NSSortDescriptor - 按位置排序

Question

我在Core Data中有一个地理位置列表(实体名称是"Stops").

我想按当前位置对它们进行排序,这样我就可以向用户显示哪些位置在附近.我正在使用NSFetchedResultsController,以便可以在UITableView中轻松显示结果.

我使用以下代码尝试此类:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"stop_lat" ascending:YES comparator:^NSComparisonResult(Stops *obj1, Stops *obj2) {
    CLLocation *currentLocation = locationManager.location;

    CLLocation *obj1Location = [[CLLocation alloc]initWithLatitude:[obj1.stop_lat doubleValue] longitude:[obj1.stop_lon doubleValue]];
    CLLocation *obj2Location = [[CLLocation alloc]initWithLatitude:[obj2.stop_lat doubleValue] longitude:[obj2.stop_lon doubleValue]];

    CLLocationDistance obj1Distance = [obj1Location distanceFromLocation:currentLocation];
    CLLocationDistance obj2Distance = [obj2Location distanceFromLocation:currentLocation];

    NSLog(@"Comparing %@ to %@.\n  Obj1 Distance: %f\n  Obj2 Distance: %f",obj1.stop_name, obj2.stop_name, obj1Distance, obj2Distance);

    if (obj1Distance > obj2Distance) {
        return (NSComparisonResult)NSOrderedDescending;
    }

    if (obj1Distance < obj2Distance) {
        return (NSComparisonResult)NSOrderedAscending;
    }

    return (NSComparisonResult)NSOrderedSame;
}];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:[NSEntityDescription entityForName:[Stops entityName] inManagedObjectContext:context]];

frcNearby = [[NSFetchedResultsController alloc]
          initWithFetchRequest:fetchRequest
          managedObjectContext:context
          sectionNameKeyPath:nil
          cacheName:nil];

NSError *error;
BOOL success = [frcNearby performFetch:&error];
if (error) NSLog(@"ERROR: %@ %@", error, [error userInfo]);

但是,我的NSFetchedResultsController只返回我指定的键("stop_lat")排序的所有项目,而不是按用户的当前位置排序.

看起来我的比较器块没有被调用,因为那里的NSLog从未打印过.

我在这里错过了什么？

Answer 1

基于Objective-C的排序描述符不能与获取请求一起使用.

来自"核心数据编程指南":

...但是,总而言之,如果直接执行提取,通常不应将基于Objective-C的谓词或排序描述符添加到提取请求中.相反,您应该将这些应用于获取的结果.