Wait for query to fetch Firestore data before continue - swift

Question

I would like to fetch the user/author of the article into a computed property. But this never happens, below code would print 1, 2, 4.
struct Article {
private let database = Firestore.firestore()
var authorId: String
var message: String
var author: User? {
print(1)
var fetchedAuthor: User?
let semaphore = DispatchSemaphore(value: 0)
print(2)
database.collection("users").document(authorId).getDocument { (document, error) in
print(3)
guard let user = document.flatMap({User(dictionary: $0.data())}) else {
print("Could not get author: \(error.debugDescription)")
semaphore.signal()
return
}
fetchedAuthor = user
semaphore.signal()
}
print(4)
semaphore.wait()
print(5)
return fetchedAuthor
}
}

Answer 1

Adding pseudo/ sample code to the comment by #Connor, since I just went through this.
extension User {
static func getBy(documentID id: String, completion: #escaping (User)->()) {
database.collection("users").document(id).getDocument { (document, error) in
guard let user = document.flatMap({User(dictionary: $0.data())}) else {
print("Could not get author: \(error.debugDescription)")
return
}
completion(user)
}
}
Then in your Article struct:
struct Article {
....
init(){
....
// Get the user
User.getBy(documentID: self.userID) { user in
self.author = user
}
}
}

Answer 2

semaphore.wait() //is for requesting the resource(semaphore)
semaphore.singal() //is for releasing the resource(semaphore)
Change code as per below.
var author: User? {
print(1)
var fetchedAuthor: User?
let semaphore = DispatchSemaphore(value: 1)
print(2)
database.collection("users").document(authorId).getDocument { (document, error) in
semaphore.wait()
print(3)
guard let user = document.flatMap({User(dictionary: $0.data())}) else {
print("Could not get author: \(error.debugDescription)")
return
}
fetchedAuthor = user
semaphore.signal()
}
print(4)
print(5)
return fetchedAuthor
}