Firebase & Swift: Asynchronous calls, Completion Handlers

I have read up a lot on this subject but have still been stumped on this specific problem. I have many Firebase calls that rely on each other. This is a kind of simplified example of my code. I had trouble making it any shorter and still getting the point across:

class ScoreUpdater {
static let ref = Database.database().reference()
var userAranking = Int?

var userBranking = Int?

var rankingAreceived = false

var rankingBreceived = false

var sum = 0
// Pass in the current user and the current meme

static func beginUpdate(memeID: String, userID: String) {
// Iterate through each user who has ranked the meme before
ScoreUpdater.ref.child("memes/\(memeID)/rankings")observeSingleEvent(of: .value) {

    let enumerator = snapshot.children
    while let nextUser = enumerator.nextObject() as? DataSnapshot {

        // Create a currentUpdater instance for the current user paired with each other user
        let currentUpdater = ScoreUpdater()

This is where the asynchronous calls start. Multiple gatherRankingValues functions can run at one time. This function contains a Firebase call which is asynchronous, which is okay for this function. The updateScores however cannot run until gatherRankingValues is finished. That is why I have the completion handler. I think this area is okay based on my debug printing.

            // After gatherRankingValues is finished running,

// then updateScores can run

currentUpdater.gatherRankingValues(userA: userID, userB: nextUser.key as! String) {

currentUpdater, userA, userB in

currentUpdater.updateScores(userA: userA, userB:userB)

}

    }

}

}
func gatherRankingValues(userA: String, userB: String, completion: @escaping (_ currentUpdater: SimilarityScoreUpdater, _ userA: String, _ userB: String) -> Void) {
// Iterate through every meme in the database
ScoreUpdater.ref.child("memes").observeSingleEvent(of: .value) {

    snapshot in
    let enumerator = snapshot.children

    while let nextMeme = enumerator.nextObject() as? DataSnapshot {

Here is where the main problem comes in. The self.getRankingA and self.getRankingB never run. Both of these methods need to run before the calculation method. I try to put in the “while rankingReceived == false” loop to keep the calculation from starting. I use the completion handler to notify within the self.rankingAreceived and self.rankingBreceived when the values have been received from the database. Instead, the calculation never happens and the loop becomes infinite.

If I remove the while loop waiting for the rankings to be received, the calculations will be “carried out” except the end result ends up being nil because the getRankingA and getRankingB methods still do not get called.

self.getRankingA(userA: userA, memeID: nextMeme.key) {

self.rankingAreceived = true

}

        self.getRankingB(userB: userB, memeID: nextMeme.key) {
            self.rankingBreceived = true
        }

        while self.rankingAreceived == false || self.rankingBreceived == false {
            continue
        }

        self.calculation()

    }

So yes, every meme gets looped through before the completion is called, but the rankings don’t get called. I can’t figure out how to get the loop to wait for the rankings from getRankingA and getRankingB and for the calculation method to run before continuing on to the next meme. I need completion of gatherRankingValues (see below) to be called after the loop has been through all the memes, but each ranking and calculation to complete also before the loop gets called again… How can I within the getRankingA and getRankingB completion handlers tell the meme iterating loop to wait up?

        // After every meme has been looped through for this pair of users, call completion

completion(self, userA, userB)

}

}
function getRankingA(userA: String, memeID: String, completion: @escaping () -> Void) {
ScoreUpdater.ref.child("memes/\(memeID)\rankings\(userA)").observeSingleEvent(of: .value) {
    snapshot in
    self.userAranking = snapshot.value
    completion()

}

}
function getRankingB(userB: String, memeID: String, completion: @escaping () -> Void) {
ScoreUpdater.ref.child("memes/\(memeID)\rankings\(userB)").observeSingleEvent(of: .value) {
    snapshot in
    self.userBranking = snapshot.value
    completion()

}

}
func calculation() {

self.sum = self.userAranking + self.userBranking

self.userAranking = nil

self.userBranking = nil

}
func updateScores() {

ScoreUpdater.ref.child(…)…setValue(self.sum)

}
}

#ios #swift #firebase