Proper way to call asynchronous function in swift singleton - swift

Question

I want to build an analytics class for my application, and i am using singleton.
If I run this, the tagEvent function immediately runs rather than first running the openSession(), so sessionId returns nil.
How can I create a class like this with proper initialisation and use it application wide like singleton instances.
Analytics.swift
final class Analytics {
public static let instance = Analytics()
private var baseUrl = "http://localhost"
public var deviceId: String?
private init(){
self.deviceId = SomeFunctionGetsDeviceID()
}
func openSession(){
// make an API call to create a session and save sessionId to UserDefaults
if let url = URL(string: self.baseUrl + "/session"){
let params:[String:Any] = ["deviceId": "\(self.deviceId!)"]
var request = URLRequest(url: url)
request.setValue("application/json", forHTTPHeaderField:"Content-Type")
request.httpMethod = "POST"
request.httpBody = try! JSONSerialization.data(withJSONObject: params, options: [])
AnalyticsSessionManager.sharedManager.request(request as URLRequestConvertible).validate().responseObject(completionHandler: { (response: DataResponse<SessionOpenResponse>) in
if response.result.value != nil {
UserDefaults.standard.set(response.result.value?.sessionId, forKey: "sessionId")
}
})
}
}
func closeSession(){
// make an API call to close a session and delete sessionId from UserDefaults
...
}
func tagEvent(eventName: String, attributes: [String : String]? = nil) {
if let url = URL(string: self.baseUrl + "/event"),
let sessionId = UserDefaults.standard.string(forKey: "sessionId"){
...
// make an API call to create an event with that sessionId
}
}
}
AppDelegate.swift
func application(_ application: UIApplication,
didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
Analytics.instance.openSession()
Analytics.instance.tagEvent(eventName: "App Launch", attributes:
["userID":"1234"])
}

Answer 1

My best guess is the openSession function is doing work asynchronously and the tagEvent call comes in BEFORE the asynchronous code has completed. There are a couple of ways around this:
1) Add synchronization so the tagEvent code will wait for the openSession call to complete (if in progress). If not in progress, perhaps it should automatically call openSession, wait for completion, then execute the code in that function
2) Add a completion handler from openSession and inside that enclosure you can call tagEvent such as:
func openSession(completionHandler: #escapaing (Bool) -> ()){
// make an API call to create a session and save sessionId to UserDefaults
...
UserDefaults.standard.set(someSessionID, forKey: "sessionId")
// when done with async work
completionHandler(true)
}
Then in your app delegate:
Analytics.instance.openSession() { (success)
Analytics.instance.tagEvent(eventName: "App Launch", attributes:["userID":"1234"])
}
3) * This is the way I would fix it * I would not make a call to openSession to be required outside of the class. I would add a flag to the Analytics class:
private var initialized = false
In the openSession function, set this after everything is done
initialized = true
In the tagEvent function:
func tagEvent(eventName: String, attributes: [String : String]? = nil) {
// Check for initialization
if (!initialized) {
openSession() { (success) in
// perform your tagEvent code
})
} else {
if let url = URL(string: self.baseUrl + "/event"),
let sessionId = UserDefaults.standard.string(forKey: "sessionId"){
...
// make an API call to create an event with that sessionId
}
}
}

Answer 2

You can implement an "internal" class to provide a static reference to your Analytics class, as follows (note: remove final keyword):
class Analytics {
// properties ...
class var sharedInstance: Analytics {
struct Static {
static let instance: Analytics = Analytics()
}
return Static.instance
}
// other methods (init(), openSession(), closeSession(), tagEvent())
}
Next call methods from your Analytics class in any other class, as follows:
Analytics.sharedInstance.openSession()
Analytics.sharedInstance.closeSession()
Analytics.sharedInstance.tagEvent( ... )