insertItemsWithIdentifiers:afterItemWithIdentifier: and - insertSectionsWithIdentifiers:afterSectionWithIdentifier:, their behavior seems to be inconsistent. (void)testInsertItemsWithIdentifiersAfterItemWithIdentifier_IfHasExist {     NSArray *items = @[@"1",@"2",@"3",@"5"];     NSArray *insertItems = @[@"3",@"4"];     NSArray *sections = @[@1,@2];     [self.snapshot appendSectionsWithIdentifiers:sections];     [self.snapshot appendItemsWithIdentifiers:items];     [self.snapshot insertItemsWithIdentifiers:insertItems afterItemWithIdentifier:@"5"];     NSArray *expect = @[@"1",@"2",@"3",@"5",@"4"];     XCTAssertEqualObjects([self.snapshot itemIdentifiers], expect); } It has a failure,because [self.snapshot itemIdentifiers] is 1,2,5,3,4 but on the other hand     NSArray *sections = @[@1,@2,@3,@5];     NSArray *insertSections = @[@3,@4];     [self.snapshot appendSectionsWithIdentifiers:sections];     [self.snapshot insertSectionsWithIdentifiers:insertSections afterSectionWithIdentifier:@5];     NSArray *expect = @[@1,@2,@3,@5,@4];     XCTAssertEqualObjects([self.snapshot sectionIdentifiers], expect); } It is success.