如何设置从其他属性计算的NSManagedObject的自定义属性？

问题描述

我使用核心数据框架来管理对象。我有一个实体有几个属性的十进制类型。其中是从其他属性数学计算的属性。示例：

I am using core data framework to manage objects. I have an entity which has several attributes of decimal types. Among them is an attribute which is mathematically calculated from other attributes. Example:

@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，那么如何？

If it is possible to do like this, then how?

推荐答案

Core Data方法是将total添加为模型的属性并将其标记为transient。然后在一个子类中提供实现。

The Core Data way is to add 'total' as an attribue to the model and mark it 'transient'. You then provide the implementation in a subclass.

@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计。

This will ensure proper caching and updating of total.

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