Trouble with @escaping Competion Handler when passing data in a shared data container

Question

Created Jun ’21

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Hi,

I am coding my first App and have never coded before and need some guidance with the @escaping completion handler logic.

I am trying to get the count of fires from a third party API and then pass the count using a shared Data Container class to my main View Controller.

The issue is that when I call the func loadInitialFireMapData on my main View Controller, I am getting an error "Expected expression in list of expressions"
This is because I don't quite understand yet how to call a function when there is a completion handler
The @escaping completion handler is necessary since the main View Controller needs to wait for the API call (func loadInitialFireMapData) to finish in order to get the fire count.
Any advice would be greatly appreciated!

See the code from the Main View Controller and the API call with the @ escaping function from my FireDataManager.swift file.

Here is the code from my Main View Controller:

// ViewController.swift
class DataContainer {
    static let shared = DataContainer()

    var fireCount: Int = 0
    var fires: [Fire] = []
    var totalFireCount: Int = 0
}



class ViewController: UIViewController, CLLocationManagerDelegate {

    let dataContainer = DataContainer.shared
    let fireDataManager = FireDataManager ()

    override func viewDidLoad() {
        super.viewDidLoad()

//Retrieve Fire API data from Fire Data Manager
        fireDataManager.loadInitialFireMapData (completion: ([Fire]) -> return)  {

            self.FireStatusLabel.text = (String(DataContainer.shared.totalFireCount) + " fires within 100 miles of your location.")
        }

Here is the FireDataManager.swift file with the API call/@escaping completion handler function.

//  FireDataManager.swift  

class FireDataManager {

func loadInitialFireMapData(completion: @escaping () -> Swift.Void) {
        if let url = URL(string:

        "https://opendata.arcgis.com/datasets/68637d248eb24d0d853342cba02d4af7_0.geojson")

        {

            URLSession.shared.dataTask(with: url) {data, response, error in
                if let data = data {
                       do {
                        let features = try MKGeoJSONDecoder().decode(data)
                               .compactMap { $0 as? MKGeoJSONFeature }
                            let validWorks = features.compactMap(Fire.init)
                        DataContainer.shared.fires.append(contentsOf: validWorks)
                        DataContainer.shared.totalFireCount = validWorks.count
                        print("Fire Data Manager Count of Total Fires: ", DataContainer.shared.totalFireCount)
                        DispatchQueue.main.async {
                            completion()
                        }

                       }
                       catch let error {
                        print("FireMap URL Session error: ", error.localizedDescription)
                       }
                    }
                 }
           .resume()
        }

    }
}

Boost

Answer 1

OOPer OP

Jun ’21

Accepted Answer

Any advice would be greatly appreciated!

If you can target your app for iOS 15+ and are planning to release it after the released version of Xcode 13 is out, you may try using new async/await feature.

(There are more session videos about async/await and I recommend to watch all if you have enough time.)

If you need to make your app target iOS 14.x and earlier, working with completion handler would be necessary.

I assume your loadInitialFireMapData is working fine in normal cases.

(Hope you are not ignoring any warnings.)

But it has some flaws considering error cases:

It ignores some error cases without showing any debug info
It does not call completion handler on errors

You should better pass an Optional<Error> to the completion handler to indicate error cases.

I would write it as follows:

class FireDataManager {
    
    enum Errors: Error {
        case urlInvalid
        case dataIsNil
    }
    
    func loadInitialFireMapData(completion: @escaping (Error?) -> Void) { //<-
        guard let url = URL(string:
                                "https://opendata.arcgis.com/datasets/68637d248eb24d0d853342cba02d4af7_0.geojson") else {
            completion(Errors.urlInvalid)
            return
        }
        URLSession.shared.dataTask(with: url) {data, response, error in
            if let error = error {
                print("FireMap URL Session error: ", error) //Use `error` instead of `error.localizedDescription` to show debug info
                completion(error)
                return
            }
            guard let data = data else {
                completion(Errors.dataIsNil)
                return
            }
            do {
                let features = try MKGeoJSONDecoder().decode(data)
                    .compactMap { $0 as? MKGeoJSONFeature }
                let validWorks = features.compactMap(Fire.init)
                DataContainer.shared.fires.append(contentsOf: validWorks)
                DataContainer.shared.totalFireCount = validWorks.count
                print("Fire Data Manager Count of Total Fires: ", DataContainer.shared.totalFireCount)
                DispatchQueue.main.async {
                    completion(nil) //<- no error here
                }
            } catch let error {
                print("FireMap decoding error: ", error)
                completion(error)
            }
        }
        .resume()
    }
}

In this case, meaning the type of completion is (Error?)->Void, you need to write a closure like { (error: Error?)->Void in ... }

or in a simplified form { error in ... } .

So, the caller side code would be something like this:

class ViewController: UIViewController, CLLocationManagerDelegate {
    @IBOutlet weak var fireStatusLabel: UILabel!

    let dataContainer = DataContainer.shared
    let fireDataManager = FireDataManager()

    override func viewDidLoad() {
        super.viewDidLoad()

        //Retrieve Fire API data from Fire Data Manager
        fireDataManager.loadInitialFireMapData (completion: { error in
            if let error = error {
                print(error)
                return
            }
            self.fireStatusLabel.text = "\(DataContainer.shared.totalFireCount) fires within 100 miles of your location."
        })
        //...
    }
    
    //...
}

(I renamed FireStatusLabel to fireStatusLabel as only type names start with Capital letter in Swift. If you have any reasons you cannot rename it, please re-interpret the lines with fireStatusLabel.)

Or you can use the trailing closure notation like this:

        fireDataManager.loadInitialFireMapData { error in //<- no opening parenthesis here
            if let error = error {
                print(error)
                return
            }
            self.fireStatusLabel.text = "\(DataContainer.shared.totalFireCount) fires within 100 miles of your location."
        } //<- no closing parenthesis

Please try.

1