Return two arrays from observeSingleEvent - swift

Question

I have questions and answers in Firebase Realtime Database, which I need to be downloaded and put in to arrays. I have created a function which takes a parameter and should return two arrays
let data = fetchQuestions(category: "Animals")
qAni = data.ques
aAni = data.ans
fetchQuestions method:
func fetchQuestions(category: String) -> (ques: [String], ans: [String]) {
var q = [String]()
var a = [String]()
ref.child("QA/Category").observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? [String: Any]
for (k, v) in value! {
if k == category {
let QA = v as! NSArray
for i in QA {
let question = i as! [String: String]
q.append(question["question"]!)
a.append(question["answer"]!)
}
return (q, a) ** Unexpected non-void return value in void function
}
}
})
}
I have tried to use dispatch group, but then I receive a different error.

Answer 1

You are currently returning from the completion block of observeSingleEvent and its return type is void, What you need is you need to create completionHandler with your fetchQuestions. Now with Swift instead of using NSArray use Swift native array. Also instead of maintaining two array maintain one array with custom objects of struct or class.
So first create one struct like this.
struct Category {
let question: String
let answer: String
}
Now make completionHandler with your fetchQuestions like this way.
func fetchQuestions(category: String, completion: #escaping([Category]) -> Void) {
var array = [Category]
ref.child("QA/Category").observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? [String: Any]
for (k, v) in value! {
if k == category {
let QA = v as! [[String:String]]
for i in QA {
let que = i["question"]!
let ans = i["answer"]!
array.append(Category(question: que, answer: ans))
}
}
}
completion(array)
})
}
Now call fetchQuestions this way.
self.fetchQuestions(category: "Animals") { categories in
//access categories here
print(categories)
}
Note: Instead of comparing value in for loop you can also use queryEqual(toValue:) with your request to get specific result check this SO thread for more details on it.

Answer 2

Its because you are returning a value from a closure instead you should use different closure for that. Try it:
func fetchQuestions(category: StringcompletionBlock:#escaping (_ ques:[String], _ ans :[String]) -> ()) {
var q = [String]()
var a = [String]()
ref.child("QA/Category").observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? [String: Any]
for (k, v) in value! {
if k == category {
let QA = v as! NSArray
for i in QA {
let question = i as! [String: String]
q.append(question["question"]!)
a.append(question["answer"]!)
}
}
return completionBlock(q,a)
}
})
}
#escaping is required : when the closure is passed as an argument to
the function, but is called after the function returns.