控制NSSortDescriptor对Core Data中的nil值进行排序的方式

给出具有核心数据的字符串的以下NSSortDescriptor ：

 [NSSortDescriptor sortDescriptorWithKey:@"series" ascending:true selector:@selector(caseInsensitiveCompare:)] 

结果按字母顺序正确排序。 但是，在series为nil情况下，具有nil值的字符串放在顶部，之后对非nil值进行排序，EG：

 [nil, nil, nil, A, B, C, D...] 

有没有办法控制这种行为？ 核心数据不允许自定义选择器。 这是我的一个类似的问题（但不是解决Core Data的限制）：

NSSortDescriptor和nil值

Answer 1

虽然您无法将自定义选择器与Core Data一起使用，但您可以将NSSortDescriptor子类NSSortDescriptor以更改默认行为。 像这样的东西应该工作：

 #define NULL_OBJECT(a) ((a) == nil || [(a) isEqual:[NSNull null]]) @interface NilsLastSortDescriptor : NSSortDescriptor {} @end @implementation NilsLastSortDescriptor - (id)copyWithZone:(NSZone*)zone { return [[[self class] alloc] initWithKey:[self key] ascending:[self ascending] selector:[self selector]]; } - (id)reversedSortDescriptor { return [[[self class] alloc] initWithKey:[self key] ascending:![self ascending] selector:[self selector]]; } - (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2 { if (NULL_OBJECT([object1 valueForKeyPath:[self key]]) && NULL_OBJECT([object2 valueForKeyPath:[self key]])) return NSOrderedSame; if (NULL_OBJECT([object1 valueForKeyPath:[self key]])) return NSOrderedDescending; if (NULL_OBJECT([object2 valueForKeyPath:[self key]])) return NSOrderedAscending; return [super compareObject:object1 toObject:object2]; } @end 

Answer 2

Swift 4.1版本：

 class NilsLastSortDescriptor: NSSortDescriptor { override func copy(with zone: NSZone? = nil) -> Any { return NilsLastSortDescriptor(key: self.key, ascending: self.ascending, selector: self.selector) } override var reversedSortDescriptor: Any { return NilsLastSortDescriptor(key: self.key, ascending: !self.ascending, selector: self.selector) } override func compare(_ object1: Any, to object2: Any) -> ComparisonResult { if (object1 as AnyObject).value(forKey: self.key!) == nil && (object2 as AnyObject).value(forKey: self.key!) == nil { return .orderedSame } if (object1 as AnyObject).value(forKey: self.key!) == nil { return .orderedDescending } if (object2 as AnyObject).value(forKey: self.key!) == nil { return .orderedAscending } return super.compare(object1, to: object2) } }