Pagination Problem

import Foundation

// Here is the codable model that I am trying to decode the feed into
struct Feed: Codable {
    let items: [Items]
    let pageInfo: PageInfo
    let nextPageToken: String?
}

struct PageInfo: Codable {
    let totalResults: Int
    let resultsPerPage: Int
}

struct Items: Codable, Identifiable {
    struct Snippet: Codable {
        struct Thumbnails: Codable {
            struct Default: Codable {
                let url: URL
            }
            let `default`: Default?
        }

        struct ResourceId: Codable {
            let videoId: String
        }

        let publishedAt: Date
        let title: String
        let description: String
        let thumbnails: Thumbnails
        let resourceId: ResourceId?
        let channelId: String?
    }

    let id: String
    let snippet: Snippet

}

// Here is a sample endpoint. I have specified maxResults here as 20, because I know that there are 50items in this playlist and so there will be pagination involved meaning that for the first 4 requests, the Feed object's nextPageToken will not be nil
let endpoint = "https://www.googleapis.com/youtube/v3/playlistItems?part=snippet%2CcontentDetails&maxResults=20&playlistId=PLBn01m5Vbs4C8jeAmLZxk9kZ59lYtxnHW&key=AIzaSyAfF93xIFQEVYXjci2P3VYdhIWNFG9n7cw"

// This is what I currently use for my APIServce to make a request to get the JSON
enum APIService {
    struct ErrorType: Identifiable {
        let id = UUID()
        let error: APIError
    }
        public enum APIError: Error {
             case error(_ errorString: String)
         }
         public static func getJSON<T: Decodable>(urlString: String,
                                    dateDecodingStategy: JSONDecoder.DateDecodingStrategy = .deferredToDate,
                                    keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys,
                                    completion: @escaping (Result<T,APIError>) -> Void) {
             guard let url = URL(string: urlString) else {
                completion(.failure(.error("Invalid URL")))
                 return
             }
             let request = URLRequest(url: url)
             URLSession.shared.dataTask(with: request) { (data, response, error) in
                 if let error = error {
                     completion(.failure(.error("URLError: \(error.localizedDescription)")))
                     return
                 }
                 guard let data = data else {
                     completion(.failure(.error("Error: Data is corrupt.")))
                     return
                 }
                 let decoder = JSONDecoder()
                 decoder.dateDecodingStrategy = dateDecodingStategy
                 decoder.keyDecodingStrategy = keyDecodingStrategy
                 guard let decodedData = try? decoder.decode(T.self, from: data) else {
                     completion(.failure(.error("Error: decoding data.")))
                     return
                 }
                 completion(.success(decodedData))
             }.resume()
         }
    }

// So, when when I want to retrieve the data, I do this.

var allItems = [Items]()
// Here is a function that I use to retrieve the items and populate the allItems array
func getAllItems(nextPageToken:String? = nil, completion: @escaping (Feed) -> Void) {
    var urlString = endpoint
    if let nextPageToken = nextPageToken {
        urlString = endpoint + "&pageToken=\(nextPageToken)"
    }
    APIService.getJSON(urlString: urlString,dateDecodingStategy: .iso8601) {  (result: Result<Feed, APIService.APIError>) in
        switch result {
        case .success(let feed):
            completion(feed)
        case .failure(let error):
            print(error.localizedDescription)
        }
    }
}


// Example processing of retrieved items

func printAllTitles(completion: @escaping ([Items]) -> Void) {
    var foundItems = [Items]()
    getAllItems { feed in
        foundItems += feed.items
        let nextPageToken = feed.nextPageToken
        if nextPageToken != nil {
            // Here is where I am stuck.  I need to keep calling getAllItems until nextPageToken is no longer nil so that I can append all of the items to the allItems array and then execute the completion
            // I am forcing completion here so that you can see that I get the first 20 in the set
            completion(foundItems)
        } else {
            completion(foundItems)
        }
    }
}

printAllTitles { items in
   allItems = items
    for item in allItems {
        print(item.snippet.title)
    }
}