Uploading Streams of Data

Send a stream of data to a server.


Streaming media apps and long-running apps that send continual updates use an ongoing stream to upload data, rather than sending a single block of data or a flat file. You can configure an instance of URLSessionUploadTask (a subclass of URLSessionTask) to work with a stream that you provide, and then fill this stream with data indefinitely.

The task gets the stream by calling your session’s delegate, so you need to create a session and set your own code as its delegate.

Create a URL Session

Begin by creating a URLSession and providing it with a delegate. Listing 1 creates a URL session with the default URLSessionConfiguration and sets self as the delegate. You’ll implement URLSessionTaskDelegate later, in Provide the Stream to the Upload Task.

Listing 1

Creating a URLSession with a delegate

lazy var session: URLSession = URLSession(configuration: .default,
                                          delegate: self,
                                          delegateQueue: .main)

Create a Streaming Upload Task

Create the upload task with the URLSession method uploadTask(withStreamedRequest:). This takes a URLRequest specifying the URL you want to upload to, along with other parameters. You start the task by calling resume(). Listing 2 shows how to create and start an upload task, connecting to a server on the local machine ( listening on port 12345.

Listing 2

Creating an upload task

let url = URL(string: "")!
var request = URLRequest(url: url,
                         cachePolicy: .reloadIgnoringLocalCacheData,
                         timeoutInterval: 10)
request.httpMethod = "POST"
let uploadTask = session.uploadTask(withStreamedRequest: request)

Use a Bound Pair of Streams to Provide an Input Stream

You provide the streaming data to the upload task as an InputStream. The task reads data from this stream and uploads it to the destination.

A good way to provide data to the input stream is to use a bound pair of streams. The bound pair contains an OutputStream that you write data to. Thanks to the binding of the streams, the data you write to the output stream is made available to the input stream, which the task can then read from. Figure 1 shows this arrangement.

Figure 1

Providing data to an upload task with a bound pair of streams

Flow diagram showing how data written by an app to the output stream of a bound pair goes into a buffer, then to the bound pair's input stream, then to the upload task, which sends it to the destination.

Listing 3 shows a structure called Streams that consists of an InputStream and an OutputStream. The listing creates a property of this type, called boundStreams, by calling the getBoundStreams(withBufferSize:inputStream:outputStream:) method of the Stream class, passing in in-out references for the input and output streams.

Listing 3

Creating a bound pair of input and output streams

struct Streams {
    let input: InputStream
    let output: OutputStream
lazy var boundStreams: Streams = {
    var inputOrNil: InputStream? = nil
    var outputOrNil: OutputStream? = nil
    Stream.getBoundStreams(withBufferSize: 4096,
                           inputStream: &inputOrNil,
                           outputStream: &outputOrNil)
    guard let input = inputOrNil, let output = outputOrNil else {
        fatalError("On return of `getBoundStreams`, both `inputStream` and `outputStream` will contain non-nil streams.")
    // configure and open output stream
    output.delegate = self
    output.schedule(in: .current, forMode: .default)
    return Streams(input: input, output: output)

When you create the bound pair, make sure you specify a buffer size large enough to hold any data you write to the output stream, prior to the data being read from the input stream. Listing 3 uses a 4096-byte buffer.

The listing also sets self as the output stream’s delegate. Declare that your class implements the StreamDelegate protocol in order to receive events that indicate when the output stream is ready to receive new data. You’ll provide the implementation of StreamDelegate later, in Write Data to the Stream When It’s Ready.

Provide the Stream to the Upload Task

You provide the input stream to the upload task in your implementation of the URLSessionTaskDelegate method urlSession(_:task:needNewBodyStream:), which is called after you start the upload task by calling resume(). The callback passes in a completion handler, which you call directly, passing in the boundStreams.input stream you created earlier. Listing 4 shows an implementation of this method.

Listing 4

Providing the input stream to the upload task in the delegate callback

func urlSession(_ session: URLSession, task: URLSessionTask,
                needNewBodyStream completionHandler: @escaping (InputStream?) -> Void) {

Write Data to the Stream When It’s Ready

Write data to an output stream only when the stream is ready for it. You get notified of the stream’s readiness in the StreamDelegate method stream(_:handle:). When this callback sends hasSpaceAvailable as its eventCode parameter, the stream is ready to accept more data.

If you’re not ready to write while handling the event, and would prefer to write on your own schedule, you can set a flag variable and check it later to determine whether it’s is safe to write to the stream. Listing 5 illustrates this technique. It handles the hasSpaceAvailable event by just setting a private canWrite property to true.

While handling stream events, also check whether eventCode is errorOccurred. This means that the stream has failed. When this happens, close the streams and abandon the upload.

Listing 5

Handling StreamDelegate events

func stream(_ aStream: Stream, handle eventCode: Stream.Event) {
    guard aStream == boundStreams.output else {
    if eventCode.contains(.hasSpaceAvailable) {
        canWrite = true
    if eventCode.contains(.errorOccurred) {
        // Close the streams and alert the user that the upload failed.

Once you’re handling the hasSpaceAvailable event, you can write to the stream whenever you know it’s ready to receive more data. You write to the stream by calling its write(_:maxLength:) method, providing a reference to the raw bytes to be written, and the maximum number of bytes to write.

Listing 6 uses a timer to wait for the private canWrite property to become true. Once this is the case, the code creates a string representing the current date and converts it to raw bytes. The listing then calls write(_:maxLength:) to send these bytes to the output stream. Because this output stream is bound to an input stream, the upload task can then automatically read these bytes from the input stream and send them to the destination URL.

Listing 6

Creating a timer to write to the output stream when the stream has space available

timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) {
    [weak self] timer in
    guard let self = self else { return }

    if self.canWrite {
        let message = "*** \(Date())\r\n"
        guard let messageData = .utf8) else { return }
        let messageCount = messageData.count
        let bytesWritten: Int = messageData.withUnsafeBytes() { (buffer: UnsafePointer<UInt8>) in
            self.canWrite = false
            return self.boundStreams.output.write(buffer, maxLength: messageCount)
        if bytesWritten < messageCount {
            // Handle writing less data than expected.

Once you write to the output stream, you can’t write again until your StreamDelegate receives a new hasSpaceAvailable event. This example enforces this constraint by setting the class’ canWrite property to false. It will be reset to true when a new hasSpaceAvailable event is received by the output stream's delegate, as shown earlier in Listing 5.

See Also


Uploading Data to a Website

Post data from your app to servers.