Closure.swift

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        fetchdata()
        doSomethingAfterDataFetchIsFinished()
    }

    func fetchdata() {
      //get data
    }
  
    func doSomethingAfterDatFetchIsFinished() {
      print("data fetch is done")
    }


}

import UIKit

class ViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    fetchdata(onCompletion: { [weak self] in
      self?.doSomethingAfterDataFetchIsFinished()
    })
  }
  
  func fetchdata(onCompletion: ()->()) {
    //get data
    //Make sure to call the on completion block from the completion block of your data fetcher if there is one
    onCompletion()
  }
  
  func doSomethingAfterDataFetchIsFinished() {
    print("data fetch is done")
  }
}