Trying to mutate array and object passed as parameters to an asynchronous function

Question

So if you look at the code below you will see the following line fails with:

cannot use mutating member on immutable value: 'forecasts' is a 'let' constant

caused by forecasts.append(forecast) . I have a service file which you see below called ForecastService and I am using the function to asynchronously grab JSON in my view controller from an API. I am trying to pass an object and an array of objects to the function which I then want to mutate on so I can use the updated values in my view controller to update my UI. I want to construct forecast objects and push them into the the array which I initialized in the view controller.

Any clarification on how I can get around this problem would be great.

import Foundation

import Alamofire

class ForecastService {

    static let sharedInstance = ForecastService()

    private init() {}

    func downloadForecastDetails(forecast: Forecast, forecasts: [Forecast], completed: @escaping DownloadComplete) {
        let url = URL(string: FORECAST_URL)!

        Alamofire.request(url).responseJSON { response in
            let result = response.result.value

            if let dict = result as? Dictionary<String, AnyObject> {
                if let list = dict["list"] as? [Dictionary<String, AnyObject>] {

                    for obj in list {
                        let forecast = Forecast(weather: obj)
                        forecasts.append(forecast)
                    }
                }

            }

            completed()
        }

    }
}

Answer 1

The problem is not the asynchronous block, but the mechanism of parameter forecasts . It's copied when you call the function, even if you declare it as var. Because Array is Swift is value type, it's the Swift's policy. ( https://developer.apple.com/swift/blog/?id=10 ) You can try to call forecasts.append outside the asynchronous block, it will still fail. So you need another way to pass a reference type (not value type) into the function.