Demo code for Strongly-Typed GET and POST Calls With Alamofire at https://grokswift.com/strongly-typed-api-calls/. Uses Swift 3.0 and Alamofire 4.0.

Todo+Networking.swift

import Foundation
import Alamofire

enum BackendError: Error {
  case objectSerialization(reason: String)
}

extension Todo {
  class func endpointForID(_ id: Int) -> String {
    return "https://jsonplaceholder.typicode.com/todos/\(id)"
  }
  class func endpointForTodos() -> String {
    return "https://jsonplaceholder.typicode.com/todos/"
  }
  
  convenience init?(json: [String: Any]) {
    guard let title = json["title"] as? String,
      let userId = json["userId"] as? Int,
      let completed = json["completed"] as? Bool
      else {
        return nil
    }
    
    let idValue = json["id"] as? Int
    
    self.init(title: title, id: idValue, userId: userId, completedStatus: completed)
  }
  
  func toJSON() -> [String: Any] {
    var json = [String: Any]()
    json["title"] = title
    if let id = id {
      json["id"] = id
    }
    json["userId"] = userId
    json["completed"] = completed
    return json
  }
  
  class func todoByID(_ id: Int, completionHandler: @escaping (Result<Todo>) -> Void) {
    Alamofire.request(Todo.endpointForID(id))
      .responseJSON { response in
        let result = Todo.todoFromResponse(response: response)
        completionHandler(result)
    }
  }
  
  func save(completionHandler: @escaping (Result<Todo>) -> Void) {
    let fields = self.toJSON()
    Alamofire.request(Todo.endpointForTodos(),
                      method: .post,
                      parameters: fields,
                      encoding: JSONEncoding.default,
                      headers: nil)
      .responseJSON { response in
        let result = Todo.todoFromResponse(response: response)
        completionHandler(result)
    }
  }
  
  private class func todoFromResponse(response: DataResponse<Any>) -> Result<Todo> {
    guard response.result.error == nil else {
      // got an error in getting the data, need to handle it
      print(response.result.error!)
      return .failure(response.result.error!)
    }
    
    // make sure we got JSON and it's a dictionary
    guard let json = response.result.value as? [String: Any] else {
      print("didn't get todo object as JSON from API")
      return .failure(BackendError.objectSerialization(reason:
        "Did not get JSON dictionary in response"))
    }
    
    // turn JSON in to Todo object
    guard let todo = Todo(json: json) else {
      return .failure(BackendError.objectSerialization(reason:
        "Could not create Todo object from JSON"))
    }
    return .success(todo)
  }
}

Todo.swift

import Foundation

class Todo {
  var title: String
  var id: Int?
  var userId: Int
  var completed: Bool
  
  required init?(title: String, id: Int?, userId: Int, completedStatus: Bool) {
    self.title = title
    self.id = id
    self.userId = userId
    self.completed = completedStatus
  }
  
  func description() -> String {
    return "ID: \(self.id), " +
      "User ID: \(self.userId), " +
      "Title: \(self.title)\n" +
    "Completed: \(self.completed)\n"
  }
}

ViewController.swift

import UIKit

class ViewController: UIViewController {
  override func viewWillAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    
    // MARK: Get Todo #1
    Todo.todoByID(1) { result in
      if let error = result.error {
        // got an error in getting the data, need to handle it
        print("error calling POST on /todos/")
        print(error)
        return
      }
      guard let todo = result.value else {
        print("error calling POST on /todos/ - result is nil")
        return
      }
      // success!
      print(todo.description())
      print(todo.title)
    }
    
    // MARK: Create new todo
    guard let newTodo = Todo(title: "My first todo",
                             id: nil,
                             userId: 1,
                             completedStatus: true) else {
                              print("error: newTodo isn't a Todo")
                              return
    }
    newTodo.save { result in
      guard result.error == nil else {
        // got an error in getting the data, need to handle it
        print("error calling POST on /todos/")
        print(result.error!)
        return
      }
      guard let todo = result.value else {
        print("error calling POST on /todos/. result is nil")
        return
      }
      // success!
      print(todo.description())
      print(todo.title)
    }
  }
}