iphone - 如何设置从其他属性计算的 NSManagedObject 的自定义属性？

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。其中有一个属性是根据其他属性进行数学计算的。例子：

@interface Marks :  NSManagedObject  
{  
}

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

这里我想要属性total = 3xanswerGradeA + 2xanswerGradeB + 1xanswerGradeC。

如果可以这样做，那怎么做？

Answer 1

Core Data 的方法是将“total”添加为模型的属性并将其标记为“transient”。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

这将确保正确缓存和更新总数。

Answer 2

为什么不将其设为一个类别并将其编译到单独的文件中呢？（严格来说，total 不应该是 CoreData 的一部分。）

@interface Marks (Calculated)
@property (nonatomic, readonly) NSDecimalNumber* total;
@end

@implementation Marks (Calculated)
- (NSDecimalNumber*) total { 
  return whatEverYouLike; 
}
@end

Answer 3

我想对 Looji 的答案进行一些修改。

@interface Marks :  NSManagedObject  
{
}
@property (nonatomic, retain) NSDecimalNumber * answerGradeA;
@property (nonatomic, retain) NSDecimalNumber * answerGradeB;
@property (nonatomic, retain) NSDecimalNumber * answerGradeC;
@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