Swift: Recursive async func with completion handler that does not get called

Question

I need to fetch large amounts of data from an endpoint in an async way. The API endpoint serves a predefined amount of data at a time. After the first request I must check to see if I get a "next" url from the response and visit that link in order to continue the download. This recursive behaviour continues until all available data has been served, in other words paging functionality (HAL links). At this point I have implemented a func that download recursively, however: problem is that the final completion handler does not seem to get called.

Demo code: The ThingsApi is a class that encapsulates the actual API call. The important thing is that this class has an initial url and during recursion will get specific url's to visit asynchronously. I call the downloadThings() func and need to get notified when it is finished. It works if I leave recursion out of the equation. But when recursion is in play then nothing!

I have created a simplified version of the code that illustrate the logic and can be pasted directly into the Playground. The currentPage and pages var's are just there to demo the flow. The last print() statement does not get called. Leave the currentPage += 1 to experience the problem and set currentPage += 6 to avoid recursion. Clearly I am missing out of some fundamental concept here. Anyone?

import UIKit

let pages = 5
var currentPage = 0

class ThingsApi {
    var url: URL?
    var next: URL?

    init(from url: URL) {
        self.url = url
    }

    init() {
        self.url = URL(string: "https://whatever.org")
    }

    func get(completion: @escaping (Data?, HTTPURLResponse?, Error?) -> Void) {
        // *** Greatly simplified
        // Essentially: use URLSession.shared.dataTask and download data async.
        // When done, call the completion handler.

        // Simulate that the download will take 1 second.
        sleep(1)

        completion(nil, nil, nil)
    }
}

func downloadThings(url: URL? = nil, completion: @escaping (Bool, Error?, String?) -> Void) {
    var thingsApi: ThingsApi

    if let url = url {
        // The ThingsApi will use the next url (retrieved from previous call).
        thingsApi = ThingsApi(from: url)
    } else {
        // The ThingsApi will use the default url.
        thingsApi = ThingsApi()
    }

    thingsApi.get(completion: { (data, response, error) in
        if let error = error {
            completion(false, error, "We have nothing")
        } else {

            // *** Greatly simplified
            // Parse the data and save to db.

            // Simulate that the thingsApi.next will have a value 5 times.
            currentPage += 1
            if currentPage <= pages {
                thingsApi.next = URL(string: "https://whatever.org?page=\(currentPage)")
            }

            if let next = thingsApi.next {

                // Continue downloading things recursivly.
                downloadThings(url: next) { (success, error, feedback) in
                    guard success else {
                        completion(false, error, "failed")
                        return
                    }
                }

            } else {
                print("We are done")
                completion(true, nil, "done")
                print("I am sure of it")
            }

        }
    })
}

downloadThings { (success, error, feedback) in
    guard success else {
        print("downloadThings() failed")
        return
    }

    // THIS DOES NOT GET EXECUTED!
    print("All your things have been downloaded")
}

Answer 1

It seems like this is simply a case of "you forgot to call it yourself" :)

In this if statement right here:

if let next = thingsApi.next {

    // Continue downloading things recursivly.
    downloadThings(url: next) { (success, error, feedback) in
        guard success else {
            completion(false, error, "failed")
            return
        }
    }

} else {
    print("We are done")
    completion(true, nil, "done")
    print("I am sure of it")
}

Think about what happens on the outermost call to downloadThings, and execution goes into the if branch, and the download is successful. completion is never called!

You should call completion after the guard statement!