Completion handlers in Swift 3.0

A completion or closure is just a function wrapped up into a parameter...

You can create a function with a closure like so...

func doSomethingAsync(completion: () -> ()) {
}

The parameter completion is of type () -> () that is... it is a function -> that takes no input parameters () and returns void ().

You could also make a function like...

// (inputs) -> (outputs)
(String) -> ()

Or with any inputs or outputs you want.

Now, like you have in your question. This function may call some other async function...

func myAsyncFunction(completion: () -> ()) {

    someOtherAsyncFunction() {

        // This is the completion of "someOtherAsyncFunction"

        // Call YOUR completion here...
        completion()
    }

}

To make sure that YOUR completion is called AFTER the other async method is done put it inside the completion of the other method. Like above.

Now, to call this you can do...

self.myAsyncFunction() {
    // your completion block code here.
}

Your completion block code will now be called AFTER the other async method has finished.

Of course, if you have several paths in the other completion (like errors etc...) then you have to call your completion at each end point...

func myAsyncFunction(completion: () -> ()) {

    someOtherAsyncFunctionWithAPossibleError() {
        error in

        if error != nil {
            completion()
            // this return means the other completion won't be run
            return
        }

        completion()
    }

}

Swift 4:

1. Create a completion block.

   func getDataFromJson(url: String, parameter: String, completion: @escaping (_ success: [String : AnyObject]) -> Void) {
   
       //@escaping...If a closure is passed as an argument to a function and it is invoked after the function returns, the closure is @escaping.
   
       var request = URLRequest(url: URL(string: url)!)
       request.httpMethod = "POST"
       let postString = parameter
   
       request.httpBody = postString.data(using: .utf8)
       let task = URLSession.shared.dataTask(with: request) { Data, response, error in
   
           guard let data = Data, error == nil else {  // check for fundamental networking error
   
               print("error=\(error)")
               return
           }
   
           if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {  // check for http errors
   
               print("statusCode should be 200, but is \(httpStatus.statusCode)")
               print(response!)
               return
   
           }
   
           let responseString  = try! JSONSerialization.jsonObject(with: data, options: .allowFragments) as! [String : AnyObject]
           completion(responseString)
   
   
   
       }
       task.resume()
   }
   

2. Call method

   getDataFromJson(url: "http://example.com", parameter: "vehicle_type=Car", completion: { response in
           print(response)
   
       })