如何总结NSPredicate筛选的coreData属性？

Question

嗨我希望能够在我的CoreData数据库中对一个属性(rowTotal)求和,之后它被我在获取结果控制器中的谓词过滤掉了.结果将是订单的小计.

最初我以为我可以在某种程度上只是总结tableCells中显示的值,但我已经读到这不能很好地工作,因为可能只有10个tableCells用于100个项目.

我已经阅读了一个类似的问题(链接如下),该问题涉及将总和放入节标题中.我想要一个变体,我希望在managedObjectContext发生任何更改后,显示和更新总和为UILabel.

求和CoreData属性

我试过以下代码:

float subtotal = 0;
for (NSManagedObject *object in [self.fetchedResultsController fetchedObjects]) {
NSNumber *objectRowTotalNumber = [object valueForKey:@"rowTotal"];
float objectRowTotal = [objectRowTotalNumber floatValue];
subtotal = subtotal + objectRowTotal;
}
NSLog(@"Subtotal: %f", subtotal);

我已经在我的代码中的多个位置尝试过,看看我是否可以让它工作.我没有成功,并且使用此代码在tableView中不显示数据.我认为它的下面一行给了我悲伤.

for (NSManagedObject *object in [self.fetchedResultsController fetchedObjects])

我感觉它实际上创建了一个新的托管对象并干扰了我所拥有的对象.

这是我获取的结果控制器的代码

- (NSFetchedResultsController *)fetchedResultsController {
NSLog(@"I'm inside the method fetchedResultsController");


/*
 Set up the fetched results controller.
 */
// Create the fetch request for the entity.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
OrderAppDelegate *appDelegate = 
(OrderAppDelegate*) [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *managedObjectContext = appDelegate.managedObjectContext;
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Order" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

// Set the Predicate
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"orderNumber == %@", orderNumberLabel.text];
NSLog(@"predicate is: %@",predicate);
[fetchRequest setPredicate:predicate];

// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];

// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"timeStamp" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];

// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;


[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];
[sortDescriptors release];

return fetchedResultsController;
} 

我是否为此应用程序使用了正确的代码？你能看到我做错的事吗？我应该在哪里实现代码？

非常感谢任何指导,谢谢.

Answer 1

顺便说一句,你可以改变这个:

float subtotal = 0;
for (NSManagedObject *object in [self.fetchedResultsController fetchedObjects]) {
    NSNumber *objectRowTotalNumber = [object valueForKey:@"rowTotal"];
    float objectRowTotal = [objectRowTotalNumber floatValue];
    subtotal = subtotal + objectRowTotal;
}
NSLog(@"Subtotal: %f", subtotal);

对此:

CGFloat subtotal = [[[[self fetchedResultsController] fetchedObjects] valueForKeyPath:@"@sum.rowTotal"] floatValue]