How do I write a completion handler for firebase data? - ios

Question

So I had issues previously working with 'observe' from firebase, and I realised I could not bring the variable values from inside the code block that was working asynchronously. A user told me to use completion handlers to resolve this issue, and his example was:
func mapRegion(completion: (MKCoordinateRegion)->()) {
databaseHandle = databaseRef.child("RunList").child(runName).observe(.value, with: { (snapshot) in
let runData = snapshot.value as? [String: AnyObject]
self.minLat = runData?["startLat"] as? Double
self.minLng = runData?["startLong"] as? Double
self.maxLat = runData?["endLat"] as? Double
self.maxLng = runData?["endLong"] as? Double
print("testing")
print(self.minLat!)
print(self.maxLng!)
let region = MKCoordinateRegion(
center: CLLocationCoordinate2D(latitude: (self.minLat! + self.maxLat!)/2,
longitude: (self.minLng! + self.maxLng!)/2),
span: MKCoordinateSpan(latitudeDelta: (self.maxLat! - self.minLat!)*1.1,
longitudeDelta: (self.maxLng! - self.minLng!)*1.1))
completion(region)
})
}
and to use the code:
mapRegion() { region in
mapView.region = region
// do other things with the region
}
So I've tried to recreate this for another method that I need to return an array of object type RunDetail:
func loadRuns(completion: ([RunDetail]) -> ()) {
// we need name, distance, time and user
databaseHandle = databaseRef.child("RunList").observe(.value, with: { (snapshot) in
self.count = Int(snapshot.childrenCount)
print(self.count!)
// more stuff happening here to add data into an object called RunDetail from firebase
// add RunDetail objects into array called 'run'
})
completion(runs)
}
I am not sure if I am setting this up correctly above^.
I still cannot get my head around getting the completion handler working (I really don't understand how to set it up). Can someone please help me and let me know if I am setting this up properly? Thanks.

Answer 1

You need to move the completion(region) to inside the Firebase completion block and add #escaping after completion:.
Also, you should not force unwrap optionals. It is easy enough to check that they are not nil and this will prevent the app from crashing.
func mapRegion(completion: #escaping (MKCoordinateRegion?) -> Void) {
let ref = Database.database().reference()
ref.child("RunList").child(runName).observe(.value, with: { (snapshot) in
guard
let runData = snapshot.value as? Dictionary<String,Double>,
let minLat = runData["startLat"],
let minLng = runData["startLong"],
let maxLat = runData["endLat"],
let maxLng = runData["endLong"]
else {
print("Error! - Incomplete Data")
completion(nil)
return
}
var region = MKCoordinateRegion()
region.center = CLLocationCoordinate2D(latitude: (minLat + maxLat) / 2, longitude: (minLng + maxLng) / 2)
region.span = MKCoordinateSpanMake((maxLat - minLat) * 1.1, (maxLng - minLng) * 1.1)
completion(region)
})
}
Then update your code to this.
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
mapRegion { (region) in
if let region = region {
self.mapView.setRegion(region, animated: true)
}
}
}
For your loadRuns
func loadRuns(completion: #escaping (Array<RunDetail>) -> Void) {
let ref = Database.database().reference()
ref.child("RunList").observe(.value, with: { (snapshot) in
var runs = Array<RunDetail>()
// Populate runs array.
completion(runs) // This line needs to be inside this closure.
})
}