How to infer a generic paramater with an async/await function - swiftui

Question

I have an async/await function to make sure that the data gets passed along first;
func downloadFirebaseData() async -> String {
let group = DispatchGroup()
group.enter() // stop the thread/enter the function
let db = Firestore.firestore()
withUnsafeThrowingContinuation { continuation in
db.collection("annotations")
.getDocuments { (querySnapshot, error) in
defer {
group.leave() // << end on any return
}
if let Lng = i.document.get("lng") as? String {
DispatchQueue.main.async {
annotationLng.append(Lng) //edit the array
print("downloadLngServerData ()\(annotationLng)")
}
}
if let Lat = i.document.get("lat") as? String {
DispatchQueue.main.async {
annotationLat.append(Lat) //edit the array
print("downloadLatServerData ()\(annotationLat)")
}
}
}
}
group.wait() // clear up the thread now, exit the function
}
And its called under my view with;
.task {
try await downloadFirebaseData() //error 1
}
#State annotationLat: [String] = []
#State annotationLng: [String] = []
Inside of firebase database:
annotationLat = ["42.828392","29.18273","97.27352"]
annotationLng = ["42.828392","29.18273","97.27352"]
I have 2 errors;
Invalid conversion from throwing function of type '#Sendable () async throws -> Void' to non-throwing function type '#Sendable () async -> Void'
This was under the .task
My second error:
Generic parameter 'T' could not be inferred
This was under withUnsafeThrowingContinuation
The first error I somewhat get, but even after I modified from my original code, the error still persisted.
For the second error, I know that I might have to define that this is a string somewhere, because I don't think that the app knows that I'm trying to work with a string.

Answer 1

This assumes that the Firestore path for the documents is
annotations/{id}
and that each document has variables lat and lng of type String
import Foundation
import FirebaseFirestoreSwift
import FirebaseFirestore
import CoreLocation
//struct to keep the latitude and longitude together, they should not be in separate arrays
struct Annotation: Codable, Identifiable{
#DocumentID var id: String?
var lat: String?
var lng: String?
}
extension Annotation{
//Safely unwrap the Strings into doubles and then create the coordinate
var coordinate: CLLocationCoordinate2D? {
guard let latStr = lat, let lngStr = lng, let latitude = Double(latStr), let longitude = Double(lngStr) else{
print("Unable to get valid latitude and longitude")
return nil
}
let coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
return coordinate
}
}
struct CustomFirestoreService{
let store: Firestore = .firestore()
init(){}
func getAnnotations() async throws -> [Annotation]{
let ANNOTATIONS_PATH = "annotations"
return try await retrieve(path: ANNOTATIONS_PATH)
}
///retrieves all the documents in the collection at the path
private func retrieve<FC : Codable>(path: String) async throws -> [FC]{
//Firebase provided async await.
let querySnapshot = try await store.collection(path).getDocuments()
return querySnapshot.documents.compactMap { document in
do{
return try document.data(as: FC.self)
}catch{
print(error)
return nil
}
}
}
}
Then in your View
import SwiftUI
struct AnnotationsView: View {
let service: CustomFirestoreService = CustomFirestoreService()
#State private var annotations: [Annotation] = []
var body: some View {
if annotations.isEmpty{
Text("Hello, World!")
.task {
do{
annotations = try await service.getAnnotations()
//Do any other work here, this line won't run unless the annotations are populated.
}catch{
print(error)
}
}
}else{
List(annotations){ annotation in
if let coord = annotation.coordinate{
VStack{
Text("Latitude = \(coord.latitude)")
Text("Longitude = \(coord.longitude)")
}
}else{
Text("Invalid Coordinate Value. Check firestore values for document \(annotation.id ?? "no id")")
}
}
}
}
}
struct AnnotationsView_Previews: PreviewProvider {
static var previews: some View {
AnnotationsView()
}
}
This makes some assumptions but if you paste it into your project you should get some working code.
You don't need this for your code but this is what a conversion from the "old" closures to the new async await would look like.
public func retrieve<FC : Codable>(path: String) async throws -> [FC]{
typealias MyContinuation = CheckedContinuation<[FC], Error>
return try await withCheckedThrowingContinuation { (continuation: MyContinuation) in
store.collection(path)
.getDocuments() { (querySnapshot, err) in
if let err = err {
//This throws an error
continuation.resume(throwing: err)
} else {
let array = querySnapshot?.documents.compactMap { document in
try? document.data(as: FC.self)
} ?? []
//This returns an array
continuation.resume(returning: array)
}
}
}
}
If you aren't calling continuation there is no point in returning a continuation of any kind.