Add isLast/FirstSection API to IGListSectionController

Summary: Product needs show that section controllers need some sort of awarenes when they are the first/last section in a list (e.g. for cell borders). Adding this simple API, non-breaking.

Reviewed By: jessesquires

Differential Revision: D4016023

fbshipit-source-id: c75f9298fc73875a1cabad191fe2db6cb9ee4376

CHANGELOG.md

@@ -5,7 +5,7 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 ## Master
 
 - Fixed `-[IGListAdapter reloadDataWithCompletion:]` not returning early when `collectionView` or `dataSource` is nil and `completion` is nil. [Ben Asher](https://github.com/benasher44) [#51](https://github.com/Instagram/IGListKit/pull/51)
-
+- Added `-isFirstSection` and `-isLastSection` APIs to `IGListSectionController`
 
 1.0.0
 -----

Source/IGListAdapter.m

@@ -386,6 +386,9 @@ - (void)updateObjects:(NSArray *)objects {
     // for IGListSectionController subclasses after calling [super init]
     IGListSectionControllerPushThread(self.viewController, self);
 
+    id firstObject = objects.firstObject;
+    id lastObject = objects.lastObject;
+
     for (id object in objects) {
         // infra checks to see if a controller exists
         IGListSectionController <IGListSectionType> *sectionController = [map sectionControllerForObject:object];
@@ -403,6 +406,8 @@ - (void)updateObjects:(NSArray *)objects {
         // in case the section controller was created outside of -listAdapter:sectionControllerForObject:
         sectionController.collectionContext = self;
         sectionController.viewController = self.viewController;
+        sectionController.isFirstSection = (object == firstObject);
+        sectionController.isLastSection = (object == lastObject);
 
         // check if the item has changed instances or is new
         const NSUInteger oldSection = [map sectionForObject:object];

Source/IGListSectionController.h

@@ -34,6 +34,16 @@
  */
 @property (nonatomic, weak, nullable, readonly) id <IGListCollectionContext> collectionContext;
 
+/**
+ Returns YES if the section controller is the first section in the list.
+ */
+@property (nonatomic, assign, readonly) BOOL isFirstSection;
+
+/**
+ Returns YES if the section controller is the last section in the list.
+ */
+@property (nonatomic, assign, readonly) BOOL isLastSection;
+
 /**
  The margins used to lay out content in the section controller.
 

Source/Internal/IGListSectionControllerInternal.h

@@ -19,4 +19,8 @@ FOUNDATION_EXTERN void IGListSectionControllerPopThread(void);
 
 @property (nonatomic, weak, readwrite) UIViewController *viewController;
 
+@property (nonatomic, assign, readwrite) BOOL isFirstSection;
+
+@property (nonatomic, assign, readwrite) BOOL isLastSection;
+
 @end

Tests/IGListAdapterTests.m

@@ -452,4 +452,29 @@ - (void)test_whenAdapterReleased_withSectionControllerStrongRefToCollectionView_
     XCTAssertNil(weakSectionController);
 }
 
+- (void)test_whenAdapterUpdatedTwice_withThreeSections_thatSectionsUpdatedFirstLast {
+    self.dataSource.objects = @[@0, @1, @2];
+    [self.adapter reloadDataWithCompletion:nil];
+
+    XCTAssertTrue([[self.adapter sectionControllerForObject:@0] isFirstSection]);
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@1] isFirstSection]);
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@2] isFirstSection]);
+
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@0] isLastSection]);
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@1] isLastSection]);
+    XCTAssertTrue([[self.adapter sectionControllerForObject:@2] isLastSection]);
+
+    // update and shift objects to test that first/last flags are also updated
+    self.dataSource.objects = @[@2, @0, @1];
+    [self.adapter performUpdatesAnimated:NO completion:nil];
+
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@0] isFirstSection]);
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@1] isFirstSection]);
+    XCTAssertTrue([[self.adapter sectionControllerForObject:@2] isFirstSection]);
+
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@0] isLastSection]);
+    XCTAssertTrue([[self.adapter sectionControllerForObject:@1] isLastSection]);
+    XCTAssertFalse([[self.adapter sectionControllerForObject:@2] isLastSection]);
+}
+
 @end