Completion handlers in iOS

It is extremely important to understand how asynchronous networking works. We are encountering it in almost all of our projects. There are three major types of asynchronous networking calls in iOS:

  1. the Delegate pattern
  2. the Notification Center
  3. the Closure type

In previous posts we already looked at the delegate pattern as well as NSNotificationCenter, so we are going to take a look at closures, which can take many forms, most common being blocks or completion handlers. To start, create a new project and in View Controller let’s write a function that geocodes a location, that is, returns the geographical coordinates (latitude and longitude) for a city we give it as input.

var coordinates = (0.0 , 0.0)

func geocoding(location: String, completion: () -> ()) {
    CLGeocoder().geocodeAddressString(location) { (placemarks, error) in
        if placemarks?.count > 0 {
            let placemark = placemarks?[0]
            let location = placemark!.location
            let coordinate = location?.coordinate
            self.coordinates = (coordinate!.latitude, coordinate!.longitude)
            print("Inside completion handler: \(self.coordinates)")
    print("Outside completion handler: \(self.coordinates)")

The Core Location framework provides the CLGeocoder class whose method named geocodeAddressString() does the forward geocoding for us. It requires that we provide a string and a completion handler as input parameters. But since the completion handler is the last argument of the function, the compiler lets us modify the function signature, from having the completion handler inside the parentheses:

CLGeocoder().geocodeAddressString(location, completionHandler: { ... })

to taking it outside of the parentheses, and even omit its name altogether:

CLGeocoder().geocodeAddressString(location) { ... }

Now let’s call the fuction inside viewDidLoad() and see the different behaviors:

override func viewDidLoad() {
    print("At load time: \(coordinates)")
    geocoding("New York, NY") {
        print("After handler completes: \(self.coordinates)")

If you build and run the project you should see the following output:

At load time: (0.0, 0.0)
Outside completion handler: (0.0, 0.0)
Inside completion handler: (40.713054, -74.007228)
After handler completes: (40.713054, -74.007228)

So first we print the coordinates before even calling the geocoding function and the result is (0.0, 0.0) as expected, because we have not yet changed our global variable. Then the fun begins! Once we called the function, the code that finished first was the print statement outside the completion handler block, because the code inside the block was still running, so obviously the result is still (0.0, 0.0) as the block has not yet updated the global variable. However, we notice that inside the block, the result is correct. What we had to do then was to pass the result to the caller line inside viewDidLoad() and we achieved that by calling the completion handler which only has a print statement, as an argument for the geocoding() call. To break this call, just comment out the completion() line inside the geocoding() function and see the difference.

Until next time!