Orestis Papadopoulos

Orestis Papadopoulos

Software, Sound, Swift

How To Upload Data To A Server: Multipart/Form-Data HTTP Requests in Swift

So you are making your first face beautifier© app and it’s about time to upload some images to a server. The backend person asks you to do it via a type of HTTP POST request known as multipart/form-data. Soon you come to realise that URLSession does not provide you with an out of the box URLRequest or DataTask for this specific task, despite the fact that this is a very standard way of uploading data.

And that’s when you reach for Alamofire.

Or you can stick with URLSession and make your own MultipartFormDataRequest

It works like this:

func uploadImage(imageData: Data) {
    let request = MultipartFormDataRequest(url: URL(string: "https://server.com/uploadPicture")!)
    request.addDataField(named: "profilePicture", data: imageData, mimeType: "img/jpeg")
    URLSession.shared.dataTask(with: request, completionHandler: {

MultipartFormDataRequest is very similar to a URLRequest in that it is initialised by a URL. The URL endpoint that you will be sending data to.

Use addTextField to send text and addDataField to send anything that can be converted to Data (images, audio, Grand Theft Auto 2 savefiles, you name it)

struct MultipartFormDataRequest {
    private let boundary: String = UUID().uuidString
    private var httpBody = NSMutableData()
    let url: URL

    init(url: URL) {
        self.url = url

    func addTextField(named name: String, value: String) {
        httpBody.append(textFormField(named: name, value: value))

    private func textFormField(named name: String, value: String) -> String {
        var fieldString = "--\(boundary)\r\n"
        fieldString += "Content-Disposition: form-data; name=\"\(name)\"\r\n"
        fieldString += "Content-Type: text/plain; charset=ISO-8859-1\r\n"
        fieldString += "Content-Transfer-Encoding: 8bit\r\n"
        fieldString += "\r\n"
        fieldString += "\(value)\r\n"

        return fieldString

    func addDataField(named name: String, data: Data, mimeType: String) {
        httpBody.append(dataFormField(named: name, data: data, mimeType: mimeType))

    private func dataFormField(named name: String,
                               data: Data,
                               mimeType: String) -> Data {
        let fieldData = NSMutableData()

        fieldData.append("Content-Disposition: form-data; name=\"\(name)\"\r\n")
        fieldData.append("Content-Type: \(mimeType)\r\n")

        return fieldData as Data

extension NSMutableData {
  func append(_ string: String) {
    if let data = string.data(using: .utf8) {

You also need a function to convert a MultipartFormDataRequest to a URLRequest in order to be able to create URLSessionDataTasks

func asURLRequest() -> URLRequest {
    var request = URLRequest(url: url)

    request.httpMethod = "POST"
    request.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")

    request.httpBody = httpBody as Data
    return request

Bonus round:

A handy URLSession extension. You can also make this an upload task, or use background sessions etc.

extension URLSession {
    func dataTask(with request: MultipartFormDataRequest,
                  completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void)
    -> URLSessionDataTask {
        return dataTask(with: request.asURLRequest(), completionHandler: completionHandler)

And this is it. You can now upload your image to the server. A very good example of how simple (or scary) HTTP can be.

I didn’t go into much detail about the HTTP specifics because:

  1. This is a sort of tl;dr blog post where I’m sharing some (I hope) useful code with you
  2. Donny Wals has done an excellent job at explaining this in his blog post
  3. You can read about it here

Feel free to contact me or tweet to me on Twitter for tips, feedback, opinions.

Thank you for reading!