MessageSender.swift

import UIKit
import Foundation

final class MessageSender {

    private let api: API = TestAPI()

    func send(images: [UIImage], text: String, completion: @escaping () -> Void) {
        // TODO: please implement this method
    }
}


// MARK: API definition

typealias ImageId = String

protocol API: AnyObject {
    func uploadImage(_: UIImage, completion: @escaping (ImageId) -> Void)
    func sendMessage(imageIds: [ImageId], text: String, completion: @escaping () -> Void)
}

// MARK: API implementation

class TestAPI: API {

    func uploadImage(_ image: UIImage, completion: @escaping (ImageId) -> Void) {
        DispatchQueue.global().asyncAfter(deadline: .now() + Double.random(in: 0..<1)) {
            completion(image.description)
        }
    }

    func sendMessage(imageIds: [ImageId], text: String, completion: @escaping () -> Void) {
        DispatchQueue.global().async {
            completion()
        }
    }
}