Completion handler needed for calling data from firebase? - ios

Question

Im trying to call data from firebase. The problem is, the data is deeply nested and I don't think I can change that.
So I'm attempting to call values from firebase, which I can then use to reference new values.
The problem arises when my for loop is not finished before the next stage is called, meaning my dictionary count for the next stage is 0, so my next function is not called?
Is there a way to do this sufficiently?
Please help?
Heres my code:
func fetchBuyer(search: String, user: String, completion: #escaping ([Post]) -> (), withCancel cancel: ((Error) -> ())?) {
let ref = Database.database().reference().child("posts").child(user).child(search).child("purchases")
ref.observeSingleEvent(of: .value, with: { (snapshot) in
guard let dictionaries = snapshot.value as? [String: Any] else {
completion([])
return
}
let keys: [String] = dictionaries.map({ $0.key })
var newdictionaries = [String: String]()
for i in keys {
let newref = Database.database().reference().child("posts").child(user).child(search).child("purchases").child(i).child("purchaser")
newref.observeSingleEvent(of: .value, with: { (snapshot) in
newdictionaries[i] = snapshot.value as? String
print("THESE ARE MY PURCHASES ID-->", newdictionaries.values)///prints out ["-M0pTHtZXYUVQT7DCLj-", "-M0pU79uQCCnBunAEkJN"]
})
}
var buyerPosts = [Post]()
print("newdictionaries.count--->", newdictionaries.count)//this print is 0
newdictionaries.forEach({ (postId, value) in
Database.database().fetchPost(withUID: user, postId: postId, completion: { (post) in
buyerPosts.append(post)
if buyerPosts.count == newdictionaries.count{
completion(buyerPosts)
}
})
})
}) { (err) in
print("Failed to fetch posts for buyers:", err)
cancel?(err)
}
}
Attempted answer:
let g = DispatchGroup() //// 1
for i in keys{
g.enter() //// 2
let newref = Database.database().reference().child("posts").child(user).child(search).child("purchases").child(i).child("purchaser")
print("now")
newref.observeSingleEvent(of: .value, with: { (snapshot)
newdictionaries[i] = snapshot.value as? String
print("print new dictionaries-->", newdictionaries)
// complete here
Database.database().fetchPost(withUID: user, postId: newdictionaries[i]!, completion: { (post) in
buyerPosts.append(post)
g.leave() //////// 3
})
})
}
g.notify(queue: DispatchQueue.main) {
print("finished!!!")
completion(buyerPosts)
}

Answer 1

You need a dispatch group and nest the calls
let g = DispatchGroup() //// 1
for i in keys{
g.enter() //// 2
let newref = Database.database().reference().child("posts").child(user).child(search).child("purchases").child(i).child("purchaser")
newref.observeSingleEvent(of: .value, with: { (snapshot)
newdictionaries[i] = snapshot.value as? String
// complete here
Database.database().fetchPost(withUID: user, postId: postId, completion: { (post) in
buyerPosts.append(post)
g.leave() //////// 3
})
})
}
/// 4
g.notfiy(queue.main) {
completion(buyerPosts)
}