如何从NSFetchedResultsController获取有限制的最新对象？

Question

Maybe the title of question is not apriopriate, but here is what I need to achieve with my NSFetchedResultsController :

private func setupOnceFetchedResultsController() {

    if fetchedResultsController == nil {
        let context = NSManagedObjectContext.MR_defaultContext()
        let fetchReguest = NSFetchRequest(entityName: "WLComment")
        let createdAtDescriptor = NSSortDescriptor(key: "createdAt", ascending: true)

        fetchReguest.predicate = NSPredicate(format: "item.identifier = %d", item.identifier)
        fetchReguest.sortDescriptors = [createdAtDescriptor]
        fetchReguest.fetchLimit = 10

        fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchReguest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
        fetchedResultsController.delegate = self

        try! fetchedResultsController.performFetch()
        tableView.reloadData()
    }
}

Suppose I have about 100 comments. The oldest on the top, and the latest on the bottom. There is no problem when I need to display all of them, but here I need only 10 of them. The above NSFetchedResultsController will display the first 10 of 100 messages, but I need to display the latest 10.

Is there a way to let it know what I need?

Answer 1

Depending on how you are going to use the data later on, you can choose from at least two options:

• revert the sort descriptor, will return the end of the sorted list with reverted order;

• get the number of items first and set the appropriate fetchOffset , will return the required number of items, ordered as needed.