"multipart/form-data" in Swift

URLRequestExtension.swift

//
//  URLRequestExtension.swift
//
//  Created by Cenk Bilgen on 2019-01-30.
//  Copyright © 2019 abc. All rights reserved.
//

import Foundation

extension URLRequest {
    
  // functions to create the http body if you want to use multipart/form-data
  
  // since the http connection is likely compressesd, the network inefficiency of transmitting text vs binary is moot
  // and just base64 encoding through JSON good enough,
  // but multipart/form-data is also straight forward if you know the spec
  
  // see https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html

  ///////////////// Example Usage
  
//  var request = URLRequest(url: url)
//  request.httpMethod = "POST"
//
//  let boundary = "-----BOUNDARY"
//  request.addValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
//
//  // 1. single-part form (to send just a single image file for example)
//  let data = try? request.formEncoded(data: data, filename: "file1.jpg", boundary: boundary)
//
//  //--- OR ---
//
//  // 2. multi-part form (to send an image file plus other metadata for example)
//  let formParts: [URLRequest.FormPart] = [FormPart(data: data1, filename: nil, mimetype: "text/plain")
//                    FormPart(data: data2, filename: "file1.jpg", mimetype: "image/jpeg")]
//  let data = try? request.formEncoded(parts: formParts, boundary: boundary)
  
// let task = URLSession.shared.uploadTask(with: request, from: data) { }
  
  ///////////////////////////////////
 

  struct FormPart {
    let data: Data
    let filename: String? // not included if nil
    let mimetype: String? // "application/octet-stream" if nil. see https://www.w3.org/Protocols/rfc1341/4_Content-Type.html
  }
  
  // Convert an array of form structures into the proper http request body form
  
  func formEncoded(parts: [FormPart], boundary: String = "-----BOUNDARY") throws -> Data {
    
    var body = Data()
    
    for part in parts {
      body += try encode(formPart: part, boundary: boundary)
    }
    
    guard body.isEmpty == false else {
      throw FormEncodingError.noParts
    }
    
    body += "\r\n--\(boundary)--\r\n".data(using: .ascii)!  // if there's a bad char in boundary, would've failed earlier
   
    // make this function mutating
//    self.addValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
//    self.httpBody = body
   
    return body
    
  }
  
  // Create data from a single form part, it's not enough to send by itself
  
  fileprivate func encode(formPart part: FormPart, boundary: String) throws -> Data {
    
    var boundaryHead = "--\(boundary)\r\n"
    
    boundaryHead += "Content-Disposition: form-data; name=\"file\""
    if let filename = part.filename {
      boundaryHead += "; filename=\"\(filename)\"\r\n"
    }
    
    boundaryHead += "Content-Type: \(part.mimetype ?? "application/octet-stream")\r\n\r\n"
    
    // can encode as as .utf8 if you add ,charset=UTF-8 to Content-Type
    guard let head = boundaryHead.data(using: .ascii) else {
      throw FormEncodingError.badboundaryValue
    }
    
    return head + part.data + "\r\n".data(using: .ascii)!
    
  }
  
  // This is alternative function if you just want to send a single-part form with data (like just one file)
  // doesn't need resizing of mutable Data structure like the multi-part version does
  
  func formEncoded(data: Data, filename: String, mimetype: String? = nil, boundary: String = "-----BOUNDARY") throws -> Data {
    
    var boundaryHead = ""
    
    boundaryHead += "--\(boundary)\r\n"
    boundaryHead += "Content-Disposition: form-data; name=\"file\"; filename=\"\(filename)\"\r\n"
    
    boundaryHead += "Content-Type: \(mimetype ?? "application/octet-stream")\r\n\r\n"
    
//    boundaryHead += "Content-Transfer-Encoding: binary\n\n" // is this necessary sometimes?
    
    let boundaryTail = "\r\n\r\n--\(boundary)--\r\n"
    
    // put it together: head + data + tail
    
    guard let head = boundaryHead.data(using: .ascii),
      let tail = boundaryTail.data(using: .ascii) else {
        throw FormEncodingError.badboundaryValue
    }
    
    let body = head + data + tail
    
    // make this function mutating
    //    self.addValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
    //    self.httpBody = body
    
    return body
    
  }
  
}

// MARK: Errors

enum FormEncodingError: Error {
  case badboundaryValue
  case noParts
}