在表格视图部分中排序核心数据对象

Question

I am using NSSortDescriptors to order core data objects and create table view sections depending on a date attribute. Using this kind of SortDescriptor, the table view sections are ordered as expected, but the rows inside the sections are also ordered by the same date attribute. Is there a way to have another ordering system inside each section? I guess the main problem is that core data stores date objects with date+time values, that way there are no objects with exactly the same date value. But I would need to order the objects inside the same section based on another attribute. Thank you.

And here is my code for the NSFetchedResultsController:

-(NSFetchedResultsController*)fetchedResultsController{

    if (_fetchedResultsController != nil){
        return _fetchedResultsController;
    }
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc]init];
    NSManagedObjectContext *context = self.managedObjectContext;
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]initWithKey:@"itemDate" ascending:YES];
    NSSortDescriptor *sortDescriptor1 = [[NSSortDescriptor alloc]initWithKey:@"itemName" ascending:NO];

    NSArray *sortDescriptors = [[NSArray alloc]initWithObjects:sortDescriptor,sortDescriptor1, nil];
    fetchRequest.sortDescriptors = sortDescriptors;
    _fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:@"sectionIdentifier" cacheName:nil];
    _fetchedResultsController.delegate = self;
    return _fetchedResultsController;
}

EDITED QUESTION

This is the piece of code added to the cellForRowAtIndexPath method:

int indexpathsection = indexPath.section;
    [self sortedSectionForIndex:indexpathsection];
    NSLog(@"INDEXPATH= %ld", (long)indexPath.section);

And this is the method proposed y Marcus:

- (NSArray*)sortedSectionForIndex:(NSInteger)index
{
    NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"priority" ascending:YES];
    id section = [[self fetchedResultsController] sections][index];
    NSLog(@"INDEX********* = %ld", (long)index);
    NSArray *objects = [section objects];
    NSArray *sorted = [objects sortedArrayUsingDescriptors:@[sort]];
    return sorted;
}

Answer 1

The NSFetchedResultsController is meant for the sort order to be controlled via a single sort or set of sorts. It is not intended to do what you are trying to do.

But you can do it, it is just not as clean.

First, the discussion of sorting a section. This does not require you to write your own sorting algorithm:

- (NSArray*)sortedSectionForIndex:(NSInteger)index
{
    NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"MyKey" ascending:YES];
    id section = [[self fetchedResultsController] sections][index];
    NSArray *objects = [section objects];
    NSArray *sorted = [objects sortedArrayUsingDescriptors:@[sort]];
    return sorted
}

With that method, you can ask for a section to be sorted whenever you want to retrieve an object. However that is inefficient as you are sorting every time you want to touch an object which in a UITableViewController is a LOT .

Instead, take this example and integrate it with your NSFetchedResultsController and its delegate methods so that you are storing the sorted arrays and keeping them in sync. That way you are only doing the sort when the data changes instead of on each method call.

Update

The sort code I provided to you does not sort what is inside of the NSFetchedResultsController . It takes what is in the section, sorts it and returns you the sorted array. So you need to use what it returns:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSArray *sorted = [self sortedSectionForIndex:[indexPath section]];
    id object = [sorted objectAtIndex:[indexPath row]];

    id cell = ...; //Now get your cell reference

    //Now update your cell with the data from object

    return cell;
}

Answer 2

You can normalize the time component of your attribute in the awakeFromFetch method of your NSManagedObject .

- (void) awakeFromFetch {
    [super awakeFromFetch];
    NSDateComponents* comps = [[NSCalendar currentCalendar] components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit  fromDate:self.itemDate];
    self.itemDate = [[NSCalendar currentCalendar] dateFromComponents:comps];     
}

In addition, looking at your code, you have to tell the NSFetchedResultsController on what it should base the section breaks. You do this by providing a keyPath for the section breaks when you initialize it. For your case, instead of using sectionNameKeyPath:@"sectionIdentifier" , use the first sort key:

    _fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:@"itemDate" cacheName:nil];