Proper way to load data to TableView - ios

Question

I have an application that loads list of questions from JSON data and shows them on TableView.
Everything is working fine most of the time but it seems to be that I am doing something wrong and that is why - app crashes.
It happens rarely so it is hard to detect but I am sure that there must a problem with the logic.
So I have model class for question and array for question items :
class questionItem {
var id = 0
var title : String = ""
var question : String = ""
}
var questions = [questionItem]()
Inside my ViewController I have IBOutlet for TableView and I placed data loading inside viewDidLoad
class QuestionsListVC: UIViewController, UITableViewDataSource, UITableViewDelegate {
#IBOutlet weak var questionsTableView: UITableView!
override func viewDidLoad() {
super.viewDidLoad()
get_questions()
}
func get_questions()
{
let request = NSMutableURLRequest(URL:myURL!)
request.HTTPMethod = "POST"
let postString = ""
request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)
let task = NSURLSession.sharedSession().dataTaskWithRequest(request)
{
data, response, error in
if error != nil {
print("error=\(error)")
return
}
//clearing array for new items
questions.removeAll(keepCapacity: false)
dispatch_async(dispatch_get_main_queue(),{
var json = JSON(data: data!)
if let items = json["questions"].array {
for item in items {
let question = questionItem()
question.id = item["id"].int!
question.title = item["title"].string!;
question.question = item["question"].string!
questions.append(question)
}
}
self.questionsTableView.reloadData()
});
}
task.resume()
}
}
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int{
return questions.count
}
Error is shown inside cellForRowAtIndexPath
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell : QuestionsListCell = self.questionsTableView.dequeueReusableCellWithIdentifier("QuestionsListCell") as! QuestionsListCell
//error happens here - Index out of range
print(questions[indexPath.row].title)
It happens one time in six cases and there is no errors in other 5 of 6 tests - but I don't understand why.

Answer 1

This points to a problem with the
numberOfSectionsInTableView
and/or
numberOfRowsInSection
Can you post your current implementation of these?
If you only displaying one continuous list, the numberOfSectionsInTableView should always return 1, and you need to check numberOfRowsInSection is accurately returning the number of items in the datasource.
Edit:
Can you try clearing the existing datasource on the main thread immediately before updating with the new items as in the code below:
dispatch_async(dispatch_get_main_queue(),{
questions.removeAll(keepCapacity: false)
var json = JSON(data: data!)
if let items = json["questions"].array {
for item in items {
let question = questionItem()
question.id = item["id"].int!
question.title = item["title"].string!;
question.question = item["question"].string!
questions.append(question)
}
}
self.questionsTableView.reloadData()
});

Answer 2

The call to questions.removeAll in your code makes the following sequence of events possible:
numberOfRowsInSection is called before questions.removeAll, returning the old non-zero capacity
questions.removeAll clears questions
cellForRowAtIndexPath is called before questions are re-populated, causing index out of range exception
One way to fix is is relatively straightforward: make a newQuestions array, populate it in get_questions, and swap it in when numberOfRowsInSection is called:
// Add this to your class
var newQuestions : [questionItem]
// Change get_questions:
dispatch_async(dispatch_get_main_queue(), {
var json = JSON(data: data!)
if let items = json["questions"].array {
var tmpQuestions = [questionItem]()
for item in items {
let question = questionItem()
question.id = item["id"].int!
question.title = item["title"].string!;
question.question = item["question"].string!
tmpQuestions.append(question)
}
newQuestions = tmpQuestions
self.questionsTableView.reloadData()
}
});
// Change numberOfRowsInSection
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int{
if newQuestions != nil {
questions = newQuestions
newQuestions = nil
}
return questions.count
}
Note how get_questions does not populate newQuestions directly. Instead, it builds tmpQuestions, and sets it to newQuestions only when it is fully built.

Answer 3

Try with below code,
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
{
let cell : QuestionsListCell = self.questionsTableView.dequeueReusableCellWithIdentifier("QuestionsListCell") as! QuestionsListCell
if let objModel : questionItem = questions[indexPath.row] as? questionItem
{
print(objModel.title)
}
}
Change your cellForRow with this method. Hope this helps you.

Answer 4

Just to avoid crash, I would have added following three safe checks.
1. Check Array count before clearing out.
if questions.count > 0
{
questions.removeAll()
}
2. Check array count before table Reload
if questions.count > 0
{
self.questionsTableView.reloadData()
}
3. In cellForRowAtIndex method Check for value in array of object, before putting on Cell
if let objModel : questionItem = questions[indexPath.row] as? questionItem
{
print(" title is \(objModel.title)")
}