dispatch_async() block didn't finish before UITableViewDataSource methods - ios

Question

There is a UITableView which its cells will be filled by data got with HTTP post request. But UITableView functions executed before the data comes. When the application starts, all three tableView methods executed and then application throws a runtime error. I guess it's because in cellForRowAtIndexPath, messageList is still empty.
Here is the code:
class messageViewController: UIViewController, UITableViewDelegate, UITableViewDataSource{
var authData : NSDictionary = [:]
var funcLib = functionLibrary()
var messagesList : NSArray = []
var messageCount: Int = 0
#IBOutlet weak var messageTableView: UITableView!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
var authCode = self.authData["auth"] as! String
var userID = self.authData["user_id"] as! String
var messageRequsetBodyData: AnyObject = ["op":"users","op2":"getThisWeekMessages","id":"\(userID)","id2":"","id3":"","authCode":"\(authCode)"] as AnyObject
funcLib.HTTPPostRequest("http://asdasd.asdasdasd.com/services/index.php", bodyData: messageRequsetBodyData){data in
dispatch_async(dispatch_get_main_queue()){
if let data = data{
var messaggesListDic = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil) as! NSDictionary
println("------MESSAGGES---------")
self.messageCount = messaggesListDic["count"] as! Int
//self.messages = messaggesListDic["messages"] as! NSDictionary
self.messagesList = messaggesListDic["messages"] as! NSArray
println("\(self.messagesList)")
self.messageTableView.reloadData()
}
}
}
self.messageTableView.delegate = self
self.messageTableView.dataSource = self
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
#IBAction func dismissMessageVC(sender: AnyObject) {
self.dismissViewControllerAnimated(true, completion: nil)
}
func numberOfSectionsInTableView(tableView: UITableView) -> Int {
println("asdasd")
return 1
}
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
println("asdasd")
println("\(self.messageCount)")
return 1
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
println("bdbsdbsdb")
var cell = self.messageTableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as? UITableViewCell
let row = indexPath.row
cell!.textLabel!.text = self.messagesList[0]["content"] as? String
return cell!
}
Runtime error description:
Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayI objectAtIndex:]: index 0 beyond bounds for empty array'
I tried assign the cell label with using cell.textLabel?.text = "asdasd" command and it works. So I think there is no problem with outlets or methods.
How can I assign the data to messageList before cellForRowAtIndexPath executed with using different way?

Answer 1

Yes, if you have a table view that needs to make an asynchronous call to retrieve the data, you should expect the table view data source methods to be called before the asynchronous request is done. But, when the asynchronous request is done, when you simply call tableView.reloadData(), the table view data sources methods will be called a second time. This is a very common pattern.
The issue here, though, is that this code is not gracefully handling the situation that there is no data to display when the table view data sources methods are called the first time. If numberOfRowsForSection returned 0 until data was retrieved (as described by the others, notably by returning messagesList.count(), as suggested by John and Yedidya, rather than returning a fixed number), all would be good.

Answer 2

You return constant value for number of rows even if your message count is zero. Better to return message count.
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
println("asdasd")
println("\(self.messageCount)")
return self.messageCount;
}

Answer 3

Replace the numberOfRows function return value with messagesList.count.