title: iOS application have multiple main thread
body: <p>Can we create multiple main thread in single application?
  Why all UI changes made in main thread?</p>
  
  <p>I have faced these question in interviews. I am unable to find any solution. Can any one help me to find the answers of this question? That will help me in future interviews and implementations in iOS app.</p>
  
  <p>Thanks in advance!</p>