How can I chain completion handlers if one method also has a return value?

Question

I have 2 methods I need to call, the second method must be executed using the result of the first method and the second method also returns a value.

I have put together a simple playground that demonstrates a simple version of the flow

import UIKit

protocol TokenLoader {
    func load(_ key: String, completion: @escaping (String?) -> Void)
}

protocol Client {
    func dispatch(_ request: URLRequest, completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) -> URLSessionTask
}


class AuthTokenLoader: TokenLoader {
    func load(_ key: String, completion: @escaping (String?) -> Void) {
        print("was called")
        completion("some.access.token")
    }
}

class Networking: Client {
    
    private let loader: TokenLoader
    
    init(loader: TokenLoader) {
        self.loader = loader
    }
    
    func dispatch(_ request: URLRequest, completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) -> URLSessionTask {
        
        let task = URLSession.shared.dataTask(with: request, completionHandler: { data, response, error in
            if let error = error {
                completion(.failure(error))
            } else if let data = data, let response = response as? HTTPURLResponse {
                completion(.success((data, response)))
            }
        })
        
        task.resume()
        
        return task
    }
}

let loader = AuthTokenLoader()
let client = Networking(loader: loader)

let request = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
client.dispatch(.init(url: request), completion: { print($0) })

I need to use the token returned by AuthTokenLoader as a header on the request sent by dispatch method in my Networking class.

Networking also returns a task so this request can be cancelled.

As I cannot return from inside the completion block of the AuthTokenLoader load completion, I unsure how to achieve this.

Answer 1

You can create a wrapper for your task and return that instead.

protocol Task {
    func cancel()
}

class URLSessionTaskWrapper: Task {
    private var completion: ((Result<(Data, HTTPURLResponse), Error>) -> Void)?
    
    var wrapped: URLSessionTask?
    
    init(_ completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) {
        self.completion = completion
    }
    
    func complete(with result: Result<(Data, HTTPURLResponse), Error>) {
        completion?(result)
    }
    
    func cancel() {
        preventFurtherCompletions()
        wrapped?.cancel()
    }
    
    private func preventFurtherCompletions() {
        completion = nil
    }
}

Your entire playground would become

protocol TokenLoader {
    func load(_ key: String, completion: @escaping (String?) -> Void)
}

protocol Client {
    func dispatch(_ request: URLRequest, completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) -> Task
}


class AuthTokenLoader: TokenLoader {
    func load(_ key: String, completion: @escaping (String?) -> Void) {
        print("was called")
        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            completion("some.access.token")
        }
    }
}


protocol Task {
    func cancel()
}

class URLSessionTaskWrapper: Task {
    private var completion: ((Result<(Data, HTTPURLResponse), Error>) -> Void)?
    
    var wrapped: URLSessionTask?
    
    init(_ completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) {
        self.completion = completion
    }
    
    func complete(with result: Result<(Data, HTTPURLResponse), Error>) {
        completion?(result)
    }
    
    func cancel() {
        preventFurtherCompletions()
        wrapped?.cancel()
    }
    
    private func preventFurtherCompletions() {
        completion = nil
    }
}

class Networking: Client {
    
    private let loader: TokenLoader
    
    init(loader: TokenLoader) {
        self.loader = loader
    }
    
    func dispatch(_ request: URLRequest, completion: @escaping (Result<(Data, HTTPURLResponse), Error>) -> Void) -> Task {
        let task = URLSessionTaskWrapper(completion)

        loader.load("token") { token in
            
            task.wrapped = URLSession.shared.dataTask(with: request, completionHandler: { data, response, error in
                if let error = error {
                    task.complete(with: .failure(error))
                } else if let data = data, let response = response as? HTTPURLResponse {
                    task.complete(with: .success((data, response)))
                }
            })
             
            task.wrapped?.resume()
        }
    
        return task
    }
}

let loader = AuthTokenLoader()
let client = Networking(loader: loader)

let request = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
client.dispatch(.init(url: request), completion: { print($0) })