Expand/collapse UITableView sections with a backing NSFetchedResultsController

Question

    let frc = NSFetchedResultsController(
        fetchRequest: alertsFetchRequest,
        managedObjectContext: self.moc,
        sectionNameKeyPath: "formattedDateDue",
        cacheName: nil)

How can I expand and collapse the sections on my table view when I've used the NSFetchedResultsController to section my records?

I have seen a lot of tutorials that explain expanding and collapsing cells themselves but not anything on sections produced using the fetched results controller.

Answer 1

First, you need an array to keep track of whether each section is expanded or collapsed:

var sectionExpandedInfo : [Bool] = []

After the fetched results controller has done its initial performFetch, populate this array with true for each section (assuming you want sections expanded by default):

sectionExpandedInfo = []
for _ in frc.sections! {
    sectionExpandedInfo.append(true)
}

Amend the numberOfRowsInSection method to return zero if the section is collapsed:

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    if sectionExpandedInfo[section] { // expanded
        let sectionInfo = self.frc.sections![section]
        return sectionInfo.numberOfObjects
    } else { // collapsed
        return 0
    }
}

To toggle whether a section is expanded or not, I've used a button as the viewForHeaderInSection, with the section name as the title:

override func tableView(tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
    if (self.frc.sections!.count > 0) {
        let sectionInfo = self.frc.sections![section]
        let sectionHeaderButton = UIButton(type: .Custom)
        sectionHeaderButton.backgroundColor = UIColor.redColor()
        sectionHeaderButton.setTitle(sectionInfo.name, forState: .Normal)
        sectionHeaderButton.addTarget(self, action: #selector(MasterViewController.toggleSection(_:)), forControlEvents: .TouchUpInside)
        return sectionHeaderButton
    } else {
        return nil
    }
}

and in the toggleSection method I then use the title to determine which header button has been tapped, and expand/collapse the corresponding section:

func toggleSection(sender: UIButton) {
    for (index, frcSection) in self.frc.sections!.enumerate() {
        if sender.titleForState(.Normal) == frcSection.name {
            sectionExpandedInfo[index] = !sectionExpandedInfo[index]
            self.tableView.reloadSections(NSIndexSet(index: index), withRowAnimation: .Automatic)
        }
    }
}

If your FRC inserts or deletes sections, you need to update the sectionExpandedInfo array to include/remove the extra section:

func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {
    switch type {
        case .Insert:
            self.sectionExpandedInfo.insert(true, atIndex: sectionIndex)
            self.tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        case .Delete:
            self.sectionExpandedInfo.removeAtIndex(sectionIndex)
            self.tableView.deleteSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        default:
            return
    }
}

Again this assumes you want sections expanded by default.