使用部分日期的iOS coredata排序描述符？

Question

在我的应用程序中,我有一个表格视图,显示一个人的活动随着时间的推移.我想按天排序数据,然后按名字排序.问题是,由于其他原因,我需要在时间中存储NSDate.

现在,我有这个:

August 18, 2014 11:02 AM  Bill B.
August 18, 2014 10:20 AM  John D.
August 18, 2014 10:05 AM  Adam S.
August 11, 2014 3:47 PM   Debbie B.
August 11, 2014 2:13 PM   Adam S.

我想要忽略时间,只评估日期.像这样:

August 18, 2014  Adam S.
August 18, 2014  Bill B.
August 18, 2014  John D.
August 11, 2014  Adam S.
August 11, 2014  Debbie B.

是否可以构建一个仅查看核心数据日期属性的月,日和年的排序描述符？

Answer 1

我会使用一个额外的属性sortDate.您可以在对象的setter startDate(或者您调用日期属性)中计算该属性.如果您在核心数据中存储多个对象,这是最快的方法.您可以使用瞬态属性并计算getter中的值,但这会对性能产生很大影响.

我在我的一个应用程序中使用了类似的东西:

- (void)setStartDate:(NSDate *)startDate {
    [self willChangeValueForKey:@"startDate"];
    [self setPrimitiveStartDate:startDate];
    [self didChangeValueForKey:@"startDate"];

    if (startDate) {
        NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
//      might want to save sortDate in UTC
//      calendar.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];
        NSDate *sortDate;
        if ([calendar rangeOfUnit:NSDayCalendarUnit startDate:&sortDate interval:NULL forDate:startDate]) {
            self.sortDate = sortDate;
        }
        else {
            NSLog(@"Can't create sort date from date %@", startDate);
        }
    }
    else {
        self.sortDate = nil;
    }
}

然后fetchRequest将使用sortDate和startDate进行排序和sortDate切片.

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"MyEntity"];
NSSortDescriptor *sdSortDate = [NSSortDescriptor sortDescriptorWithKey:@"sortDate" ascending:NO];
NSSortDescriptor *sdStartDate = [NSSortDescriptor sortDescriptorWithKey:@"startDate" ascending:NO];
request.sortDescriptors = @[sdSortDate, sdStartDate];

NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"sortDate" cacheName:@"MyFancyViewControllerCache"];