How to use completion handler correctly [duplicate

Question

This question already has an answer here:

• NSURLConnection sendAsynchronousRequest can't get variable out of closure 1 answer
• How do I return the response from an asynchronous call? 35 answers

I understand how completion handlers work, but im a bit confused on the syntax. Below is a function that, given a username, calls a parse query to find out the corresponding userId. The query ends after the function is returned (so it returns nil), which is why we need the completion handler. How do i implement it?

func getUserIdFromUsername(username: String) -> String {
    var returnValue = String()
    let query = PFQuery(className: "_User")
    query.whereKey("username", equalTo: username)
    query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
        if let objects = objects {
            for object in objects {
                returnValue = object.objectId!
            }
        }
    }
    return returnValue

}

NOTE: I know examples similar to this exist, but they are either not swift, or extremely lengthy. This is a short and concise version that contains Parse.

Answer 1

Here's how to implement it:

func getUserIdFromUsername(username: String, completionHandler: String -> Void) {

    let query = PFQuery(className: "_User")
    query.whereKey("username", equalTo: username)
    query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
        if let objects = objects {
            for object in objects {
                completionHandler(object.objectId!)
            }
        }
    }
}

And here's how to use it:

getUserIdFromUsername("myUser") { id in
    doSomethingWithId(id)
}