Parsing JSON using Swift - ios

Question

I have an async method in a custom class that returns an NSArray containing dictionaries. I am trying to display this data in a UITableView. One issue is that the getGenres method call is async so the data gets loaded after the table is displayed. The main problem is figuring out how to implement the 3 datasource methods. In Objective C this was so simple...
var genreList:NSArray?
override func viewDidLoad() {
super.viewDidLoad()
Bookshop.getGenres {
genres in
self.genreList = genres // copies data to the public NSArray
self.tableView.reloadData() // this returns the data async so need to reload the tableview
println("records: \(self.genreList?.count)") // displays the array of dictionaries correctly
}
}
// #pragma mark - Table view data source
override func numberOfSectionsInTableView(tableView: UITableView!) -> Int {
return 1
}
override func tableView(tableView: UITableView!, numberOfRowsInSection section: Int) -> Int {
if self.genreList != nil {
println("Rows in section: \(self.genreList?.objectAtIndex(0))")
return self.genreList?.count! // Value of optional 'Int?' not wrapped; did you mean to use '!'
}
return 3
}
override func tableView(tableView: UITableView!, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell? {
let cell:UITableViewCell = tableView.dequeueReusableCellWithIdentifier("GenreCell", forIndexPath: indexPath) as UITableViewCell
cell.textLabel.text = "Hello World!"
println("ROW: \(indexPath.row)")
println("data: \(self.genreList?.objectAtIndex(indexPath.row))")
var item = self.genreList?.objectAtIndex(indexPath.row) Variable 'item' inferred to have type 'AnyObject?', which may be unexpected.
//cell.textLabel.text = item.title
//var item:NSDictionary = self.genreList?.objectAtIndex(indexPath.row) as NSDictionary
//println("item: \(item)")
return cell
}
What am I doing wrong? It seems so unnecessarily complicated to do what is a really simple task.
This is my utility class:
import Foundation
class Bookshop {
class func getGenres(completionHandler: (genres: NSArray) -> ()) {
//println("Hello inside getGenres")
let urlPath = "http://creative.coventry.ac.uk/~bookshop/v1.1/index.php/genre/list"
//println(urlPath)
let url: NSURL = NSURL(string: urlPath)
let session = NSURLSession.sharedSession()
var resultsArray:NSArray!
let task = session.dataTaskWithURL(url) {
data, response, error in
//println("Task completed")
if(error) {
println(error.localizedDescription)
}
//println("no error")
var err: NSError?
var options:NSJSONReadingOptions = NSJSONReadingOptions.MutableContainers
var jsonResult = NSJSONSerialization.JSONObjectWithData(data, options: options, error: &err) as NSDictionary
if(err != nil) {
println("JSON Error \(err!.localizedDescription)")
}
//NSLog("jsonResults %#", jsonResult)
let results: NSArray = jsonResult["genres"] as NSArray
//NSLog("jsonResults %#", results)
//resultsArray = results
//println("calling completion handler")
completionHandler(genres: results)
}
task.resume()
}
}

Answer 1

What swift has done is just eliminating a bunch of error-prone functionality we really shouldn't have been depending on anyway, specifically relying on messages set to zero was always error prone. Now it's a little bit more verbose, but significantly more explicit, and hence safe.
The equivalent of return self.genreList.count in objective-C is:
override func tableView(tableView: UITableView!, numberOfRowsInSection section: Int) -> Int {
if let genres = self.genreList {
return genres.count
}
else {
// Note: this is the case when you haven't yet downloaded your genre
// list, do whatever is appropriate, this just shows an empty list
return 0
}
}
Since cellForRowAtIndexPath will only be called if you have a non-zero cell count, that method is simpler, but really for consistency/safety, you follow the same general flow:
override func tableView(tableView: UITableView!, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell? {
if let genres = self.genreList as? [NSDictionary] {
let cell:UITableViewCell = tableView.dequeueReusableCellWithIdentifier("GenreCell", forIndexPath: indexPath) as UITableViewCell
var item = genres[indexPath.row]
cell.textLabel.text = item["title"] as String
return cell
}
else {
// Note: again, this is the case where you don't have your genre list
// list yet, so you can create whatever default cells needed.
return nil
}
}

Answer 2

Change your declaration of self.genreList from var genreList:NSArray? to var genreList:NSArray!. Then you can access the members like you did it in Obj-C (without the quesitonmark). And maybe, if the handler from the getGenres-method is in another thread you need to write
Bookshop.getGenres {
genres in
self.genreList = genres // copies data to the public NSArray
dispath_asynch(dispatch_get_main_queue) {
self.tableView.reloadData() // this returns the data async so need to reload the tableview
println("records: \(self.genreList.count)") // displays the array of dictionaries correctly
}
}
And in your cellForRowAtIndexPath you need to cast the item like this:
var item = self.genreList[indexPath.row] as NSDictionary
dump(item) // just to show the dictionary in this scenario
cell.textLabel.text = item["title"] as String
ALSO: you have to check if self.genreList != nil before accessing it in the cellForRowAtIndexPath:
if genreList != nil {
var item = self.genreList[indexPath.row] as NSDictionary
dump(item) // just to show the dictionary in this scenario
cell.textLabel.text = item["title"] as String
}
else {
cell.textLabel.text = "Loading..."
}