Swift: Asynchronous callback

Question

How do I make asynchronous callbacks in swift? I'm writing a little Framework for my app because it's supposed to run on both, iOS und OS X. So I put the main code that is not device-specific into this framework that also handles requests to my online api. And obviously I also want the app's GUI and therefore my ViewControllers to react as soon as a api request has finished. In Objective-C I've done this by saving the view containing the function that had to be called in an id variable and the function itself in a selector variable. Then I invoked the function using the following code:

SEL selector = callbackMethod;
((void (*)(id, SEL))[callbackViewController methodForSelector:selector])(callbackViewController, selector);

How can I accomplish this in swift? Or is there a better way of doing this?

I really appreciate all your help!

Answer 1

I'd like to recommend use a block or closure callback instead of using NSThread and selectors.

For example, in my API I have follow method:

Swift:

Below you will find an updated implementation.

func getUsers(completion: (result: NSArray?, error: NSError?)->())
{
    var session = NSURLSession.sharedSession()
    var task = session.dataTaskWithRequest(request){
     (data, response, error) -> Void in
       if error != nil {
         completion(nil, error)
       } else {
         var result:NSArray = data to NSArray;
         completion(result, nil)
       }
    }
    task.resume()
}

Objective-C:

...
typedef void (^CBSuccessBlock)(id result);
typedef void (^CBFailureBlock)(NSError *error);
...

- (void)usersWithSucces:(CBSuccessBlock)success failure:(CBFailureBlock)failure
{
    NSURLSession *session = [NSURLSession sharedSession];
    [[session dataTaskWithURL:[NSURL URLWithString:url]
            completionHandler:^(NSData *data,
                                NSURLResponse *response,
                                NSError *error) {

                NSArray *users = //convert data to array

                if(error)
                    failure(error);
                else
                    success(users);
            }] resume];
}

Then, just make a call to api from view controller:

Objc:
[api usersWithSucces:^(id result)
{
   //Success callback
} failure:^(NSError *error)
{
   //Failure callback
}];

Swift:
api.getUsers({(result: AnyObject?, error: NSError?) -> Int in
    // callback here
})

UPDATE:

Meanwhile, I see that the question and answers are still being useful and interested. Well, here is an updated version of swift implementation using generic enum as a result object:

//Generic enum that represents the result
enum AsyncResult<T>
{
    case Success(T)
    case Failure(NSError?)
}


class CustomUserObject
{

}

func getUsers(completion: (AsyncResult<[CustomUserObject]>)->())
{
    let request = NSURLRequest()
    let session = NSURLSession.sharedSession()
    let task = session.dataTaskWithRequest(request){
        (data, response, error) -> Void in
        if let error = error
        {
            completion(AsyncResult.Failure(error))
        } else {
            let result: [CustomUserObject] = []//deserialization json data into array of [CustomUserObject]
            completion(AsyncResult.Success(result))
        }
    }
    task.resume()
}

//Usage:

getUsers { (result) in
    switch result
    {
    case .Success(let users):
        /* work with users*/
        break
    case .Failure(let error):
        /* present an error */
        break
    }
}

Answer 2

Can NSThread help you? :

NSThread.detachNewThreadSelector(<#selector: Selector#>, toTarget: <#AnyObject?#>, withObject: <#AnyObject?#>) 

Answer 3

I've shared the pattern that I use for this scenario in the following gist: https://gist.github.com/szehnder/84b0bd6f45a7f3f99306

Basically, I create a singleton DataProvider.swift that setups an AFNetworking client. Then the View Controllers call methods on that DataProvider, each of which is terminated by a closure that I've defined as a typealias called ServiceResponse. This closure returns either a dictionary or an error.

It allows you to very cleanly (imo) call for an async data action from the VC's with a very clear indication of what you want performed when that async response returns.

DataProvider.swift

typealias ServiceResponse = (NSDictionary?, NSError?) -> Void

class DataProvider: NSObject {

    var client:AFHTTPRequestOperationManager?
    let LOGIN_URL = "/api/v1/login"

    class var sharedInstance:DataProvider {
        struct Singleton {
            static let instance = DataProvider()
        }
        return Singleton.instance
    }

    func setupClientWithBaseURLString(urlString:String) {
        client = AFHTTPRequestOperationManager(baseURL: NSURL.URLWithString(urlString))
        client!.operationQueue = NSOperationQueue.mainQueue()
        client!.responseSerializer = AFJSONResponseSerializer()
        client!.requestSerializer = AFJSONRequestSerializer()
    }

    func loginWithEmailPassword(email:String, password:String, onCompletion: ServiceResponse) -> Void {
        self.client!.POST(LOGIN_URL, parameters: ["email":email, "password":password] , success: {(operation:AFHTTPRequestOperation!, responseObject:AnyObject!) -> Void in

            self.setupClientWithBaseURLString("http://somebaseurl.com")

            let responseDict = responseObject as NSDictionary
                // Note: This is where you would serialize the nsdictionary in the responseObject into one of your own model classes (or core data classes)
                onCompletion(responseDict, nil)
            }, failure: {(operation: AFHTTPRequestOperation!, error:NSError!) -> Void  in
                onCompletion(nil, error)
            })
    }
}

MyViewController.swift

import UIKit

class MyViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func viewWillAppear(animated: Bool)  {
        super.viewWillAppear(animated)
        DataProvider.sharedInstance.loginWithEmailPassword(email:"[e-mail]", password:"somepassword") { (responseObject:NSDictionary?, error:NSError?) in

            if (error) {
                println("Error logging you in!")
            } else {
                println("Do something in the view controller in response to successful login!")
            }
        }
    }  
}

Answer 4

I've just made this little example: Swift: Async callback block pattern example

Basically there is ClassA:

//ClassA it's the owner of the callback, he will trigger the callback when it's the time
class ClassA {
    //The property of that will be associated to the ClassB callback
    var callbackBlock : ((error : NSError?, message : String?, adress : String? ) -> Void)?

    init() {
        //Do Your staff
    }

    //Define your function with the clousure as a parameter
    func yourFunctionWithCallback(#functionCallbackParameter : (error : NSError?,message : String?, adress : String?) -> ()) {
        //Set the calback with the calback in the function parameter
        self.callbackBlock = functionCallbackParameter
    }

    //Later On..
    func callbackTrigger() {
        self.callbackBlock?(error: nil,message: "Hello callback", adress: "I don't know")

    }
}

And ClassB:

//ClassB it's the callback reciver the callback
class ClassB {
    @IBAction func testCallbackFunction(sender: UIButton) {
        let classA = ClassA()
        classA.yourFunctionWithCallback { (error, message, adress) -> () in
            //Do your stuff
        }
    }
}

ClassA: it's the owns a property witch is the callbackBlock. ClassB will set this property by Call the yourFunctionWithCallback function. Later on then ClassA it's ready, will trigger the callback by calling the callBackBlock inside the callbackTrigger function.

ClassB: will call the ClassA method to set the callback block and wait until the block has been triggered.