background_thread.swift

gistfile1.txt

// Without threading
func slow() {
  let methodStart = Date()
  let url = URL(string: "some string url goes here for an image")
  
  do {
    let data = try Data(contentsOf: url!)
    self.imageView.image = UIImage(data: data)
  } catch {
    print("error"
  }
  let methodFinish = Date()
  let executionTime = methodFinish.timeIntervalSince(methodStart)
  print("Execution Time: \(executionTime)")
}


// With threading 
func fast() {
     DispatchQueue.global(qos: .background).async {
        let methodStart = Date()
        let url = URL(string: "same string as one above")
        
        do {
             let data = try Data(contentsOf: url!)
             // go back to main thread to update user interface 
             DispatchQueue.main.async {
                self.imageView.image = UIImage(data: data)
                let methodFinish = Date() 
                let executionTime = methodFinish.timeIntervalSince(methodStart)
                
        }
    }
}