Timer initialised in 'for in' loop firing twice for each loop - swift

Question

I'm building an application that displays a custom driving route on a map using MapKit. It takes an array of coordinates and goes through a for loop, in each loop the corresponding coordinates (i.e [0] and [1] or [7] and [8]) are assembled into an individual request and then drawn on the map.
In order to bypass MapKit's throttling error, I have a timer that is set so that each request is spaced out 1 second apart.
My issue is that the timer is firing twice for each individual request, which is resulting in double the number of necessary requests being made.
I'm using Xcode 10 and Swift 4, this is the function where I believe the issue is occurring.
func requestDirections(arrays coordinates: [CLLocationCoordinate2D]) {
var dest = 1
let coordinatesArray = coordinates
let end = coordinatesArray.count - 2
var timerDelay:Double = 1
for origin in 0...end {
if dest <= coordinatesArray.count {
let startCoord = MKPlacemark(coordinate: coordinatesArray[origin])
let destCoord = MKPlacemark(coordinate: coordinatesArray[dest])
let request = MKDirections.Request()
request.source = MKMapItem(placemark: startCoord)
request.destination = MKMapItem(placemark: destCoord)
request.transportType = .automobile
request.requestsAlternateRoutes = false
print("Starting timer for \(dest) at \(timerDelay) seconds")
Timer.scheduledTimer(withTimeInterval: timerDelay, repeats: false) { timer in
self.createIndividualDirectionsRequest(request)
print("Timer fired")
}
dest = dest + 1
timerDelay = timerDelay + 1
}
}
}
I'm expecting the timer to fire once for each loop, if this is happening the expected console output would be
"Starting timer for 'dest' at 'timerDelay' seconds" printed 18 times (or whatever the size of the array is)
"Timer fired" being printed 18 times as well
While "Starting timer for 'dest' at 'timerDelay' seconds" is in fact being printed the correct number of times, "Timer fired" is being printed twice as often as it should.
Thank you very much for your help and your patience, I am quite new to programming and am struggling to wrap my head around this issue.

Answer 1

Actually I do not know why "Timer fired" is printed twice for each individual request :).
But if you are using Timer.scheduledTimer just to delay the execution of the block, you can use DispatchQueue.main.asyncAfter instead of Timer
DispatchQueue.main.asyncAfter(deadline: .now() + timerDelay) {
self.createIndividualDirectionsRequest(request)
print("Timer fired")
}
You can also use DispatchQueue.global.asyncAfter but I think you want to execute the block in main ui thread