets(
   getFrameMetrics: (index: number) => {length: number, offset: number},
 ): Array<number> {
   const out = [];
+  let outLength = 0;
   for (let ii = 0; ii < itemCount; ii++) {
     const frame = getFrameMetrics(ii);
     const trailingOffset = frame.offset + frame.length;
     for (let kk = 0; kk < offsets.length; kk++) {
       if (out[kk] == null && trailingOffset >= offsets[kk]) {
         out[kk] = ii;
+        outLength++;
         if (kk === offsets.length - 1) {
           invariant(
-            out.length === offsets.length,
-            'bad offsets input, should be in increasing order ' +
-              JSON.stringify(offsets),
+            outLength === offsets.length,
+            'bad offsets input, should be in increasing order: %s',
+            JSON.stringify(offsets),
           );
           return out;
         }

Libraries/Lists/VirtualizedList.js

@@ -535,6 +535,11 @@ class VirtualizedList extends React.PureComponent<Props, State> {
         'to support native onScroll events with useNativeDriver',
     );
 
+    invariant(
+      props.windowSize > 0,
+      'VirtualizedList: The windowSize prop must be present and set to a value greater than 0.',
+    );
+
     this._fillRateHelper = new FillRateHelper(this._getFrameMetrics);
     this._updateCellsToRenderBatcher = new Batchinator(
       this._updateCellsToRender,

Libraries/Lists/__tests__/VirtualizeUtils-test.js

@@ -81,4 +81,15 @@ describe('elementsThatOverlapOffsets', function() {
       elementsThatOverlapOffsets(offsets, frames.length, ii => frames[ii]),
     ).toEqual([1]);
   });
+  it('errors on non-increasing offsets', function() {
+    const offsets = [150, 50];
+    const frames = [
+      {offset: 0, length: 50},
+      {offset: 50, length: 150},
+      {offset: 250, length: 100},
+    ];
+    expect(() => {
+      elementsThatOverlapOffsets(offsets, frames.length, ii => frames[ii]);
+    }).toThrowErrorMatchingSnapshot();
+  });
 });

Libraries/Lists/__tests__/__snapshots__/VirtualizeUtils-test.js.snap

@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`elementsThatOverlapOffsets errors on non-increasing offsets 1`] = `"bad offsets input, should be in increasing order: [150,50]"`;