NSObject+Weakifiable.swift

import UIKit
import PlaygroundSupport

protocol Weakifiable: AnyObject { }

extension NSObject: Weakifiable { }

extension Weakifiable {
  func weakify(_ block: @escaping (Self) -> Void) -> () -> Void {
    { [weak self] in self.map(block) }
  }

  func weakify<T>(_ block: @escaping (Self, T) -> Void) -> (T) -> Void {
    { [weak self] in if let self = self { block(self, $0) } }
  }
}

class MyVC: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()

    loadData(weakify { (strongSelf, data) in
      print(strongSelf)
      print(data)
    })
  }
}

PlaygroundPage.current.liveView = MyVC()

func loadData(_ completion: @escaping ([String]) -> Void) {
  DispatchQueue.global(qos: .background).async {
    Thread.sleep(forTimeInterval: 2)
    DispatchQueue.main.async {
      completion((0...100).map { String(describing: $0) })
    }
  }
}