ios - 具有复杂谓词的 NSFetchedResultsController

Question

我正在使用 UICollectionView 开发应用。
它是按日期排列的事件集合。我正在使用 NSFetchedResultsController 从数据库中获取数据。但我需要获取当前和 future 的事件 + 2 个过去的事件。对于 NSFetchRequest，我正在使用 NSCompoundPredicate

NSPredicate *nextPredicate = [NSPredicate predicateWithFormat: @"startAt >= %@", [NSDate date]];
NSPredicate *previousPredicate = [NSPredicate predicateWithFormat:@"startAt < %@", [NSDate date]];
NSCompoundPredicate *resultPredicate = [NSCompoundPredicate andPredicateWithSubpredicates: @[nextPredicate, previuosPredicate]];
fetchRequest.predicate = resultPredicate;

而且我不知道如何限制 previousPredicate 只接收 2 个项目。

Answer 1

如果您知道当前事件的日期，那么创建一个方法返回要获取的最后一个事件的日期，然后将主NSFetchRequest 上的谓词设置为该日期.

 -(NSDate *)dateOfPastEvent{
    NSFetchRequest *request = [[NSFetchRequest alloc]init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"EntityName" inManagedObjectContext:_mainContext];
    [request setEntity:entity];

    NSSortDescriptor *sortDesc = [NSSortDescriptor sortDescriptorWithKey:@"startAt" ascending:NO];
    [request setSortDescriptors:@[sortDesc]];

    NSError *error = nil;
    NSArray *array = [_mainContext executeFetchRequest:request error:&error];


    __block NSDate *pastEventDate;

    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop){

        if ([obj valueForKey:@"startAt"] == currentEventDate) {
            if (array.count >= idx+2) {
                pastEventDate = [[array objectAtIndex:idx+2] valueForKey:@"startAt"];
                *stop = YES;
            }

        }
    }];


    return pastEventDate;

}

现在为上述返回日期设置谓词

  NSFetchRequest *requestForController = [[NSFetchRequest alloc]init];
  NSEntityDescription *entity = [NSEntityDescription entityForName:@"EntityName" inManagedObjectContext:_mainContext];
    [requestForController setEntity:entity];

  NSPredicate *predicate = [NSPredicate predicateWithFormat: @"startAt >= %@", [self dateOfPastEvent]];