Synchronous v/s Asynchronous API call in Swift

In Swift, the main difference between a synchronous API call and an asynchronous one is how you handle the response data.

Synchronous call

With a synchronous API call, the app freezes while the call is being made. The app remain unresponsive until a response is available.

Here is an example using the URLSession:

let url = URL(string: "https://www.boredapi.com/api/activity")!
let request = URLRequest(url: url)

do {
    let data = try URLSession.shared.data(request: request)
    let activity = try JSONDecoder().decode(Activity.self, from: data)
    self.activities = activity
} catch {
    print("Error decoding data")
}

Asynchronous call

With an asynchronous API call, the app remain responsive while the API call is being made.

Here is an example using the URLSession:

let url = URL(string: "https://www.boredapi.com/api/activity")!
let request = URLRequest(url: url)

URLSession.shared.dataTask(with: request) { data, response, error in
    if let error = error {
	print("Error fetching data: \(error)")        
	return
    }

    guard let data = data else {
        print("No data")
        return
    }

    do {
      let activity = try JSONDecoder().decode(Activity.self, from: data)
      DispatchQueue.main.async {
	self.activities = activity
      }
    } catch {
        print("Error decoding data: \(error)")
    }
}.resume()

As can be noted, the asynchronous API call uses a closure that gets called when the response comes back. This closure allows you to handle the response data without blocking the main thread of your app.