How to wait for two callbacks in swift.

callbacks.swift

/*
 *
 * I recommend using RxSwift, Combine or PromiseKit for this kind of flow.
 *
*/

import UIKit
import Foundation

func wait<A,B>(_ blockA: @escaping (@escaping (Result<A,Error>) -> Void) -> Void,
               _ blockB: @escaping (@escaping (Result<B,Error>) -> Void) -> Void,
                 then execute: @escaping (Result<(A,B),Error>) -> Void) {
    
    var resultA: A?
    var resultB: B?
    var firstError: Error?
    let group = DispatchGroup()
    
    group.enter()
    blockA { (result) in
        switch result {
        case .success(let payload):
            resultA = payload
        case .failure(let error):
            firstError = error
        }
        group.leave()
    }
    
    group.enter()
    blockB { (result) in
        switch result {
        case .success(let payload):
            resultB = payload
        case .failure(let error):
            firstError = error
        }
        group.leave()
    }
    
    group.notify(queue: .main) {
        if let error = firstError {
            execute(.failure(error))
        } else if let resultA = resultA,
                  let resultB = resultB {
            execute(.success((resultA, resultB)))
        }
    }
}

func getUsersCount(then execute: @escaping (Result<Int, Error>) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) {
        execute(.success(10))
    }
}

func getUsername(then execute: @escaping (Result<String, Error>) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2)) {
        execute(.success("@manasshr"))
    }
}

wait(getUsersCount, getUsername) { (result) in
    switch result {
      case .success((let result1, let result2)):
       print(result1, result2)
         
     case .failure(let error):
        print(error)
     }
}

/*
 *
 * Methods that have paramaters other than the callback cannot be used which makes this approach very inflexible.
 * For example:
 *
 * func getPost(id: Int, then execute: @escaping (Result<String, Error>) -> Void) {}
 * 
 * // error here
 * wait(getUsersCount, getPost(id: 20, then:) ) { (_) in }
 * 
 *
*/