Closures return value (previously completionBlock)

Question

I'd like to return some values after the long term operation is completed. But furthermore I'd like to split the logic and the gui.

For example; I have two classes

1. SomeServices.swift which has a method named "getDataFromService..."
2. MyTableViewController.swift which will display the result from "getDataFromService"

So, previously in Objective-C I've just add a method in SomeServices like this:

(void)getDataFromService:(void (^)(NSArray *, NSError *))completionBlock{ ...... }

In this method I've just called completionBlock(myData, myError) to return my values to the tableviewcontroller.

What would be the equivalent closure which I have to define in SomeServices.swift and how will it be called in MyTableViewController?

I know how to call a simple closures like this one:

   ....({
            responseData, error  in
            if(!error){
                //Do something
            }
        })

But I don't have any ideas how to define a closure with a completionBlock equivalent.

Any help would be appreciated

Answer 1

The plus of closures are, that you can pass everything you want. Methods or functions - it doesn't matter.

You can pass a function within the parameters and just call it.

func someFunctionThatTakesAClosure(completionClosure: () -> ()) {
    // function body goes here
    if(error = false) {
       completionClosure()
    }

}

//Call it
someFunctionThatTakesAClosure({
    //Completions Stuff
    println("someFunctionThatTakesAClosure")
});

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/ch/jEUH0.l

Answer 2

The answer is in the language guide:

Assume you want to return a String. This is the syntax

({(responseData: DataClass, error: ErrorClass) -> String in
//do stuff - calculations etc..
return calculatedString
})

Here is an example that takes two strings and concatenates them, and returns the result:

let sumStrings = ({(first: String, second: String) -> String in
    return first + " " + second
})

then you can do the following:

sumStrings("Hello","Swift")             // "Hello Swift"

Answer 3

Here is how I use a singleton ServiceManager to achieve this.

class ServiceManager: NSObject {

//  Static Instance variable for Singleton
static var sharedSessionManager = ServiceManager()

//  Function to execute GET request and pass data from escaping closure
func executeGetRequest(with urlString: String, completion: @escaping (Data?) -> ()) {

    let url = URL.init(string: urlString)
    let urlRequest = URLRequest(url: url!)

    URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
        //  Log errors (if any)
        if error != nil {
            print(error.debugDescription)
        } else {
            //  Passing the data from closure to the calling method
            completion(data)
        }
    }.resume()  // Starting the dataTask
}

//  Function to perform a task - Calls executeGetRequest(with urlString:) and receives data from the closure.
func downloadMovies(from urlString: String, completion: @escaping ([Movie]) -> ()) {
    //  Calling executeGetRequest(with:)
    executeGetRequest(with: urlString) { (data) in  // Data received from closure
        do {
            //  JSON parsing
            let responseDict = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any]
            if let results = responseDict!["results"] as? [[String:Any]] {
                var movies = [Movie]()
                for obj in results {
                    let movie = Movie(movieDict: obj)
                    movies.append(movie)
                }
                //  Passing parsed JSON data from closure to the calling method.
                completion(movies)
            }
        } catch {
            print("ERROR: could not retrieve response")
        }
    }
  }
}

Below is the example how I use the singleton class.

ServiceManager.sharedSessionManager.downloadMovies(from: urlBase) { (movies : [Movie]) in   // Object received from closure
self.movies = movies
      DispatchQueue.main.async {
            //  Updating UI on main queue
            self.movieCollectionView.reloadData()
      }
}

I hope this helps anybody looking for the same solution.