fetch_api_with_URLSession.swift

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

let url = URL(string: "https://jsonplaceholder.typicode.com/posts/1")!
func fetch(callback:( (String)  -> Void)? = nil) {
    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard error == nil else {
            callback?(error?.localizedDescription ?? "エラーです")
            return
        }

        guard let data = data, let contents = String(data: data, encoding: String.Encoding.utf8) else {
            callback?("変換エラーです")
            return
        }
        callback?(contents)
    }
    task.resume()
}

fetch { content in
    print(content)
}