call-web-service.swift

import Foundation

let api = "https://jsonplaceholder.typicode.com/posts/1"
let url = URL(string: api)!
let task = URLSession.shared.dataTask(with: url) { data, urlResponse, error in
    if let data = data, let json = String(data: data, encoding: .utf8) {
        print(json)        
    }
}
task.resume()

RunLoop.main.run(until: Date(timeIntervalSinceNow: 2))

download1.swift

import Foundation

let api = "https://jsonplaceholder.typicode.com/posts/1"
if let url = URL(string: api) {
  let list = try? String(contentsOf: url)
  if let data = list {
    print(type(of: data), data)
  }
}

download2.swift

import Foundation

let api = "https://jsonplaceholder.typicode.com/posts/1"
let url = URL(string: api)!
let task = URLSession.shared.downloadTask(with: url) { localURL, urlResponse, error in
    if let localURL = localURL {
        if let string = try? String(contentsOf: localURL) {
            print(string)
        }
    }
}
task.resume()
RunLoop.main.run(until: Date(timeIntervalSinceNow: 2))

download3.swift

import Foundation

let params = ["username":"john", "password":"123456"] as Dictionary<String, String>
let url = "https://jsonplaceholder.typicode.com/posts/1"
var request = URLRequest(url: URL(string: url)!)

request.httpMethod = "POST"
request.httpBody = try? JSONSerialization.data(withJSONObject: params, options: [])
request.addValue("application/json", forHTTPHeaderField: "Content-Type")

let session = URLSession.shared
let task = session.dataTask(with: request, completionHandler: { data, response, error -> Void in
    print(response)
    do {
        let json = try JSONSerialization.jsonObject(with: data!) as! Dictionary<String, AnyObject>
        print(json)
    } catch {
        print("error")
    }
})

task.resume()

RunLoop.main.run(until: Date(timeIntervalSinceNow: 2))

jsonencoder.swift

import Foundation

let dic = ["2": "B", "1": "A", "3": "C"]
let encoder = JSONEncoder()
if let jsonData = try? encoder.encode(dic) {
    if let jsonString = String(data: jsonData, encoding: .utf8) {
        print(jsonString)
    }
}