Container size doesnt use content inset, add new APIs

Summary:
The content inset of a collection view can change at any time (as it does with our refresh control) and isn't a good measure of the container size. I don't want to totally remove that API though, so I changed the default behavior, added an insets API, and also added the functionality of the original in a new API.

This makes sizes much more deterministic.

Reviewed By: jessesquires

Differential Revision: D4800758

fbshipit-source-id: 85ce843b5b1c297cea2e2ea705fa255617cbe356

Examples/Examples-iOS/IGListKitExamples/SectionControllers/ImageSectionController.m

@@ -15,6 +15,10 @@
 #import "ImageSectionController.h"
 #import "PhotoCell.h"
 
+@interface ImageSectionController () <IGListSupplementaryViewSource>
+
+@end
+
 @implementation ImageSectionController
 
 #pragma mark - IGListSectionType
@@ -39,4 +43,22 @@
     
 }
 
+- (id<IGListSupplementaryViewSource>)supplementaryViewSource {
+    return self;
+}
+
+- (NSArray<NSString *> *)supportedElementKinds {
+    return @[UICollectionElementKindSectionFooter];
+}
+
+- (CGSize)sizeForSupplementaryViewOfKind:(NSString *)elementKind atIndex:(NSInteger)index {
+    return CGSizeMake(self.collectionContext.containerSize.width, 30);
+}
+
+- (UICollectionReusableView *)viewForSupplementaryElementOfKind:(NSString *)elementKind atIndex:(NSInteger)index {
+    UICollectionReusableView *view = [self.collectionContext dequeueReusableSupplementaryViewOfKind:elementKind forSectionController:self class:[UICollectionReusableView class] atIndex:index];
+    view.backgroundColor = [UIColor yellowColor];
+    return view;
+}
+
 @end