swift parse app crashes after logout when app resumes - ios

Question

My code has a weird bug I'm having trouble solving.
I've built an app using swift and parse. You log in, and are taken to the main page thats a table view. Now at this point if you are logged in, and leave and comeback to the app, everything is fine and dandy.
However, when I log out, the user is then taken back to the log in screen. Now if the user leaves the app, but comes back, the app crashes giving me the error unexpectedly found nil while unwrapping an optional value.
It makes no sense to me what could be happening. When the app launches fresh from scratch the user is set to nil, then the user is logged in and everything is cool. When you log out, the user is then set back to nil and taken to the log in screen. If you resume the app, crash.
Is this an issue with how I'm logging out the user?
The relevant code is posted below..
On the login page:
#IBAction func loginButton(sender: AnyObject) {
//checks if there is a matching username with a matching password. if so, lets the user log in.
PFUser.logInWithUsernameInBackground(username.text, password: userPassword.text) {
(user: PFUser?, error: NSError?) -> Void in
if user != nil {
// Do stuff after successful login.
//go to main table view
//get current user
//display current users locations
println("login success")
//shows home screen after successful login.
self.performSegueWithIdentifier("showHomeFromLogin", sender: self)
} else {
// The login failed. Check error to see why.
//display error?
self.displayAlert("Login Failed", alertMessage: "Double check to make sure the username and password are correct.")
println("login failed")
}
}
}
var activeField: UITextField?
override func viewDidLoad() {
super.viewDidLoad()
username.delegate = self
userPassword.delegate = self
registerForKeyboardNotifications()
// Do any additional setup after loading the view.
//setup so when we tap outside the edit, we close keyboard.
var tap: UITapGestureRecognizer = UITapGestureRecognizer(target: self, action: "DismissKeyboard")
view.addGestureRecognizer(tap)
}
override func viewDidAppear(animated: Bool) {
if PFUser.currentUser() != nil {
self.performSegueWithIdentifier("showHomeFromLogin", sender: self)
}
}
On the home page where the user logs out:
#IBAction func logoutButton(sender: AnyObject) {
PFUser.logOut()
var currentUser = PFUser.currentUser() // this will now be nil
self.performSegueWithIdentifier("logoutSegue", sender: self)
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
NSNotificationCenter.defaultCenter().addObserver(self, selector: "launchSync", name: UIApplicationDidBecomeActiveNotification, object: nil)
//setting table view datasource and delegate.
self.tableView.dataSource = self
self.tableView.delegate = self
var currentUser = PFUser.currentUser()
println(currentUser)
}
Breaks in here I think on the line query.whereKey("User", equalTo:PFUser.currentUser()!):
func launchSync() {
var query = PFQuery(className:"ParseLighthouse")
query.whereKey("User", equalTo:PFUser.currentUser()!)
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]?, error: NSError?) -> Void in
if error == nil {
// The find succeeded.
println("Successfully retrieved \(objects!.count) lighthouses.")
// Do something with the found objects
if let light = objects as? [PFObject] {
for object in light {
println(object.objectId)
}
}
println(objects?.count)
self.syncToLighthouse(objects!)
} else {
// Log details of the failure
println("Error: \(error!) \(error!.userInfo!)")
}
}
}
This launchsync function is called in the view did appear method on my home screen. Is it possible that when the logout segue is performed, the main view controllers is still running in the background so when I resume, that code cant find the user now that its set back to nil?

Answer 1

On the following line
NSNotificationCenter.defaultCenter().addObserver(self, selector: "launchSync", name: UIApplicationDidBecomeActiveNotification, object: nil)
you say to call the launchSync function whenever the app becomes active.
As mentioned by Paulw11, inside that function you are force unwrapping (using !) on PFUser.currentUser() which will return nil when the user is logged out.
You can address this by ensuring that the current user isn't nil
func launchSync() {
if (PFUser.currentUser() != nil) {
var query = PFQuery(className:"ParseLighthouse")
query.whereKey("User", equalTo:PFUser.currentUser()!)
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]?, error: NSError?) -> Void in
if error == nil {
// The find succeeded.
println("Successfully retrieved \(objects!.count) lighthouses.")
// Do something with the found objects
if let light = objects as? [PFObject] {
for object in light {
println(object.objectId)
}
}
println(objects?.count)
self.syncToLighthouse(objects!)
} else {
// Log details of the failure
println("Error: \(error!) \(error!.userInfo!)")
}
}
}
}

Answer 2

If there is the potential for a conditional value to be nil then you need to check the conditional before you use it. When there is no currently logged in user, PFUser.currentUser() will return nil. When you force unwrap this value with ! you get an exception.
You can change your code to conditionally unwrap the value -
func launchSync() {
if let currentUser=PFUser.currentUser() {
var query = PFQuery(className:"ParseLighthouse")
query.whereKey("User", equalTo:currentUser)
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]?, error: NSError?) -> Void in
if error == nil {
// The find succeeded.
println("Successfully retrieved \(objects!.count) lighthouses.")
// Do something with the found objects
if let light = objects as? [PFObject] {
for object in light {
println(object.objectId)
}
}
println(objects?.count)
self.syncToLighthouse(objects!)
} else {
// Log details of the failure
println("Error: \(error!) \(error!.userInfo!)")
}
}
}
}