A strange behaviour of table view batch operation - ios

Question

Preparation work:
Create a new single view project
embed the view controller in a navigation controller
drag two bar button items onto the navigation bar
drag a table view onto the root view, make view controller the data source of the table view
My code is as below:
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var tableView: UITableView!
var sec = [["00", "01", "02"],
["10", "11", "12"]]
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
extension ViewController: UITableViewDataSource {
func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return sec.count
}
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return sec[section].count
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)
cell.textLabel?.text = sec[indexPath.section][indexPath.row]
return cell
}
}
extension ViewController {
#IBAction func deleteUpSection(sender: UIBarButtonItem) {
sec[1][0] = "something else"
sec.removeAtIndex(0)
let deletedIndexPaths = [NSIndexPath(forRow: 0, inSection: 0), NSIndexPath(forRow: 1, inSection: 0),NSIndexPath(forRow: 2, inSection: 0)]
let deletedIndexSet = NSIndexSet(index: 0)
let reloadedIndexPaths = [NSIndexPath(forRow: 0, inSection: 1)]
tableView.beginUpdates()
tableView.deleteRowsAtIndexPaths(deletedIndexPaths, withRowAnimation: .Fade)
tableView.deleteSections(deletedIndexSet, withRowAnimation: .Right)
tableView.reloadRowsAtIndexPaths(reloadedIndexPaths, withRowAnimation: .Automatic)
tableView.endUpdates()
}
#IBAction func deleteDownSection(sender: UIBarButtonItem) {
sec[0][0] = "something else"
sec.removeAtIndex(1)
let deletedIndexPaths = [NSIndexPath(forRow: 0, inSection: 1), NSIndexPath(forRow: 1, inSection: 1), NSIndexPath(forRow: 2, inSection: 1)]
let deletedIndexSet = NSIndexSet(index: 1)
let reloadedIndexPaths = [NSIndexPath(forRow: 0, inSection: 0)]
tableView.beginUpdates()
tableView.deleteRowsAtIndexPaths(deletedIndexPaths, withRowAnimation: .Fade)
tableView.deleteSections(deletedIndexSet, withRowAnimation: .Right)
tableView.reloadRowsAtIndexPaths(reloadedIndexPaths, withRowAnimation: .Automatic)
tableView.endUpdates()
}
}
deleteDownSection works as expected, but deleteUpSection crashed, they are nearly the same.
Something I have discovered:
If I remove deleteRowsAtIndexPath in deleteUpSection, it works as expected, both deletion and update are executed.
If I remove reloadRowsAtIndexPath in deleteUpSection, it deletes the upper section successfully.
Any opinion is welcome.

Answer 1

reloadRowsAtIndexPaths is failing because you are attempting to reload rows in a section that no longer exists (NSIndexPath(forRow: 0, inSection: 1)). You deleted the rows of section 0 and section 0 itself in the prior statements which means there is only one section left (ie. the old section 1 which is now section 0).
I'm guessing when you remove deleteRowsAtIndexPath and it works it is because even though you remove the section in the next statement, those rows are still temporarily referenced somewhere so when reloadRowsAtIndexPaths is called it does not fail to find those rows even though its section is deleted.