How to open HTTP stream on iOS using ndjson - ios

Question

Does anyone have experience opening HTTP stream on iOS? I have tried multiple solutions without any luck (examples bellow).
For better context, here's example of endpoint that will stream values (as ndjson) upon opening connection:
GET /v2/path/{id}
Accept: application/x-ndjson
Attempt #1:
Issue: The completion handler is never called
let keyID = try keyAdapter.getKeyID(for: .signHash)
let url = baseURL.appendingPathComponent("/v2/path/\(keyID)")
var urlRequest = URLRequest(url: url)
urlRequest.httpMethod = "GET"
urlRequest.setValue("application/x-ndjson", forHTTPHeaderField: "Accept")
session.dataTask(with: urlRequest) { data, response, error in
// This never gets called.
// I would expect that the completion is called every time backend emits new value.
}.resume()
Attempt #2:
Issue: Debugger displays this message: Connection 0: encountered error(12:1)
private var stream: URLSessionStreamTask? = nil
func startStream() {
let keyID = try keyAdapter.getKeyID(for: .signHash)
let url = baseURL.appendingPathComponent("/v2/path/\(keyID)")
let stream = session.streamTask(withHostName: url, port: 443)
// Not sure how to set headers.
// Header needs to be set so backend knows client wants to connect a stream.
self.stream = stream
stream.startSecureConnection()
startRead(stream: stream)
}
private func startRead(stream: URLSessionStreamTask) {
stream.readData(ofMinLength: 1, maxLength: 4096, timeout: 120.0) { data, endOfFile, error in
if let error = error {
Logger.shared.log(level: .error, "Reading data from stream failed with error: \(error.localizedDescription)")
} else if let data = data {
Logger.shared.log(level: .error, "Received data from stream (\(data.count)B)")
if !endOfFile {
self.startRead(stream: stream)
} else {
Logger.shared.log(level: .info, "End of file")
}
} else {
Logger.shared.log(level: .error, "Reading stream endup in unspecified state (both data and error are nil).")
}
}
}
Does anyone have experience with this? How can I keep HTTP connection open and listen to a new values that backend is streaming?

Answer 1

iOS can connect to HTTP stream using now deprecated API URLConnection. The API was deprecated in iOS 9, however it's still available for use (and will be in iOS 16 - tested).
First you need to create URLRequest and setup the NSURLConnection:
let url = URL(string: "\(baseURL)/v2/path/\(keyID)")!
var urlRequest = URLRequest(url: url)
urlRequest.setValue("application/x-ndjson", forHTTPHeaderField: "Accept")
let connnection = NSURLConnection(request: urlRequest, delegate: self, startImmediately: true)
connnection?.start()
Notice that the argument for delegate in the code above is of type Any which doesn't help to figure out what protocol(s) to implement. There are two - NSURLConnectionDelegate and NSURLConnectionDataDelegate.
Let's receive data:
public func connection(_ connection: NSURLConnection, didReceive data: Data) {
let string = String(data: data, encoding: .utf8)
Logger.shared.log(level: .debug, "didReceive data:\n\(string ?? "N/A")")
}
Then implement a method for catching errors:
public func connection(_ connection: NSURLConnection, didFailWithError error: Error) {
Logger.shared.log(level: .debug, "didFailWithError: \(error)")
}
And if you have custom SSL pinning, then:
public func connection(_ connection: NSURLConnection, willSendRequestFor challenge: URLAuthenticationChallenge) {
guard let certificate = certificate, let identity = identity else {
Logger.shared.log(level: .info, "No credentials set. Using default handling. (certificate and/or identity are nil)")
challenge.sender?.performDefaultHandling?(for: challenge)
return
}
let credential = URLCredential(identity: identity, certificates: [certificate], persistence: .forSession)
challenge.sender?.use(credential, for: challenge)
}
There is not much info on the internet, so hopefully it will save someone days of trial and error.