如何设置从其他属性计算的NSManagedObject的Custom属性？

Question

我正在使用核心数据框架来管理对象.我有一个实体,它有几个十进制类型的属性.其中包括从其他属性数学计算的属性.例:

@interface Marks :  NSManagedObject  
{  
}

@property (nonatomic, retain) NSDecimalNumber * answerGradeA;  
@property (nonatomic, retain) NSDecimalNumber * answerGradeB;  
@property (nonatomic, retain) NSDecimalNumber * answerGradeC;  
@property (nonatomic, retain) NSDecimalNumber * total;

我想要属性total = 3xanswerGradeA + 2xanswerGradeB + 1xanswerGradeC.

如果有可能这样做,那怎么样？

Answer 1

核心数据的方式是添加"总数"作为模型的归属并将其标记为"瞬态".然后,您在子类中提供实现.

@interface Marks :  NSManagedObject  
{
}
@property (nonatomic, readonly) NSDecimalNumber* total;
@end

@implementation Marks (Calculated)
- (NSDecimalNumber*) total { 
    return (3 * [self valueForKey:@"answerGradeA"]) + (2 * [self valueForKey:@"answerGradeB"]) + [self valueForKey:@"answerGradeC"]; 
}
+ (NSSet *)keyPathsForValuesAffectingTotal
{
    return [NSSet setWithObjects:@"answerGradeA", @"answerGradeB", @"answerGradeC", nil];
}
@end

这将确保正确的缓存和更新总数.