Question

For the purpose of asking this question about ordering. The following MyObject class returns an instance with random generated category names.

I use the following dataSource methods:

numberOfSections accessed with [dataSource count].

titleForSection accessed with [[dataSource objectAtIndex:indexPath.section] valueForKey:@"categoryName"].

numberOfRowsInSection accessed with [[[dataSource objectAtIndex:indexPath.section] valueForKey:@"myObjects"] count].

And finally, the MyObject for each row is accessed with [[[dataSource objectAtIndex:indexPath.section] valueForKey:@"myObjects"] objectAtIndex:indexPath.row] on the cellForRowAtIndexPath method.

I use the following code to create a dataSource that displays 9 section categories, however I’m a little stuck on the ordering of these categories and the data within. Assume there’s an NSDate property as part of the MyObject class.

Question: How would I go about using this to display the records in descending order?

- (void)createDatasource
{
    NSInteger numberOfObjects = 10;
    NSMutableArray *objects = [NSMutableArray arrayWithCapacity:numberOfObjects];
    NSMutableArray *categories = [NSMutableArray arrayWithCapacity:numberOfObjects];
    for (int i = 0; i < numberOfObjects; i++)
    {
        MyObject *obj = [[MyObject alloc] init];
        [objects addObject:obj];
        [categories addObject:obj.category];
        [obj release];
    }
    NSSet *set = [NSSet setWithArray:categories];
    NSMutableArray *dataSource = [[NSMutableArray alloc] initWithCapacity:[set count]];
    for (NSString *categoryString in set)
    {
        NSMutableDictionary *mainItem = [[NSMutableDictionary alloc] initWithObjectsAndKeys:nil, @"categoryName", nil, @"myObjects", nil];
        NSMutableArray *mainItemMyObjects = [NSMutableArray array];
        [mainItem setValue:categoryString forKey:@"categoryName"];
        for (MyObject *obj in objects)
        {
            if ([obj.category isEqualToString:categoryString])
            {
                [mainItemMyObjects addObject:obj];
            }
        }
        [mainItem setValue:mainItemMyObjects forKey:@"myObjects"];
        [dataSource addObject:mainItem];
        [mainItem release];
    }
    NSLog (@"objects = %@\ncategories = %@\nset = %@\ndatasource = %@", objects, categories, set, dataSource);
}

Answer 1

Easiest would be to sort your arrays, using NSMutableArray’s sorting mutators or NSArray’s sorting methods. Otherwise you’d have to construct some sort of mapping from input indices to dataSource indices for use by the various data source methods.

---

Edit Requested sample code for sorting, something like this should work. I assume you are wanting to sort everything by a property named date on the MyObject.

// First, sort the myObject mutable array in each category
for (NSDictionary *d in dataSource) {
    [[d valueForKey:@"myObjects"] sortUsingComparator:^NSComparisonResult(id o1, id o2){
        // Compare dates. NSDate's 'compare:' would do ascending order, so if we just
        // reverse the order of comparison they'll come out descending.
        return [[o2 date] compare:[o1 date]];
    }];
}

// Second, sort the categories by the earliest dated object they contain
[dataSource sortUsingComparator:^NSComparisonResult(id o1, id o2){
    // Extract the first object from each category's array, which must be the
    // earliest it contains due to the previous sort.
    MyObject *myObject1 = [[o1 valueForKey:@"myObjects"] objectAtIndex:0];
    MyObject *myObject2 = [[o2 valueForKey:@"myObjects"] objectAtIndex:0];

    // Compare dates, as above.
    return [[myObject2 date] compare:[myObject1 date]];
}];