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。如果这是一个普通的属性，我会简单地覆盖setter / getters，但由于它们是由Core Data生成的，所以我不知道该怎么做。

纠正方法是什么？我不想仅仅为了计算它们而取出患者，而且我不想将KVO用于这么简单的事情。

Answer 1

您是否知道您可以直接从hospital.patients.count获得患者数量？为什么要为此保留一个属性？

但如果必须，那么在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];
    }

在关系中添加/删除对象时更新属性

1 个答案: