我无法理解NSManagedObjectContextConcurrencyType的文档。在以下情况下我会使用哪种类型，为什么？-(void)viewDidLoad{self.managedObjectContext=[[NSManagedObjectContextalloc]initWithConcurrencyType:???];[selfloadRecordsFromCoreData];}-(void)loadRecordsFromCoreData{[self.managedObjectContextperformBlockAndWait:^{//[self.managedOb