Array is null after setting data in it - swift

Question

I have a JSON request that gets data from the Darksky API, I get the data properly and it is showing on the screen. However, When i'm trying to set the data from the array I get from the JSON call in another array, it stays empty.
This is my code:
just declaring the array:
var mForecastArray = [Weather]()
this is the function that calls the API:
func getForecast(){
Weather.forecast(withLocation: "37.8267,-122.4233") { (arr) in
DispatchQueue.main.async {
self.mForecastArray = arr
self.mTodayWeather = arr[0]
self.mCollectionView.reloadData()
}
}
}
The weird part is that it does work, and the data do shows on screen, but still, mForecastArray seems null.
This is the API call itself:
static func forecast(withLocation location: String, completion: #escaping ([Weather]) -> ()){
let url = basePath + location
let request = URLRequest(url: URL(string: url)!)
let task = URLSession.shared.dataTask(with: request) { (data: Data?, response: URLResponse?, error: Error?) in
var forecastArray: [Weather] = []
if let data = data{
do{
if let json = try JSONSerialization.jsonObject(with: data, options: []) as? [String:Any]{
if let dailyForecast = json["daily"] as? [String:Any]{
if let dailyData = dailyForecast["data"] as? [[String:Any]]{
for dataPoint in dailyData{
if let weatherObject = try? Weather(json: dataPoint){
forecastArray.append(weatherObject)
}
}
}
}
}
}catch{
print(error.localizedDescription)
}
completion(forecastArray)
}
}
task.resume()
}

Answer 1

It's a visual asynchronous illusion.
The static method forecast works asynchronously.
Most likely your code looks like
getForecast()
print(self.mForecastArray)
This cannot work because the array is populated much later.
Move the print line into the completion handler of the static method
func getForecast(){
Weather.forecast(withLocation: "37.8267,-122.4233") { (arr) in
DispatchQueue.main.async {
self.mForecastArray = arr
print(self.mForecastArray)
self.mTodayWeather = arr[0]
self.mCollectionView.reloadData()
}
}
}