passing a struct type as a parameter throwing initialization error - swift

Question

I have the following code which has 3 objects which are then part of a 4th object. I'm getting errors trying to create the init method for the aggregate object (GTFS) because I'm passing (or trying to pass) the type of the 3 component objects (Stop, Route, Trip). I'm not sure why those have to be initialized before just their types being used.
protocol GTFSObject {
static var fileName: String { get }
init(csvRow: [String: String])
}
struct Stop: GTFSObject {
static let fileName = "stops.txt"
let stopID: String
let stopCode: String
let stopName: String
let stopDesc: String
let stopLat: Double
let stopLon: Double
let locationType: Int
let parentStation: String
init(csvRow: [String: String]) {
self.stopID = csvRow["stop_id"]!
self.stopCode = csvRow["stop_code"]!
self.stopName = csvRow["stop_name"]!
self.stopDesc = csvRow["stop_desc"]!
self.stopLat = Double(csvRow["stop_lat"]!)!
self.stopLon = Double(csvRow["stop_lon"]!)!
self.locationType = Int(csvRow["location_type"]!)!
self.parentStation = csvRow["parent_station"]!
}
}
struct Trip: GTFSObject {
static let fileName = "trips.txt"
let routeID: String
let serviceID: String
let tripID: String
init(csvRow: [String: String]) {
tripID = csvRow["trip_id"] ?? ""
routeID = csvRow["route_id"] ?? ""
serviceID = csvRow["service_id"] ?? ""
}
}
struct Route: GTFSObject {
static let fileName = "trips.txt"
let routeID: String
let agencyID: String
let routeShortName: String
let routeLongName: String
let routeDesc: String
let routeType: Int
let routeURL: String
let routeColor: String
let routeTextColor: String
init(csvRow: [String: String]) {
routeID = csvRow["route_id"] ?? ""
agencyID = csvRow["agency_id"] ?? ""
routeShortName = csvRow["route_short_name"] ?? ""
routeLongName = csvRow["route_long_name"] ?? ""
routeDesc = csvRow["route_desc"] ?? ""
routeType = Int(csvRow["route_type"] ?? "0") ?? 0
routeURL = csvRow["route_url"] ?? ""
routeColor = csvRow["route_color"] ?? ""
routeTextColor = csvRow["route_text_color"] ?? ""
}
}
class GTFS {
let routes: [Route]
let stops: [Stop]
let trips: [Trip]
init(gtfsFolderUrl: URL) {
self.stops = init_struct_from_url(gtfsFolderUrl: gtfsFolderUrl, type: Stop.self)
self.trips = init_struct_from_url(gtfsFolderUrl: gtfsFolderUrl, type: Trip.self)
self.routes = init_struct_from_url(gtfsFolderUrl: gtfsFolderUrl, type: Route.self)
}
private func init_struct_from_url<GTFSType>(gtfsFolderUrl: URL, type: GTFSType.Type) -> [GTFSType] where GTFSType : GTFSObject{
var returnList: [GTFSType] = []
let rows = try! NamedCSV(url: GTFS_FOLDER_URL.appendingPathComponent(type.fileName), delimiter: CSVDelimiter.comma, loadColumns: false).rows
for row in rows {
returnList.append(type.init(csvRow: row))
}
return returnList
}
}
The error I get is
'self' used in method call 'init_struct_from_url' before all stored properties are initialized
I don't know why I have to initialize the properties of the struct just to pass the type of the struct to this other function. What am I missing?

Answer 1

It's not about the three types that you are passing to init_struct_from_url. It's about the call init_struct_from_url itself.
You are actually calling:
self.init_struct_from_url(...)
^^^^^
The error is saying that that use of self is not allowed, because self is not initialised. If init_struct_from_url uses a property in self, it could see an uninitialised value.
Since init_struct_from_url doesn't actually use self at all and is just a helper function, you can make it an inner function of init:
init(gtfsFolderUrl: URL) {
func structFromUrl<GTFSType>(gtfsFolderUrl: URL, type: GTFSType.Type) -> [GTFSType] where GTFSType : GTFSObject{
var returnList: [GTFSType] = []
let rows = try! NamedCSV(url: GTFS_FOLDER_URL.appendingPathComponent(type.fileName), delimiter: CSVDelimiter.comma, loadColumns: false).rows
for row in rows {
returnList.append(type.init(csvRow: row))
}
return returnList
}
self.stops = structFromUrl(gtfsFolderUrl: gtfsFolderUrl, type: Stop.self)
self.trips = structFromUrl(gtfsFolderUrl: gtfsFolderUrl, type: Trip.self)
self.routes = structFromUrl(gtfsFolderUrl: gtfsFolderUrl, type: Route.self)
}