remove duplicate identifiers from IGListBindingSectionController objects

Summary:
I was building a new `IGListBindingSectionController` subclass and accidentally used two view models with the same ID. Was seeing strange results and realized we're not removing dups or asserting here.

Adding a call to `objectsWithDuplicateIdentifiersRemoved` when the view models are first requested.

Reviewed By: rnystrom

Differential Revision: D8303601

fbshipit-source-id: 42c62adc401feaec2c7dce2a83cfc6533599752b

CHANGELOG.md

@@ -16,6 +16,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - `[IGListAdapterUpdater performBatchUpdatesWithCollectionViewBlock:]` and `[IGListAdapterUpdater performReloadDataWithCollectionViewBlock:]` clean state and run completion blocks if their `UICollectionView` is nil. [Brandon Darin](https://github.com/jbd1030) (tbd)
 
+- Ensuring view models with duplicate diff identifiers are removed when view models are first requested by `IGListBindingSectionController` [Adam Stern](https://github.com/adamastern) (tbd)
+
 3.4.0
 -----
 

Source/IGListBindingSectionController.m

@@ -112,7 +112,8 @@ typedef NS_ENUM(NSInteger, IGListDiffingSectionState) {
     self.object = object;
 
     if (oldObject == nil) {
-        self.viewModels = [[self.dataSource sectionController:self viewModelsForObject:object] copy];
+        NSArray *viewModels = [self.dataSource sectionController:self viewModelsForObject:object];
+        self.viewModels = objectsWithDuplicateIdentifiersRemoved(viewModels);
     } else {
         IGAssert([oldObject isEqualToDiffableObject:object],
                  @"Unequal objects %@ and %@ will cause IGListBindingSectionController to reload the entire section",

Tests/IGListBindingSectionControllerTests.m

@@ -73,6 +73,19 @@
     XCTAssertEqualObjects(cell12.textField.text, @"42");
 }
 
+- (void)test_withDuplicateDiffIdentifiers_thatDuplicatesAreRemoved {
+    [self setupWithObjects:@[
+                             [[IGTestDiffingObject alloc] initWithKey:@1 objects:@[@7, @7]],
+                             ]];
+
+    XCTAssertEqual([self.collectionView numberOfSections], 1);
+    XCTAssertEqual([self.collectionView numberOfItemsInSection:0], 1);
+
+    IGTestNumberBindableCell *cell00 = [self cellAtSection:0 item:0];
+
+    XCTAssertEqualObjects(cell00.textField.text, @"7");
+}
+
 - (void)test_whenUpdating_withAddedModels_thatCellsCorrectAndConfigured {
     [self setupWithObjects:@[
                              [[IGTestDiffingObject alloc] initWithKey:@1 objects:@[@7, @"seven"]],
@@ -362,21 +375,24 @@
 
 - (void)test_whenUpdating_withMutableArrayObject_thatViewModelsDontMutate {
     NSArray *objects = @[
-                             @"foo",
+                         @"foo",
-                             @"bar"
+                         @"bar"
-                             ];
+                         ];
+
     NSMutableArray *initObjects = [NSMutableArray arrayWithArray:objects];
-    
+
     [self setupWithObjects:@[
                              [[IGTestDiffingObject alloc] initWithKey:@1 objects:initObjects]
                              ]];
     
     IGTestDiffingSectionController *section = [self.adapter sectionControllerForObject:self.dataSource.objects.firstObject];
-    
+
-    NSArray *oldModels = [section.viewModels copy];
+    XCTAssertNotEqual(initObjects, section.viewModels);
+    XCTAssertEqualObjects(initObjects, section.viewModels);
+
     [initObjects removeAllObjects];
     
-    XCTAssertEqual(oldModels, section.viewModels);
+    XCTAssertNotEqualObjects(initObjects, section.viewModels);
 }
 
 @end