Returning from threading/ GCD/ completion handler

Question

I have some logic to sign a user in from a login screen. If the login fails, I want to display a message to let the user know. If the user logs in successfully, I trigger a segue. When I test it with invalid credentials, the error alert displays as expected but the segue is still being triggered even though it's nested in an if-else statement. Why is this? How can I return out of it and avoid the else block...? I tried adding 'return' under the DispatchQ/ show alert code and that didn't have any effect.

self.login(username: usernameTextField.text!, password: passwordTextField.text!) { (error) in
        if error != nil {
            DispatchQueue.main.async {
                self.showAlert(msg: error ?? "error")
            }
        } else {
            DispatchQueue.main.async {
                // segue code
            }
        }
    }

login:

func login(username: String, password: String, completionHandler: @escaping (_ error: String?) -> ()) {
    SessionHelper.shared.logUserIn(withUsername: username, andPassword: password) { (error) in
        if let err = error {
            completionHandler(err)
        }
        completionHandler(nil)
    }
}

Your sure that’s the only way to trigger the segue? That the callback isn’t being executed twice? From the code you have here it appears it should work as expected which means the problem is most likely elsewhere. Best help would probably be to show login as well. — David Berry, Nov 18 '18 at 02:12
@DavidBerry added. Not sure if that helps or you need even more info — user7804097, Nov 18 '18 at 02:25
@davidBerry also, yes, positive its the only segue trigger. If I remove it it works as intended - if i add a print statement to the else block, the print runs... — user7804097, Nov 18 '18 at 02:44

score 4 · Accepted Answer · answered Nov 18 '18 at 03:03

You're running the completion handler either way; if you get an error back, you're calling it, but then you fall through and run it with nil. Try this:

func login(username: String, password: String, completionHandler: @escaping (_ error: String?) -> ()) {
SessionHelper.shared.logUserIn(withUsername: username, andPassword: password) { (error) in
    if let err = error {
        completionHandler(err)
    } else {
        completionHandler(nil)
    }
}

}

score 0 · Answer 2 · answered Nov 18 '18 at 03:03

0

Your login function calls completionHandler twice in the error case. The if falls through to the following statement. You should either put the following statement in an else block, or return from the true block.

answered Nov 18 '18 at 03:03

Ken Thomases

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Returning from threading/ GCD/ completion handler

2 Answers2