How to return a value within an if let statement in Swift 4?

Question

How can I return a value within an if let statement to be further returned within a function? Here is the code:

func loadUrl(url:String) -> String {
        DispatchQueue.global().async {
            do {
                let appUrl = URL(string:url)!
                let data = try Data(contentsOf:appUrl)
                let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
                print("Test from do")
                if let results = json["results"] as? [[String:Any]] {
                    print("Test from if let 1")
                    if let first = results[0] as? [String:Any] {

                        print("Test from if let 2")
                        var cityStateLocation = first["formatted_address"]!

                        return cityStateLocation
                        //What needs to be returned
                    }
                }
                DispatchQueue.main.async {
                    print("No Error")
                }
            } catch {
                DispatchQueue.main.async {
                    print("Cannot connect to the server.")
                }
            }
        }
    }

What I would like to be able to do is take cityStateLocation and return it in the func, but because it is a part of an if let statement within an .async method I don't know how to do that. Could someone please explain?

EDIT: I need the return value of cityStateLocation to equal a variable in a separate function. Here is the separate function:

@IBAction func continueButton(_ sender: Any) {
        var cityState:String
        if locationSwitch.isOn == true {
            print(location.latitude)
            print(location.longitude)
            let url = "https://maps.googleapis.com/maps/api/geocode/json?latlng=\(location.latitude),\(location.longitude)&result_type=locality&key=AIzaSyDI-ZacHyPbLchRhkoaUTDokwj--z_a_jk"
            loadUrl(url: url)
            cityState = loadUrl(url: url)
        } else {
            cityState = ""
        }
        CoreDataHandler.saveObject(locationLocality: cityState)
    }

Edit 2: The main reason why the "duplicate answer" is not a duplicate is that my code needs to call the return of this function within a separate function then save it to Core Data. Also, my code is not using an array.

Answer 1

You could modify your function to include a closure. For instance:

func loadUrl(url: String, completionHandler: @escaping (_ location: String?) -> (Void)) {

And then, where you want to return it, you'd pass it in as such.

completionHandler(cityStateLocation)

I made it an optional so that, in your fail paths, you could return nil.

Then, where you call the function would change. Using trailing closure syntax, it could look like this:

loadUrl(url: "someurl.com/filepath.txt") { optionalLocation in 
    guard let nonOptionalLocation = optionalLocation else {
        // Location was nil; Handle error case here
        return
    }
    // Do something with your location here, like setting UI or something
}

This is a fairly common pattern when dealing with asynchronous activity, such as working with network calls.

Answer 2

The simplest (perhaps no the prettiest), way of doing this would simply be to declare and instantiate a variable above the dispatch queue. Then you can set the variable equal to whatever you want, within the dispatch queue, and return it afterwards. You can change the type of ret, so that it suits your needs more directly.

func loadUrl(url:String) -> String {
    var ret = NSObject()
    DispatchQueue.global().async {
        do {
            let appUrl = URL(string:url)!
            let data = try Data(contentsOf:appUrl)
            let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
            print("Test from do")
            if let results = json["results"] as? [[String:Any]] {
                print("Test from if let 1")
                if let first = results[0] as? [String:Any] {

                    print("Test from if let 2")
                    var cityStateLocation = first["formatted_address"]!

                    ret = cityStateLocation
                    //What needs to be returned
                }
            }
            DispatchQueue.main.async {
                print("No Error")
            }
        } catch {
            DispatchQueue.main.async {
                print("Cannot connect to the server.")
            }
        }
    }
return ret
}

Answer 3

DispatchQueue.global().async will cause the coded included in the closure to be executed at some point the future, meaning you loadUrl function will return (almost) immediately.

What you need is some kind of callback which can be called when you have a result (AKA closure)

This is just another way to approach the problem, the difference between this and Josh's example is simply, I provide an additional closure to handle the errors

func loadUrl(url:String, complition: @escaping (String?) -> Void, fail: @escaping (Error) -> Void) {
    DispatchQueue.global().async {
        do {
            let appUrl = URL(string:url)!
            let data = try Data(contentsOf:appUrl)
            let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
            print("Test from do")
            if let results = json["results"] as? [[String:Any]], !results.isEmpty {
                print("Test from if let 1")
                let first = results[0]

                print("Test from if let 2")
                if let cityStateLocation = first["formatted_address"] as? String {
                    complition(cityStateLocation)
                } else {
                    complition(nil)
                }
            } else {
                complition(nil)
            }
        } catch let error {
            fail(error)
        }
    }
}

Which you might call using something like...

loadUrl(url: "your awesome url", complition: { (value) in
    guard let value = value else {
        // No value
        return
    }
    // process value
}) { (error) in
    // Handle error
}