Handle nil section controllers from data source

Summary:
Removing the assert and handling `nil` section controllers. This wont effect Swift code which demands a non-optional section controller, but Objective-C is more nuanced. There is evidence that some callers do not have a default state and are returning `nil`.

Unfortunately this will [OOB here](https://github.com/Instagram/IGListKit/blob/master/Source/Internal/IGListSectionMap.m#L63) if using the original `objects` array provided by the data source. Instead we prune the array, only allowing in objects that have a matching section controller.

Fixes t15773862 internally.

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
Closes https://github.com/Instagram/IGListKit/pull/488

Reviewed By: dshahidehpour

Differential Revision: D4553886

Pulled By: rnystrom

fbshipit-source-id: a473a99b5eb513e4b610019dba0394a014193fc4

CHANGELOG.md

@@ -30,6 +30,10 @@ This release closes the [3.0.0 milestone](https://github.com/Instagram/IGListKit
 
 - You can now manually move items (cells) within a section controller, ex: `[self.collectionContext moveInSectionController:self fromIndex:0 toIndex:1]`. [Ryan Nystrom](https://github.com/rnystrom) [(#418)](https://github.com/Instagram/IGListKit/pull/418)
 
+### Fixes
+
+- Gracefully handle a `nil` section controller returned by an `IGListAdapterDataSource`. [Ryan Nystrom](https://github.com/rnystrom) [(tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
+
 2.2.0
 -----
 

Source/IGListAdapter.m

@@ -476,7 +476,9 @@
     }
 #endif
 
-    NSMutableArray<IGListSectionController <IGListSectionType> *> *sectionControllers = [[NSMutableArray alloc] init];
+    NSMutableArray<IGListSectionController <IGListSectionType> *> *sectionControllers = [NSMutableArray new];
+    NSMutableArray *validObjects = [NSMutableArray new];
+
     IGListSectionMap *map = self.sectionMap;
 
     // collect items that have changed since the last update
@@ -498,8 +500,9 @@
             sectionController = [dataSource listAdapter:self sectionControllerForObject:object];
         }
 
-        IGAssert(sectionController != nil, @"Data source <%@> cannot return a nil section controller.", dataSource);
         if (sectionController == nil) {
+            IGLKLog(@"WARNING: Ignoring nil section controller returned by data source %@ for object %@.",
+                    dataSource, object);
             continue;
         }
 
@@ -516,12 +519,13 @@
         }
 
         [sectionControllers addObject:sectionController];
+        [validObjects addObject:object];
     }
 
     // clear the view controller and collection context
     IGListSectionControllerPopThread();
 
-    [map updateWithObjects:objects sectionControllers:sectionControllers];
+    [map updateWithObjects:validObjects sectionControllers:sectionControllers];
 
     // now that the maps have been created and contexts are assigned, we consider the section controller "fully loaded"
     for (id object in updatedObjects) {

Tests/IGListAdapterTests.m

@@ -1060,4 +1060,12 @@ XCTAssertEqual(CGPointEqualToPoint(point, p), YES); \
     [mockDelegate verify];
 }
 
+- (void)test_whenDataSourceDoesntHandleObject_thatObjectIsDropped {
+    // IGListTestAdapterDataSource does not handle NSStrings
+    self.dataSource.objects = @[@1, @"dog", @2];
+    [self.adapter reloadDataWithCompletion:nil];
+    NSArray *expected = @[@1, @2];
+    XCTAssertEqualObjects(self.adapter.objects, expected);
+}
+
 @end

Tests/Objects/IGListTestAdapterDataSource.m

@@ -20,8 +20,10 @@
 }
 
 - (IGListSectionController <IGListSectionType> *)listAdapter:(IGListAdapter *)listAdapter sectionControllerForObject:(id)object {
-    IGListTestSection *list = [[IGListTestSection alloc] init];
-    return list;
+    if ([object isKindOfClass:[NSNumber class]]) {
+        return [IGListTestSection new];
+    }
+    return nil;
 }
 
 - (nullable UIView *)emptyViewForListAdapter:(IGListAdapter *)listAdapter {