Trying to parse XML from URL to PickerView with swift - ios

Question

I'm trying to parse XML with XMLParser and put it on a PickerView. This is my code:
Main code:
import UIKit
class ViewController: UIViewController, XMLParserDelegate {
#IBOutlet weak var containerView: UIView!
#IBOutlet weak var pickerView: UIPickerView!
#IBOutlet weak var Image: UIImageView!
#IBOutlet weak var Button: UIButton!
var arrayCategorias = [Categories]()
var parser = XMLParser()
override func viewDidLoad() {
super.viewDidLoad()
let urlString = URL(string: "http://thecatapi.com/api/categories/list")
self.parser = XMLParser(contentsOf: urlString!)!
let success:Bool = self.parser.parse()
parser.delegate = self
if success {
print("success")
} else {
print("parse failure!")
}
print(arrayCategorias.count)
}
#IBAction func botonPulsado(_ sender: Any) {
}
func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String : String]) {
if(elementName=="category")
{
let categoria = Categories()
for string in attributeDict {
let strvalue = string.value as NSString
switch string.key {
case "id":
categoria.id = strvalue.integerValue
break
case "name":
categoria.name = strvalue as String
break
default:
break
}
}
arrayCategorias.append(categoria)
}
func parser(_ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {
}
func parser(_ parser: XMLParser, foundCharacters string: String) {
}
func parser(_ parser: XMLParser, parseErrorOccurred parseError: Error) {
print("failure error: ", parseError)
}
}
}
Class Categories Code:
import Foundation
class Categories{
var id: Int = 0
var name: String = ""
}
I don't know exactly what the problem is, but when I try to read arrayCategorias() just to see if there is any data, it just shows me 0. And where I catch the error, it shows me: "parse failure".
If anyone can help me, I'm just an android programmer and I don't know why it doesn't work.
I need help with putting the data on the PickerView.

Answer 1

There are a whole bunch of issues:
You have set your delegate after you parsed, whereas you must do this before you call parse().
You have some of your delegate methods implemented inside didStartElement. These need to be top level methods of your parser delegate.
Minor unrelated issue, but your class name of Categories isn't quite right, because it represents a single "category", not many. So I'd rename this to be Category. (I'd personally also make it a struct, but that's up to you.)
Your id and name values are not attributes of the category element. They are their own elements, so you have to parse them separately (using foundCharacters and build the Category object on didEndElement of category.
The id of the categories appears to be an integer, so I'd make the id of Category an Int.
You're calling parse() of a XMLParser that is using a URL from the main thread. That's inadvisable because you're blocking the main thread while it does the request. Personally, I'd use URLSession to request the data asynchronously, process it on a background queue, and only dispatch the final update of the model object and UI update to the main queue.
Personally, I'd adopt asynchronous patterns myself with #escaping completion handlers, to help isolate UI updates that I trigger after a parse from the parsing itself
A stylistic matter, but I wouldn't put all of this XMLParserDelegate code in the view controller. At the very least, put it in an extension so it's nicely organized. Even better, define a stand-alone parser delegate object in the spirit of the single responsibility principle and ensuring that you're not accidentally updating view controller model objects while the UI might be referring to it. It more safely ensures thread-safety and keeps your code more nicely encapsulated.
Pulling this all together, you could do something like:
struct Category {
let id: Int
let name: String
}
class ViewController: UIViewController {
#IBOutlet weak var pickerView: UIPickerView!
var categories = [Category]()
override func viewDidLoad() {
super.viewDidLoad()
startRequestAndParse() { categories, error in
guard let categories = categories, error == nil else {
print(error?.localizedDescription ?? "Unknown error")
return
}
// if you got here, everything is OK, so update model and UI on main thread
DispatchQueue.main.async {
self.categories = categories
print(self.categories)
// trigger whatever UI update you want here, too;
self.pickerView.reloadAllComponents()
}
}
}
/// Initiate request from server and parse results
///
/// - Parameters:
/// - completion: This is called when the request/parsing is done. This may be called
/// on background thread. If parsing failed, the array of categories
/// will be `nil` and we should have `error`.
/// - categories: First parameter of the `completion` closure is the array of `Category` objects, or `nil` on error.
/// - error: Second parameter of the `completion` closure is the resulting `Error`, if any.
private func startRequestAndParse(completion: #escaping (_ categories: [Category]?, _ error: Error?) -> Void) {
let url = URL(string: "http://thecatapi.com/api/categories/list")!
let task = URLSession.shared.dataTask(with: url) { data, _, error in
guard let data = data, error == nil else {
completion(nil, error)
return
}
// ok, now parse
let parser = XMLParser(data: data)
let delegate = ParserDelegate()
parser.delegate = delegate
parser.parse()
completion(delegate.categories, parser.parserError)
}
task.resume()
}
}
// this assumes you set the picker's `delegate` to be the view controller (either in IB or programmatically in `viewDidLoad`
extension ViewController: UIPickerViewDelegate {
func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
return categories[row].name
}
}
// this assumes you set the picker's `dataSource` to be the view controller (either in IB or programmatically in `viewDidLoad`
extension ViewController: UIPickerViewDataSource {
func numberOfComponents(in pickerView: UIPickerView) -> Int {
return 1
}
func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
return categories.count
}
}
/// Parser delegate for categories
class ParserDelegate: NSObject, XMLParserDelegate {
private var id: Int?
private var name: String?
private var value: String?
var categories: [Category]?
// initialize `categories`
func parserDidStartDocument(_ parser: XMLParser) {
categories = []
}
// if `id` or `name`, initialize `value` so we'll capture the appropriate value
func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String : String]) {
if elementName == "id" || elementName == "name" {
value = ""
}
}
// if `value` is not `nil`, go ahead and concatenate the additional characters
func parser(_ parser: XMLParser, foundCharacters string: String) {
value? += string
}
// if `id` or `name`, update the appropriate property
// if `category`, build a `Category` object and add it to our array
// regardless, reset `value` to `nil`
func parser(_ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {
switch elementName {
case "id":
if let value = value {
id = Int(value)
}
case "name":
name = value
case "category":
if let id = self.id, let name = self.name {
categories!.append(Category(id: id, name: name))
}
id = nil
name = nil
default:
()
}
value = nil
}
// if any error, reset `categories` so caller knows there was an issue
func parser(_ parser: XMLParser, parseErrorOccurred parseError: Error) {
categories = nil
}
}