Passing variable outside of a function for use in Swift 3

Question

I'm new to Swift, and I want to 1) run a function that extracts a value from a JSON array (this part works) and 2) pass that variable into another function which will play that URL in my audio player.

My issue: I can't access that string stored in a variable outside the first function. Luckily, there's a bunch of questions on this (example), and they say to establish a global variable outside the function and update it. I have tried this like so:

var audio = "" override func viewDidLoad() { super.viewDidLoad() let url = URL(string: "http://www.example.json") URLSession.shared.dataTask(with:url!, completionHandler: {(data, response, error) in guard let data = data, error == nil else { return } let json: Any? do{ json = try JSONSerialization.jsonObject(with: data, options: []) } catch{ return } guard let data_list = json as? [[String:Any]] else { return } // here's the important part if let foo = data_list.first(where: {$0["episode"] as? String == "Special Episode Name"}) { // do something with foo self.audio = (foo["audio"] as? String)! } else { // item could not be found } }).resume() print(audio) // no errors but doesn't return anything

I have confirmed the JSON extraction is working -- if I move that print(audio) inside the function, it returns the value. I just can't use it elsewhere.
I originally tried it without the self. but returned an error.

Is there a better way to store this string in a variable so I can use it in another function?

EDIT: Trying new approach based on Oleg's first answer. This makes sense to me based on how I understand didSet to work, but it's still causing a thread error with the play button elsewhere.

 var audiotest = ""{ didSet{ // use audio, start player if let audioUrl = URL(string: audiotest) { let documentsDirectoryURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first! let destinationUrl = documentsDirectoryURL.appendingPathComponent(audioUrl.lastPathComponent) //let url = Bundle.main.url(forResource: destinationUrl, withExtension: "mp3")! do { audioPlayer = try AVAudioPlayer(contentsOf: destinationUrl) } catch let error { print(error.localizedDescription) } } // end player } } override func viewDidLoad() { super.viewDidLoad() let url = URL(string: "http://www.example.com/example.json") URLSession.shared.dataTask(with:url!, completionHandler: {(data, response, error) in guard let data = data, error == nil else { return } let json: Any? do{ json = try JSONSerialization.jsonObject(with: data, options: []) } catch{ return } guard let data_list = json as? [[String:Any]] else { return } if let foo = data_list.first(where: {$0["episode"] as? String == "Houston Preview"}) { // do something with foo self.audiotest = (foo["audio"] as? String)! } else { // item could not be found } print(self.audiotest) }).resume()

Hey @Priya, that would print; however, to clarify, my goal: use this string stored in the var in another function inside ViewDidLoad. Maybe I am missing something, but if possible, I need to be able to print outside the block to make it work (as I want to pass it as a URL in another function that will let the user download the audio from the link). Does that make more sense? — darkginger
– darkginger, Commented Jan 24, 2018 at 5:54
dataTask works asynchronously. pass it as a URL in another function inside the completion block. — vadian
– vadian, Commented Jan 24, 2018 at 5:56
I think you want to use audio after you get response from API. For that you need to use method which takes parameter of completionHandler or Block — Priya
– Priya, Commented Jan 24, 2018 at 5:57
Yeah, experimenting now. The audio player keeps turning up thread errors when I run it inside the block. Not sure why. Will keep testing. — darkginger
– darkginger, Commented Jan 24, 2018 at 6:04

Oleg Sherman · Accepted Answer · 2018-01-24 06:14:28Z

The request for the data is asynchronous so the code that is inside the completionHandler block happens some time later (depending on the server or the timeout) , that’s why if you try to print outside the completionHandler actually the print func happens before you get the data.

There are couple of solution: 1. Add property observer to your audio property and start playing when it is set:

var audio = “”{ didSet{ // use audio, start player } }

2. Wrapping the request with a method that one of its parameters is a completion closure:

// the request func fetchAudio(completion:(String)->()){ // make request and call completion with the string inside the completionHandler block i.e. completion(audio) } // Usage fetchAudio{ audioString in // dispatch to main queue and use audioString }

Oleg, your solution 1 works for me and I learned something. Now understand why the sequence matters when it seemed like the code was fine to me.
Shoot, I spoke too soon. It works fine with a static value there -- I didn't update it to be dynamic yet, and then I run into a thread issue which is weird. Trying to experiment with DispatchQueue.main.async -- will report back if I figure it out. From documentation, I thought putting my play audio in the didSet would do the trick.
Added what I am working on from your first solution -- now visible in the main thread in case anyone sees this question.

Priya · Accepted Answer · 2018-01-24 06:04:08Z

Try this code. No need to take global variable if it is not being used in multiple function. you can return fetched URL in completion handler.

 func getAudioUrl(completionHandler:@escaping ((_ url:String?) -> Void)) { let url = URL(string: "http://www.example.json") URLSession.shared.dataTask(with:url!, completionHandler: {(data, response, error) in guard let data = data, error == nil else { return } let json: Any? do{ json = try JSONSerialization.jsonObject(with: data, options: []) } catch{ return } guard let data_list = json as? [[String:Any]] else { return } // here's the important part if let foo = data_list.first(where: {$0["episode"] as? String == "Special Episode Name"}) { // do something with foo let audio = (foo["audio"] as? String)! completionHandler(audio) } else { // item could not be found completionHandler(nil) } }).resume() } func useAudioURL() { self.getAudioUrl { (url) in if let strUrl = url { // perform your dependant operation print(strUrl) }else { //url is nil } } }

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