How to parse a RSS feed using NSXMLParser with variables on the feed address by pressing a button in a UIAlertController Alert in Swift?

Question

I have been trying to integrate a Pinboard bookmarks view (by parsing an RSS Feed and displaying it in a UITableView) in my browser app, and I'm encountering a few issues. You can see my previous questions here and here. I'm trying to use the variables I collected from my user in the Alert, to get the user's secret RSS feed token. However when I run the app, it causes a crash with a error of "EXC_BAD_INSTRUCTION" with the following console output:

fatal error: unexpectedly found nil while unwrapping an Optional value

I checked everything but I can't find a nil.

This is the code I'm using:

class SettingsTableViewController: UITableViewController, MFMailComposeViewControllerDelegate, NSXMLParserDelegate {

var _pinboardUsername: String?
var _pinboardAPIToken: String?

var parser: NSXMLParser = NSXMLParser()
var bookmarks: [Bookmark] = []
var pinboardSecret: String = String()
var eName: String = String()

...

    @IBAction func pinboardUserDetailsRequestAlert(sender: AnyObject) {
    //Create the AlertController
    var pinboardUsernameField :UITextField?
    var pinboardAPITokenField :UITextField?
    let pinboardUserDetailsSheetController: UIAlertController = UIAlertController(title: "Pinboard Details", message: "Please enter your Pinboard Username and API Token to access your bookmarks", preferredStyle: .Alert)
    //Add a text field
    pinboardUserDetailsSheetController.addTextFieldWithConfigurationHandler({(usernameField: UITextField!) in
        usernameField.placeholder = "Username"
        usernameField.text = self._pinboardUsername
        var parent = self.presentingViewController as! ViewController
        pinboardUsernameField = usernameField

    })
    pinboardUserDetailsSheetController.addTextFieldWithConfigurationHandler({(apiTokenField: UITextField!) in
        apiTokenField.placeholder = "API Token"
        apiTokenField.text = self._pinboardAPIToken
        var parent = self.presentingViewController as! ViewController
        pinboardAPITokenField = apiTokenField

    })
    pinboardUserDetailsSheetController.addAction(UIAlertAction(title: "Cancel", style: .Cancel, handler: nil))
    pinboardUserDetailsSheetController.addAction(UIAlertAction(title: "Done", style: .Default, handler: { (action) -> Void in
        // Now do whatever you want with inputTextField (remember to unwrap the optional)
        var valueUser = pinboardUsernameField?.text
        NSUserDefaults.standardUserDefaults().setValue(valueUser, forKey: AppDefaultKeys.PinboardUsername.rawValue)
        var valueAPI = pinboardAPITokenField?.text
        NSUserDefaults.standardUserDefaults().setValue(valueAPI, forKey: AppDefaultKeys.PinboardAPIToken.rawValue)
        let url:NSURL = NSURL(string: "https://api.pinboard.in/v1/user/secret/?auth_token=\(valueAPI)")!
        self.parser = NSXMLParser(contentsOfURL: url)!
        self.parser.delegate = self
        self.parser.parse()

        func parser(parser: NSXMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [NSObject : AnyObject]) {
            self.eName = elementName
            if elementName == "result" {
                self.pinboardSecret = String()
                NSUserDefaults.standardUserDefaults().setValue(self.pinboardSecret, forKey: AppDefaultKeys.PinboardSecret.rawValue)
            }
        }
    }))
    self.presentViewController(pinboardUserDetailsSheetController, animated: true, completion: nil)   
}

The error is in the let url: NSURL = ... line of the parser function.

What am I doing wrong?

Answer 1

The issue is that valueAPI is an optional. If, for example, valueAPI was foo, your URL string would look like:

https://api.pinboard.in/v1/user/secret/?auth_token=Optional("foo")

I'd suggest building your URL string first, looking at it, and you'll see what I mean.

Bottom line, that Optional("...") reference in the URL is not valid and thus instantiating the URL with that string will fail. And using the ! to unwrap that optional NSURL will crash as you outlined.

You have to unwrap the valueAPI optional before using it in the URL string.

let url:NSURL = NSURL(string: "https://api.pinboard.in/v1/user/secret/?auth_token=\(valueAPI!)")!

Or you could make valueAPI an implicitly unwrapped optional.

Frankly, if there's any chance that valueAPI might be nil, I'd rather use optional binding (i.e. if let clauses), so you could gracefully detect these issues in the future.

---

Regarding the completely separate XML parsing problems, there are a couple of issues:

1. I'd suggest retrieving the response asynchronously via NSURLSession. Using NSXMLParser with contentsOfURL performs the request synchronously which results in a poor UX and the iOS watchdog process may kill your app.

   Instead, use NSURLSession method dataTaskWithURL and then use the resulting NSData with the NSXMLParser object instead.

2. If you do that, you can also convert the NSData response object to a string and examine it. Often if there are API problems, the response will include some description of the nature of the error (sometimes it's text, sometimes it's HTML, sometimes it's XML; depends upon how that web service was designed). If you don't get the XML response you expected, you really will want to look at the raw response to identify what's wrong.

3. Your NSXMLParserDelegate method didStartElement is defined inside the addAction block. It must be a method of the class, itself, not buried inside this closure.

4. Your didStartElement is saving the secret. But you don't have the secret yet, so there's no point in trying to save it yet.

5. I don't see the other NSXMLParserDelegate methods (foundCharacters, didEndElement, and parseErrorOccurred, at the very least). Do you have those defined elsewhere? See https://stackoverflow.com/a/27104883/1271826

Answer 2

let url:NSURL = NSURL(string: "https://api.pinboard.in/v1/user/secret/?auth_token=\(valueAPI)")!

NSURL() is a failable initializer, meaning it may fail on initialization (malformed URL, server not responding, ...).

By forcing the unwrap with ! you declare that you are absolutely sure to get a valid object, but you don't. This leads to the fatal error.