Use of undeclared type in Swift 3

Question

I have the following class to return a list of NOAA weather observation stations. I am using it to learn how to deal with XML. However, I am getting a "Use of undeclared type 'wxObservationStations'" as an error at func returnWxStation() -> (wxObservationStations). I am using SWXMLHash to deserialize the XML, but I don't think that is my problem (though I am just learning, so it may be).

class WxObservationStations {

let wxObserStationsURL = URL(string: "http://w1.weather.gov/xml/current_obs/index.xml")

struct wxStation: XMLIndexerDeserializable {
    let stationName: String
    let stationState: String
    let latitude: Double
    let longitude: Double

    static func deserialize(_ node: XMLIndexer) throws -> wxStation {
        return try wxStation(
            stationName: node["station_name"].value(),
            stationState: node["state"].value(),
            latitude: node["latitude"].value(),
            longitude: node["longitude"].value()
        )
    }
}

public var wxObservationStations: [wxStation] = []


private func getStationNamesAndLocations(url: URL, completion:@escaping (XMLIndexer) -> ()) {

    Alamofire.request(url).responseJSON { response in
        //                print(response) // To check XML data in debug window.
        let wxStationList = SWXMLHash.parse(response.data!)
        print(wxStationList)
        completion(wxStationList)

    }
}

//The error is here:
func returnWxStation() -> (wxObservationStations) {
    getStationNamesAndLocations(url: wxObserStationsURL!, completion: { serverResponse in
        do {
            self.wxObservationStations = try serverResponse["wx_station_index"]["station"].value()

        } catch {

        }
    })
      return self.wxObservationStations
}
}

Any thoughts? The variable is declared in the class, and I would like to use it to send the data back to the requesting object. Thanks in advance.

Rob · Accepted Answer · 2016-12-24T19:18:56.913

The wxObservationStations is not a type, so it doesn't make sense to say

func returnWxStation() -> (wxObservationStations) { ... }

You're returning self.wxObservationStations, which is of type [wxStation]. So the method declaration should be

func returnWxStation() -> [wxStation] { ... }

By the way, your life will be much easier if you stick with Cocoa naming conventions, namely types should start with upper case letters. So rather than the wxStation type, I'd suggest WxStation.

Your following method will not achieve what you want:

func returnWxStation() -> [wxStation] {
    getStationNamesAndLocations(url: wxObserStationsURL!, completion: { serverResponse in
        do {
            self.wxObservationStations = try serverResponse["wx_station_index"]["station"].value()
        } catch {

        }
    })

    return self.wxObservationStations
}

The method getStationNamesAndLocations runs asynchronously and your self.wxObservationStations will not be populated by the time that returnWxStation actually returns.

The entire purpose of the getStationNamesAndLocations method is to provide you a nice asynchronous method with completion handler. I would excise returnWxStation from your code entirely. Or do something like:

func returnWxStation(completionHandler: ([wxStation]?) -> Void) {
    getStationNamesAndLocations(url: wxObserStationsURL!) { serverResponse in
        do {
            let stations = try serverResponse["wx_station_index"]["station"].value()
            completionHandler(stations)
        } catch {
            completionHandler(nil)
        }
    }
}

And you'd use it like so:

returnWxStation() { stations in
    guard let stations = stations else {
        // handle error here
        return
    }

    // use `stations` here
}

// but not here

Thanks! I do try to stick with the conventions, but as I am learning I am unsure of whether things are types or objects at time. As to whether the code works, I suspected it wouldn't for that reason, but I am trying to get it to the point where it will build and run so I can work on it from there, so your additional answer is a great help. — Yrb, Dec 24 '16 at 18:28
@Rob, Excellent answer, except it looks like you made an editing error. The code in the returnWxStation function should not return `try serverResponse...`. That line should pass that expression to the completion handler. — Duncan C, Dec 24 '16 at 19:04

Use of undeclared type in Swift 3

1 Answers1