title: Drop-down list in navigation bar iOS
body: <p>I want to implement a drop-down list in the navigation bar, which contains all categories such as in this application:
  <img src="https://i.stack.imgur.com/3jUSu.png" alt="enter image description here"></p>
  
  <p>I want to know what its name, and steps to implement it.
  <img src="https://i.stack.imgur.com/glGBO.png" alt="enter image description here"></p>