Fix a bug in IGListBindingSingleSectionController

Summary:
Originally we want to update the cell when the SectionController is asked to update to a new value, that's pretty much what the experiment `singleItemSectionUpdates` is for inside `IGListAdapterUpdater`.

However, we should not request any cell during the initial setup, otherwise it would trigger cell to render in a wrong time, in particular we shouldn't call

```
UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
```
as the data source set up is not ready, and it can get crashy to compare the initial dataSource(nil) to the valid data source(IGListAdapter), which doesn't make sense.

So tl;dr is do not access cell during the initial data source setup. We might need to audit other callsite that might have violate this invariant.

Solution here is to rely on the willDisplay/endDisplay API from `IGListDisplayDelegate` so that we can set/unset the displaying cell here before we configure the cell with the updated data. This change is also safe as it's protected by `_enabledCellConfigurationDuringUpdate` which defaults to be NO.

= Facebook =

Also currently the `singleItemSectionUpdates` is only setup for Inbox thread-bump useage for now. So I added a `enabledCellConfigurationDuringUpdate` to properly gate it behind QE so that we have a safer exit here.

Reviewed By: maxolls

Differential Revision: D19332040

fbshipit-source-id: cbca7a63b0486c053ceabe759ce316f5d841fef5

Source/IGListKit/IGListBindingSingleSectionController.h

@@ -20,6 +20,9 @@ NS_ASSUME_NONNULL_BEGIN
 NS_SWIFT_NAME(ListBindingSingleSectionController)
 @interface IGListBindingSingleSectionController<__covariant ViewModel : id<IGListDiffable>, Cell : UICollectionViewCell *> : IGListSectionController
 
+// Testing the stability of this infra. Defaults to NO.
+@property (nonatomic, readwrite, assign) BOOL enabledCellConfigurationDuringUpdate;
+
 #pragma mark - Subclass
 
 // Required to be implemented by subclass.

Source/IGListKit/IGListBindingSingleSectionController.m

@@ -9,9 +9,20 @@
 
 #import <IGListDiffKit/IGListAssert.h>
 
+@interface IGListBindingSingleSectionController () <IGListDisplayDelegate>
+
+@end
 
 @implementation IGListBindingSingleSectionController {
     id _item;
+    __weak UICollectionViewCell *_displayingCell;
+}
+
+- (instancetype)init {
+    if (self = [super init]) {
+        self.displayDelegate = self;
+    }
+    return self;
 }
 
 - (void)didSelectItemWithCell:(UICollectionViewCell *)cell {
@@ -44,6 +55,26 @@
     return CGSizeZero;
 }
 
+#pragma mark - IGListDisplayDelegate
+
+- (void)listAdapter:(nonnull IGListAdapter *)listAdapter willDisplaySectionController:(nonnull IGListSectionController *)sectionController {
+    // no-op
+}
+
+- (void)listAdapter:(nonnull IGListAdapter *)listAdapter didEndDisplayingSectionController:(nonnull IGListSectionController *)sectionController {
+    // no-op
+}
+
+- (void)listAdapter:(nonnull IGListAdapter *)listAdapter willDisplaySectionController:(nonnull IGListSectionController *)sectionController cell:(nonnull UICollectionViewCell *)cell atIndex:(NSInteger)index {
+    IGParameterAssert(index == 0);
+    _displayingCell = cell;
+}
+
+- (void)listAdapter:(nonnull IGListAdapter *)listAdapter didEndDisplayingSectionController:(nonnull IGListSectionController *)sectionController cell:(nonnull UICollectionViewCell *)cell atIndex:(NSInteger)index {
+    IGParameterAssert(index == 0);
+    _displayingCell = nil;
+}
+
 #pragma mark - IGListSectionController Overrides
 
 - (NSInteger)numberOfItems {
@@ -66,9 +97,10 @@
 - (void)didUpdateToObject:(id)object {
     _item = object;
     
-    UICollectionViewCell *cell = [self.collectionContext cellForItemAtIndex:0 sectionController:self];
-    if (cell) {
-        [self configureCell:cell withViewModel:_item];
+    if (_enabledCellConfigurationDuringUpdate) {
+        if (_displayingCell) {
+            [self configureCell:_displayingCell withViewModel:_item];
+        }
     }
 }