Synchronized a function call - ios

Question

Consider a scenario, I have a function "REFRESH TOKEN", this function is called by different methods simultaneously, let's say the methods are "A", "B", "C".
If method "A" calls "REFRESH TOKEN" firstly then methods "B" and "C" should wait until it finishes.
Does anybody have a sample swift code?
How can I attain this scenario? Appreciate your help!
let serialQueue = DispatchQueue(label: "serialQueue")
var myFlag = false
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
self.refresh(param: 1)
self.refresh(param: 2)
self.refresh(param: 3)
}
func refresh(param: NSInteger) -> Void {
let absolutePath = "MY SAMPLE API"
var headers: [String: String] = Dictionary<String, String>();
headers["Content-Type"] = "application/json"
serialQueue.sync {
print("\nEntered ", param)
Alamofire.request(absolutePath, method: .get, parameters: nil, encoding: JSONEncoding.default, headers: headers).responseString {
response in
switch response.result {
case .success:
print("SUCCESS")
break
case .failure(let error):
print(error)
}
}
}}
Output:
Entered 1
Entered 2
Entered 3
SUCCESS
SUCCESS
SUCCESS
I need an output like this:
Entered 1
SUCCESS
Entered 2
SUCCESS
Entered 3
SUCCESS

Answer 1

You can make use of DispatchGroup.
let dispatchGroup = DispatchGroup()
dispatchGroup.enter()
Method A {
//Refresh Token code here
dispatchGroup.leave()
}
dispatchGroup.wait()
dispatchGroup.enter()
Method B {
//Refresh Token code here
dispatchGroup.leave()
}
dispatchGroup.wait()
dispatchGroup.notify(queue: .main) {
print("Both functions are invoked one after other")
}

Answer 2

The easy and nasty way is to set a flag and check the flag on calling the function and changing flag on returning.
A better way, as far as I found out, is using operation queue.

Answer 3

You should create a dispatchQueue(Serial) and put that code in sync of that queue.
//Create a dispatch Queue
var dispatchQueue:DispatchQueue
func refreshToken() {
dispatchQueue.sync {
//whatever code is there in refreshToken method
}
}

Answer 4

Request chaining is one way. If you find that you have to do this a lot then maybe look into futures and promises. Another way is to use Dispatch Group.
Request Chain example
Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo": "bar"]).response { (_, _, data, error) in
if error == nil {
Alamofire.request(.GET, "http://httpbin.org/get2\(data.something)", parameters: ["foo": "bar"]).response { (_, _, something, error) in
fulfill(something)
}
} else {
reject(error)
}
}
Dispatch Group Example:
DispatchQueue.global(qos: .userInitiated).async {
var storedError: NSError?
let downloadGroup = DispatchGroup()
downloadGroup.enter()
Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo":
"bar"]).response { (_, _, data, error) in
downloadGroup.leave()
}
downloadGroup.wait()
downloadGroup.enter()
Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo":
"bar"]).response { (_, _, data, error) in
downloadGroup.leave()
}
downloadGroup.wait()
downloadGroup.notify(queue: DispatchQueue.main) {
completion?(storedError)
}
}
If you want to look into Futures and Promises Alamofire made a lib
https://github.com/PromiseKit/Alamofire-