自定义核心数据SectionNameKeyPath

Question

我是核心数据的新手,我正试图弄清楚如何sectionNameKeyPath在我的网站中创建自定义NSFetchedResultsController.我有一个带有属性的托管对象acctPeriod.这是一个NSString.我想根据此字段的前4个字符创建部分.前4个字符代表会计期间的年份,不需要保存.

我已经浏览了这个网站并看过关于瞬态属性的帖子,但我似乎无法让它们工作.基本上我想要这个然后分配periodYear给我sectionNameKeyPath.

@dynamic periodYear;

-(NSString *)periodYear
{
    return [self.acctPeriod substringToIndex:4];
}

任何帮助,将不胜感激.

**更新:使用Martin R.回答,我能够按预期工作.

- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
    return _fetchedResultsController;
}

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Billing" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

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

// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"acctPeriod" ascending:NO];
NSArray *sortDescriptors = @[sortDescriptor];

//Predicate
NSPredicate *pred = [NSPredicate predicateWithFormat:@"clients = %@", self.client];
NSLog(@"%@",pred);

//[fetchRequest setResultType:NSDictionaryResultType];
//[fetchRequest setReturnsDistinctResults:YES];

[fetchRequest setPredicate:pred];
[fetchRequest setSortDescriptors:sortDescriptors];

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"periodYear" cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;

NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error])
{
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

return _fetchedResultsController;  
}

Answer 1

以下内容应该有效:periodYear在托管对象子类的类扩展中实现该方法(将用作"节名称键路径"):

@interface Event (AdditionalMethods)
- (NSString *)periodYear;
@end

@implementation Event (AdditionalMethods)
- (NSString *)periodYear {
    return [self.acctPeriod substringToIndex:4];
}
@end

确保将acctPeriod其用作获取请求的第一个(或唯一的)排序描述符:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"acctPeriod" ascending:YES];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];

使用periodYear作为sectionNameKeyPath对取出结果控制器:

NSFetchedResultsController *_fetchedResultsController = [[NSFetchedResultsController alloc]
                  initWithFetchRequest:fetchRequest 
                   managedObjectContext:self.managedObjectContext 
                     sectionNameKeyPath:@"periodYear"
                              cacheName:nil];
_fetchedResultsController.delegate = self;
self.fetchedResultsController = _fetchedResultsController;

最后添加默认titleForHeaderInSection方法:

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo name];
}

或者,您可以将受管对象定义periodYear为临时属性.在这种情况下,它也不会存储在数据库中,但可以按需要计算和缓存值的方式实现.

Apple Developer Library中的DateSectionTitles示例项目演示了它的工作原理.

Answer 2

我建议不要使用瞬态属性,sectionNameKeyPath因为它会导致由获取请求获取的所有对象出错,因此该属性可以用作节路径.
您最好定义一个持久属性year并将其用作您的sectionNameKeyPath.
设置你FRC sectionNameKeyPath到year像这样:

[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                    managedObjectContext:self.managedObjectContext
                                      sectionNameKeyPath:@"year"
                                               cacheName:nil/*your chioce*/];

要在表格中将节名称显示为标题,请执行以下操作:

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    id<NSFetchedResultsSectionInfo> sec = [self.fetchedResultsController sections][section];
    return [sec name];
}