Uploading Data to a Website

Post data from your app to servers.


Many apps work with servers that accept uploads of files like images or documents, or use web service API endpoints that accept structured data like JSON. To upload data from your app, you use a URLSession instance to create a URLSessionUploadTask instance. The upload task uses a URLRequest instance that details how the upload is to be performed.

Prepare Your Data for Upload

The data to upload can be the contents of a file, a stream, or data, as is the case in Listing 1.

Many web service endpoints take JSON-formatted data, which you create by using the JSONEncoder class on Encodable types like arrays and dictionaries. As shown in Listing 1, you can declare a structure that conforms to Codable, create an instance of this type, and use JSONEncoder to encode the instance to JSON data for upload.

Listing 1

Preparing JSON data for upload

struct Order: Codable {
    let customerId: String
    let items: [String]

// ...

let order = Order(customerId: "12345",
                  items: ["Cheese pizza", "Diet soda"])
guard let uploadData = try? JSONEncoder().encode(order) else {

There are many other ways to create a data instance, such as encoding an image as JPEG or PNG data, or converting a string to data by using an encoding like UTF-8.

Configure an Upload Request

An upload task requires a URLRequest instance. As shown in Listing 2, set the httpMethod property of the request to "POST" or "PUT", depending on what the server supports and expects. Use the setValue(_:forHTTPHeaderField:) method to set the values of any HTTP headers that you want to provide, except the Content-Length header. The session figures out content length automatically from the size of your data.

Listing 2

Configuring a URL request

let url = URL(string: "")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")

Create and Start an Upload Task

To begin an upload, call uploadTask(with:from:completionHandler:) on a URLSession instance to create an uploading URLSessionTask instance, passing in the request and the data instances you’ve previously set up. Because tasks start in a suspended state, you begin the network loading process by calling resume() on the task. Listing 3 uses the shared URLSession instance, and receives its results in a completion handler. The handler checks for transport and server errors before using any returned data.

Listing 3

Starting an upload task

let task = URLSession.shared.uploadTask(with: request, from: uploadData) { data, response, error in
    if let error = error {
        print ("error: \(error)")
    guard let response = response as? HTTPURLResponse,
        (200...299).contains(response.statusCode) else {
        print ("server error")
    if let mimeType = response.mimeType,
        mimeType == "application/json",
        let data = data,
        let dataString = String(data: data, encoding: .utf8) {
        print ("got data: \(dataString)")

Alternatively, Upload by Setting a Delegate

As an alternative to the completion handler approach, you can instead set a delegate on a session you configure, and then create the upload task with uploadTask(with:from:). In this scenario, you implement methods from the URLSessionDelegate and URLSessionTaskDelegate protocols. These methods receive the server response and any data or transport errors.

See Also

First Steps

Fetching Website Data into Memory

Receive data directly into memory by creating a data task from a URL session.

Downloading Files in the Background

Create tasks that download files while your app is inactive.

class URLSession

An object that coordinates a group of related network data transfer tasks.

class URLSessionConfiguration

A configuration object that defines behavior and policies for a URL session.

class URLSessionTask

A task, like downloading a specific resource, performed in a URL session.