How to call performSegueWithIdentifier in Swift - ios

Question

I have created a prepareForSegue method and I am trying to call it from a button that I created by using the performSegueWithIdentifier method. The app is crashing when I load the simulator and it's not getting me a complete error message. Can someone please lead me in the right direction?
import Foundation
import UIKit
import Alamofire
import FBSDKCoreKit
import FBSDKShareKit
import FBSDKLoginKit
class PageContentViewController: UIViewController {
#IBOutlet weak var logoImageView: UIImageView!
#IBOutlet weak var contentLabel: UILabel!
#IBOutlet weak var backgroundImageView: UIImageView!
#IBOutlet weak var pageControl: UIPageControl!
#IBOutlet weak var facebookButton: UIButton!
var index : Int = 0
var logoFile: String = ""
var content: String = ""
var backgroundFile: String = ""
let facebookReadPermissions = ["public_profile", "email", "user_friends"]
override func viewDidLoad() {
super.viewDidLoad()
pageControl.currentPage = index
facebookButton.hidden = (index == 3 ) ? false : true
pageControl.hidden = (index == 3) ? true: false
logoImageView.image = UIImage(named: logoFile)
contentLabel.text = content
backgroundImageView.image = UIImage(named: backgroundFile)
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {
if let destinationController = segue.destinationViewController as? PaymentSubViewController
where segue.identifier == "payment" {
// Do something with `destinationController`
}
}
#IBAction func test(sender: AnyObject) {
self.performSegueWithIdentifier("payment", sender: self)
}
#IBAction func fbTouched(sender: AnyObject) {
FBSDKLoginManager().logInWithReadPermissions(self.facebookReadPermissions, handler: { (result:FBSDKLoginManagerLoginResult!, error:NSError!) -> Void in
if error != nil {
//According to Facebook:
//Errors will rarely occur in the typical login flow because the login dialog
//presented by Facebook via single sign on will guide the users to resolve any errors.
// Process error
FBSDKLoginManager().logOut()
} else if result.isCancelled {
// Handle cancellations
FBSDKLoginManager().logOut()
} else {
let fbToken = result.token.tokenString
Alamofire.request(Router.FacebookAuth(fbToken)).validate(statusCode: 200 ..< 300).responseJSON(completionHandler: { (request, response, JSON, error) in
if let json = JSON as? Dictionary<String, AnyObject> {
if let token = json["token"] as? String {
Router.OAuthToken = token
self.performSegueWithIdentifier("showHomeFeed", sender: self)
}
}
})
}
})
}
}

Answer 1

Because you are force unwrapping the destinationViewController using as!, if that value is nil or not a PaymentSubViewController, the app will crash.
The better way to implement this is with an optional binding (if let) and a conditional downcast (as?):
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {
if let destinationController = segue.destinationViewController as? PaymentSubViewController
where segue.identifier == "payment" {
// Do something with `destinationController`
}
}
Of course, that'll stop the crash but won't answer the question of why segue.destinationViewController is nil or of another type. Make sure that you segue is configured properly in interface builder and that the destination view controller actually has segue.destinationViewController for its Class value in the identity inspector tab.

Answer 2

I had a map object on the storyboard and I did not add an outlet for the object which was creating an error message.