Syncronize async functions - swift

Question

I try to create my first project with swift 3.
I try to get data from my API. This works pretty good if I manually start the function. I need to synchronize the async request.
I need to trigger my function 3 times and wait for the others to complete.
makeGetCall(URLstring: "api1")
wait to complete
makeGetCall(URLstring: "api2")
wait to complete
makeGetCall(URLstring: "api3")
Set this to background and trigger every 5 seconds.
func makeGetCall(URLstring: String, update: Bool) {
let completeURL = "http://myapi/" + URLstring
// Set up the URL request
guard let url = URL(string: completeURL) else {
print("Error: cannot create URL")
return
}
let urlRequest = URLRequest(url: url)
// set up the session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
// make the request
let task = session.dataTask(with: urlRequest) {
(data, response, error) in
// check for any errors
guard error == nil else {
print("error calling GET on /todos/1")
print(error as Any)
return
}
// make sure we got data
guard let responseData = data else {
print("Error: did not receive data")
return
}
// parse the result as XML
if URLstring == "devicelist.cgi" {
self.readDevice(XMLData: responseData)
}
if URLstring == "statelist.cgi" {
self.readDeviceData(XMLData: responseData, update: update)
}
if URLstring == "functionlist.cgi" {
self.readGewerke(XMLData: responseData)
}
}
task.resume()
}
Can somebody help please.
Hagen
This is what I tried with completion handler:
override func viewDidLoad() {
super.viewDidLoad()
makeGetCall(input: "statelist.cgi") {
(result: Bool) in
print("finished statelist")
}
makeGetCall(input: "devicelist.cgi") {
(result: Bool) in
print("finished devicelist")
}
makeGetCall(input: "functionlist.cgi") {
(result: Bool) in
print("finished functionlist")
}
}
func makeGetCall(input: String, completion: #escaping (_ result: Bool) -> Void) {
let completeURL = "http://192.168.0.25/addons/xmlapi/" + input
// Set up the URL request
guard let url = URL(string: completeURL) else {
print("Error: cannot create URL")
return
}
let urlRequest = URLRequest(url: url)
// set up the session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
// make the request
let task = session.dataTask(with: urlRequest) {
(data, response, error) in
// check for any errors
guard error == nil else {
print("error calling GET on /todos/1")
print(error as Any)
return
}
// make sure we got data
guard data != nil else {
print("Error: did not receive data")
return
}
completion(true)
}
task.resume()
}
If I put the 3 calls together it worked as it should.
I thing it should also work with GCD but most of examples for swift 2.
makeGetCall(input: "devicelist.cgi") {
(result: Bool) in
print("finished devicelist")
self.makeGetCall(input: "functionlist.cgi") {
(result: Bool) in
print("finished functionlist")
self.makeGetCall(input: "statelist.cgi") {
(result: Bool) in
print("finished statelist")
}
}
}
Maybe now somebody can help.
Thanks Hagen

Answer 1

Use a DispatchGroup to get notified when something happens.
override func viewDidLoad() {
super.viewDidLoad()
let stateListGroup = DispatchGroup()
stateListGroup.enter()
makeGetCall(input: "statelist.cgi") {
(result: Bool) in
print("finished statelist")
stateListGroup.leave()
}
let deviceListGroup = DispatchGroup()
deviceListGroup.enter()
// the notify closure is called when the (stateList-) groups enter and leave counts are balanced.
stateListGroup.notify(queue: DispatchQueue.main) {
self.makeGetCall(input: "devicelist.cgi") {
(result: Bool) in
print("finished devicelist")
deviceListGroup.leave()
}
}
let functionListGroup = DispatchGroup()
functionListGroup.enter()
deviceListGroup.notify(queue: DispatchQueue.main) {
self.makeGetCall(input: "functionList") {
(result: Bool) in
print("finished functionlist")
functionListGroup.leave()
}
}
functionListGroup.notify(queue: DispatchQueue.main) {
print("update ui here")
}
}
Prints:
statelist.cgi
finished statelist
devicelist.cgi
finished devicelist
functionList
finished functionlist
update ui here
Also keep in mind that the completion handler of session.dataTask() is called on a background queue, so I recommend to dispatch completion(true) on the main queue to avoid unexpected behaviour:
DispatchQueue.main.async {
completion(true)
}