ios - 在关系中添加/删除对象时更新属性

Question

考虑这个模型:

@class Patient;

@interface Hospital : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * patientCount;
@property (nonatomic, retain) NSSet *patients;
@end

@interface Hospital (CoreDataGeneratedAccessors)

- (void)addPatientsObject:(Patient *)value;
- (void)removePatientsObject:(Patient *)value;
- (void)addPatients:(NSSet *)values;
- (void)removePatients:(NSSet *)values;

@end

我想在每次添加或删除患者时更新 patientCount。如果这是一个普通属性，我会简单地覆盖 setter/getter，但由于它们是由 Core Data 生成的，我不知道该怎么做。

正确的方法是什么？我不想只是为了统计病人的人数，也不想将 KVO 用于这种简单的事情。

Answer 1

您知道您可以直接从 hospital.patients.count 获取患者数量吗？为什么要为此保留一个属性？

但如果必须，则在您的 managedObjectSubclass 中实现与这些类似的方法，并更新这些方法中的相关属性。

    - (void)addTasksObject:(TreeNode *)value {    
        NSSet *changedObjects = [[NSSet alloc] initWithObjects:&value count:1];
        [self willChangeValueForKey:@"tasks" withSetMutation:NSKeyValueUnionSetMutation usingObjects:changedObjects];
        [[self primitiveValueForKey:@"tasks"] addObject:value];
        [self didChangeValueForKey:@"tasks" withSetMutation:NSKeyValueUnionSetMutation usingObjects:changedObjects];
        // Add code to update self.patientsCount here
        self.patientsCount = [NSNumber numberWithInt:[self.patientsCount intValue] + 1];
    }

    - (void)removeTasksObject:(TreeNode *)value {
        NSSet *changedObjects = [[NSSet alloc] initWithObjects:&value count:1];
        [self willChangeValueForKey:@"tasks" withSetMutation:NSKeyValueMinusSetMutation usingObjects:changedObjects];
        [[self primitiveValueForKey:@"tasks"] removeObject:value];
        [self didChangeValueForKey:@"tasks" withSetMutation:NSKeyValueMinusSetMutation usingObjects:changedObjects];
        // Add code to update self.patientsCount here (better check not negative)
        self.patientsCount = [NSNumber numberWithInt:[self.patientsCount intValue] - 1];
    }

    - (void)addTasks:(NSSet *)value {    
        [self willChangeValueForKey:@"tasks" withSetMutation:NSKeyValueUnionSetMutation usingObjects:value];
        [[self primitiveValueForKey:@"tasks"] unionSet:value];
        [self didChangeValueForKey:@"tasks" withSetMutation:NSKeyValueUnionSetMutation usingObjects:value];
        self.patientsCount = [NSNumber numberWithInt:self.patients.count];
    }

    - (void)removeTasks:(NSSet *)value {
        [self willChangeValueForKey:@"tasks" withSetMutation:NSKeyValueMinusSetMutation usingObjects:value];
        [[self primitiveValueForKey:@"tasks"] minusSet:value];
        [self didChangeValueForKey:@"tasks" withSetMutation:NSKeyValueMinusSetMutation usingObjects:value];
        self.patientsCount = [NSNumber numberWithInt:self.patients.count];
    }