Parse.com API: How to query for objects incrementally?

Parse.com API: How to query for objects incrementally?

I have a table view controller that is fetching objects from a database
hosted by parse.com. At the moment, I have the view controller fetch all
the objects in the database for storage in an array. Right now, there are
100 objects in the database, and what I would like to do, is have the
table view controller fetch 20 of those objects and display them, then
have it fetch and display 20 more when the table scrolls to the bottom.
Here is my init method:
- (id)initWithStyle:(UITableViewStyle)style{
self = [super initWithStyle:style];
if (self) {
//create the array, set the table view size, then populate it.
listings = [[NSMutableArray alloc] init];
[self populateArrayWithListings];
}
return self;
}
[self populateArrayWithListings]; simply fills the array listings with the
100 objects in the database. I have this method to detect when the table
view controller is scrolled to at the bottom:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
if (scrollView.contentOffset.y + scrollView.bounds.size.height >
scrollView.contentSize.height * 0.9) {
}
}
Question is, what should I put inside those brackets to have it fetch the
next 20 objects in the database?