Refine and add code coverage for trait collection in collection context

Summary:
Now that the inconsistency exception is being wrapped up, I'm going to move forward with a public release some more. I'm going to go through and make sure everything is up-to-date in the GitHub repo.

This diff adds code coverage to the new trait collection exposure API in `IGListCollectionContext`. It also adds document-generating compatible commenting and positions it at a more appropriate ordering in the header.

Reviewed By: zats

Differential Revision: D51146049

fbshipit-source-id: 2d7ecc8fa042006ef4fd70451b9ab7ba6d0a66bb

Source/IGListKit/IGListAdapter.m

@@ -978,7 +978,7 @@ - (CGSize)containerSize {
     return self.collectionView.bounds.size;
 }
 
-- (UITraitCollection *_Nullable)traitCollection {
+- (UITraitCollection *)traitCollection {
     return self.collectionView.traitCollection;
 }
 

Source/IGListKit/IGListCollectionContext.h

@@ -26,8 +26,6 @@ NS_ASSUME_NONNULL_BEGIN
 NS_SWIFT_NAME(ListCollectionContext)
 @protocol IGListCollectionContext <NSObject>
 
-@property (nonatomic, nullable, readonly) UITraitCollection *traitCollection;
-
 /**
  The size of the collection view. You can use this for sizing cells.
  */
@@ -53,6 +51,11 @@ NS_SWIFT_NAME(ListCollectionContext)
  */
 @property (nonatomic, readonly) CGPoint containerContentOffset;
 
+/**
+ The trait collection of the collection view.
+ */
+@property (nonatomic, nullable, readonly) UITraitCollection *traitCollection;
+
 /**
  The current scrolling traits of the underlying collection view.
  */

Tests/IGListAdapterTests.m

@@ -1736,6 +1736,12 @@ - (void)test_whenSupplementarySourceReturnsNegativeSize_thatAdapterReturnsZero {
     XCTAssertEqual(size.height, 0.0);
 }
 
+- (void)test_whenQueryingTraitCollection_thatMatchesCollectionView {
+    self.dataSource.objects = @[@2];
+    [self.adapter reloadDataWithCompletion:nil];
+    XCTAssertEqual(self.adapter.traitCollection, self.collectionView.traitCollection);
+}
+
 - (void)test_whenQueryingContainerInset_thatMatchesCollectionView {
     self.dataSource.objects = @[@2];
     [self.adapter reloadDataWithCompletion:nil];