tableView.insertRows throws NSException, but not when `insertRows` at `[[0,0]]` - ios

Question

I am following a modified (simplified) version of the tutorial very much like what is found here:
https://developer.apple.com/library/content/referencelibrary/GettingStarted/DevelopiOSAppsSwift/ImplementNavigation.html#//apple_ref/doc/uid/TP40015214-CH16-SW1
and here is my UITableViewController:
import UIKit
class NewsTableViewController: UITableViewController {
#IBOutlet var newsTableView: UITableView!
/*
MARK: properties
*/
var news = NewsData()
override func viewDidLoad() {
super.viewDidLoad()
dummyNewData()
}
// MARK: - Table view data source
override func numberOfSections(in tableView: UITableView) -> Int {
return 1
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return news.length()
}
// here we communicate with parts of the app that owns the data
override func tableView(_ tableView: UITableView
, cellForRowAt indexPath: IndexPath
) -> UITableViewCell {
// note here we're using the native cell class
let cell = tableView.dequeueReusableCell(withIdentifier: "newsCell", for: indexPath)
// Configure the cell...
let row : Int = indexPath.row
cell.textLabel?.text = news.read(idx: row)
return cell
}
// MARK: Navigation ****************************************************************
// accept message from CreateNewViewController
#IBAction func unwindToCreateNewView(sender: UIStoryboardSegue){
if let srcViewController = sender.source as? CreateNewsViewController
, let msg = srcViewController.message {
// push into news instance and display on table
news.write(msg: msg)
let idxPath = IndexPath(row: news.length(), section: 1)
// tableView.insertRows(at: [idxPath], with: .automatic)
tableView.insertRows(at: [[0,0]], with: .automatic)
print("unwound with message: ", msg, idxPath)
print("news now has n pieces of news: ", news.length())
print("the last news is: ", news.peek())
}
}
/*
#DEBUG: debugging functions that display things on screen **************************
*/
// push some values into new data
private func dummyNewData(){
print("dummyNewData")
news.write(msg: "hello world first message")
news.write(msg: "hello world second message")
news.write(msg: "hello world third message")
}
}
The problem is in the function unwindToCreateNewView:
let idxPath = IndexPath(row: news.length(), section: 1)
tableView.insertRows(at: [idxPath], with: .automatic)
where news.length() gives me an Int that is basically someArray.count.
When I insertRows(at: [idxPath] ...), I get error:
libc++abi.dylib: terminating with uncaught exception of type NSException
But when I just hard code it to do:
tableView.insertRows(at: [[0,0]], with: .automatic)
It works just fine. And on the simulator I see new messages are inserted below the previous ones. What gives?

Answer 1

You have an "off by one" problem with the following code:
news.write(msg: msg)
let idxPath = IndexPath(row: news.length(), section: 1)
Let's say that just before this code is called, you have no items in news. This means there are 0 rows. When you want to add a new row, you need to insert at row 0 since row numbers start at 0.
Calling news.write(msg: msg) add the new item and its length is now 1.
Calling IndexPath(row: news.length(), section: 1) sets the row to a value of 1 but it needs to be 0.
One simple solution is to swap those two lines:
let idxPath = IndexPath(row: news.length(), section: 1)
news.write(msg: msg)
That will create the index path with the proper row number.
And since this is the first (and only) section, the section number needs to be changed to 0 in addition to the above change.
let idxPath = IndexPath(row: news.length(), section: 0)
news.write(msg: msg)