Add a completion to a function when firebase has finished, iOS, Swift - ios

Question

I'm trying to find out the best way to handle a completion on a function.
The function calls for data from firebase and adds them to an array of dictionaries. Because this is for maps and adding annotations the loop is adding lots of data before coming to the final appended version so its throwing loads of annotations dow in the same place. i want to know if i can call a completion on the loop when its finished and then call the function ShowSightings().
func getDatafromFB() {
DataService.ds.REF_POSTS.child("postCodes").observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? NSDictionary
let postsIds = value?.allKeys as! [String]
for postId in postsIds {
let refToPost = Database.database().reference(withPath: "posts/" + "postCodes/" + postId)
refToPost.observe(.value, with: { snapshot in
if snapshot.exists() {
let postDict = snapshot.value as? [String: AnyObject]
print("Tony: before append post \(self.posts)")
self.posts.append(postDict!)
print("Tony: post \(self.posts)")
}else {
print("Tony: Couldn't get the data")
}
})
}
print("Tony: The compleetion result \(self.posts)")
})
}

Answer 1

You can try this:
func doAsyncTask(completionHandler:#escaping (Bool) -> ()){
//do async tasks
completionHandler(true) //<- call this when the data is retrieved
//so in your case, see below
}
override func viewDidLoad{
doAsyncTask(){ succes in
//succes gives true or false
}
}
//your case
}else {
print("Tony: Couldn't get the data")
}
completionHandler(true) //<- right there
This is for 1 async task. I see you want to use multiple async task. This is a job for dispatch groups. I change some of my function to take parameters. Check this out:
func doAsyncTask(postID: String, completionHandler:#escaping (Bool) -> ()){
//do async tasks
completionHandler(true)
}
override func viewDidLoad{
var arrPostIDs = [String]()
//append to arrPostIDs here
let postIDDispatchGroup = DispatchGroup()
for postID in arrPostIDs{
postIDDispatchGroup.enter()
doAsyncTask(postID: postID){ succes in
//succes gives true or false
postIDDispatchGroup.leave()
}
}
postIDDispatchGroup.notify(queue: .main) {
//everything completed :), do whatever you want
}
}