Completion handlers in Swift 3.0

Question

I am using the code below to sync data with my server. After completing the task, I would like to call:

self.refreshControl?.endRefreshing()

However, I would like to make sure it happens after anything that might happen inside this method. Is this where I would use a completion handler? It is confusing to me because I am already running code that gets executed after getting the http response. If I add a completion handler, does it get executed after the http response is received? And could I put my endRefreshing() code there that would happen after anything that might happen in the code below? Thanks!

func syncCustomers(token: String) {
    let url:NSURL = NSURL(string: Constants.Api.BaseUrl + "api/customer")!
    let session = URLSession.shared
    let request = NSMutableURLRequest(url: url as URL)
    request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
    request.httpMethod = "GET"
    let task = session.dataTask(with: request as URLRequest) { (data, response, error) in
        guard let data = data else { return }
        do {
            if error != nil {
                self.showAlert(title: "Error", message: error!.localizedDescription)
            }
            else if let httpResponse = response as? HTTPURLResponse {
                if httpResponse.statusCode == 200 {
                    let json = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? Array<Any>
                    DispatchQueue.global().async {
                        for item in json! {
                            if let customer = Customer(json: item as! [String : Any]) {
                                _ = SqliteDB.instance.replaceCustomer(customer: customer)
                            }
                        }
                        self.customers = SqliteDB.instance.getCustomers()
                        self.tableView.reloadData()
                    }
                } else if httpResponse.statusCode == 401 {
                    self.showAlert(title: "Error", message: "Unauthorized. Please try logging in again.")

                }
            }
        } catch let error as NSError {
            self.showAlert(title: "Error", message: error.localizedDescription)
        }
    }
    task.resume()
}

Answer 1

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()
    }

}