a solution to retrieve information when they're no internet connection - swift

Question

On my app I have a form which the user can complete and send it to us with a SOAP request.
the form sended:
But I have a problem, I they're no internet connection I can't send the data by using my SOAP request.
So firstly I save the request in Core Data, so here it's ok... And when the user is on the app I do a scheduled request to see if they're a request in Core Data to send them.
But what about if the app is not running ? In Android I use background service, but with Apple it's impossible:
Android Services equivalent in iOS Swift
Someone have an idea to retrieve the data ?
Soap function:
func soapCall(fonction:String, soapMessageParamsTab:[SoapMessageParams], completion: #escaping (_ strData: NSString)-> ()/*, completion: #escaping (_ result: Int, _ resultContactWebId: Int)->()*/){
let soapRequest = AEXMLDocument()
let attributes = ["xmlns:SOAP-ENV" : "http://schemas.xmlsoap.org/soap/envelope/", "xmlns:ns1" : namespace, "xmlns:xsd" : "http://www.w3.org/2001/XMLSchema", "xmlns:xsi" : "http://www.w3.org/2001/XMLSchema-instance", "xmlns:SOAP-ENC" : "http://schemas.xmlsoap.org/soap/encoding/", "SOAP-ENV:encodingStyle" : "http://schemas.xmlsoap.org/soap/encoding/"]
let envelope = soapRequest.addChild(name: "SOAP-ENV:Envelope", attributes: attributes)
let body = envelope.addChild(name: "SOAP-ENV:Body")
let sendLead = body.addChild(name: "ns1:"+fonction)
for obj in soapMessageParamsTab {
sendLead.addChild(name: obj.soapName, value: obj.soapValue, attributes: ["xsi:type" : "xsd:"+obj.soapType])
}
//sendLead.addChild(name: "xml", value: messageSoap, attributes: ["xsi:type" : "xsd:string"])
let soapMessage = soapRequest.xmlString
let messageSoapMinify = minify(soapMessage)
// URL Soap
let urlString = soapServiceURL
let url = URL(string: urlString)
var theRequest = URLRequest(url: url!)
// Taille SoapMessage
let msgLength = String(messageSoapMinify.characters.count)
// Soap Login
let username = soapUsername
let password = soapPassword
let loginString = NSString(format: "%#:%#", username, password)
let loginData: Data = loginString.data(using: String.Encoding.utf8.rawValue)!
let base64LoginString = loginData.base64EncodedString(options: NSData.Base64EncodingOptions.lineLength64Characters)
// Requete Soap
theRequest.addValue("text/xml; charset=utf-8", forHTTPHeaderField: "Content-Type")
theRequest.addValue("Keep-Alive", forHTTPHeaderField: "Connection")
theRequest.addValue(msgLength, forHTTPHeaderField: "Content-Length")
theRequest.addValue("urn:ResponseAction", forHTTPHeaderField: "SOAPAction")
theRequest.httpMethod = "POST"
theRequest.httpBody = messageSoapMinify.data(using: String.Encoding.utf8, allowLossyConversion: true)
theRequest.addValue("Basic \(base64LoginString)", forHTTPHeaderField: "Authorization")
// print("Request is \(theRequest.allHTTPHeaderFields!)")
let session = Foundation.URLSession.shared
let task = session.dataTask(with: theRequest, completionHandler: {data, response, error -> Void in
if error != nil
{
print(error?.localizedDescription)
}
//print("Response Login: \(response)")
//print("Error Login: \(error)")
let strData = NSString(data: data!, encoding: String.Encoding.utf8.rawValue)!
//print("Body Login: \(strData)")
completion(strData)
})
task.resume()
}

Answer 1

Using NSURLSession with background configuration you should be ok.
Note: If the app is explicitly killed by the user the scheduled request will be cancelled. This trick works just if the app was killed by the system for any reason (low memory, crash etc...). There is nothing you can do if the app was killed by the user. The background fetch might be a solution, but it will be decided by iOS if and when to awake the app for the opportunistic fetch.
This could help you:
https://blog.newrelic.com/2016/01/13/ios9-background-execution/
Edit:
This tutorial could be useful as well: https://www.raywenderlich.com/110458/nsurlsession-tutorial-getting-started