Custom HTTP Headers with Swift 3.0 and Alamofire 4.0: See https://grokswift.com/custom-headers-alamofire4-swift3/ for explanations

Custom HTTP Headers with Swift 3.0 and Alamofire 4.0.swift

// MARK: - Adding a header to a single request

let headers: HTTPHeaders = [
  "X-Mashape-Key": MY_API_KEY,
  "Accept": "application/json"
]

Alamofire.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh", headers: headers)
  .responseJSON { response in
  debugPrint(response)
}

// MARK: - To make sure the headers are sent, use debugPrint on the request
let request = Alamofire.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh", headers: headers)
  .responseJSON { response in
  debugPrint(response)
}
debugPrint(request)

// MARK: - Adding headers to all requests in the session

// get the default headers
var headers = Alamofire.SessionManager.defaultHTTPHeaders
// add your custom header
headers["API-Version"] = "2.0"

// create a custom session configuration
let configuration = URLSessionConfiguration.default
// add the headers
configuration.httpAdditionalHeaders = headers

// create a session manager with the configuration
let sessionManager = Alamofire.SessionManager(configuration: configuration)

// make calls with the session manager
sessionManager.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh")
  .responseJSON { response in
  debugPrint(response)
}

// MARK: - Add headers when using a URLRequest

if let url = URL(string: "https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh") {
  var urlRequest = URLRequest(url: url)
  urlRequest.httpMethod = HTTPMethod.get.rawValue
  
  urlRequest.addValue(MY_API_KEY, forHTTPHeaderField: "X-Mashape-Key")
  urlRequest.addValue("application/json", forHTTPHeaderField: "Accept")
  
  Alamofire.request(urlRequest)
    .responseJSON { response in
      debugPrint(response)
  }
}

// Alternatively:
if let url = URL(string: "https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh") {
  var urlRequest = URLRequest(url: url)
  urlRequest.httpMethod = HTTPMethod.get.rawValue
  
  var headers: HTTPHeaders
  if let existingHeaders = urlRequest.allHTTPHeaderFields {
    headers = existingHeaders
  } else {
    headers = HTTPHeaders()
  }
  headers["X-Mashape-Key"] = MY_API_KEY
  headers["Accept"] = "application/json"
  urlRequest.allHTTPHeaderFields = headers
  
  let request = Alamofire.request(urlRequest)
    .responseJSON { response in
      debugPrint(response)
  }
  debugPrint(request)
}

// MARK: - Use an Alamofire RequestAdapter to add headers to all calls in the session

class MashapeHeadersAdapter: RequestAdapter {
  func adapt(_ urlRequest: URLRequest) throws -> URLRequest {
    var urlRequest = urlRequest
    
    urlRequest.setValue(MY_API_KEY, forHTTPHeaderField: "X-Mashape-Key")
    urlRequest.setValue("application/json", forHTTPHeaderField: "Accept")
    
    return urlRequest
  }
}

// Then to make the call:
// get a session manager and add the request adapter
let sessionManager = Alamofire.SessionManager.default
sessionManager.adapter = MashapeHeadersAdapter()

// make calls with the session manager
let request = sessionManager.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh")
  .responseJSON { response in
    debugPrint(response)
  }
debugPrint(request)