title: What does postInvalidate() do?
body: <p>What is the use of the <code>postInvalidate()</code> function in Android? I have been seeing this function in a lot of places. When I Googled, I came out with this much:</p>
  
  <blockquote>
    <p>postInvalidate-- <em>post an invalidate request on the UI-thread</em></p>
  </blockquote>
  
  <p>I don't know what the phrase "invalidate request" there means. Can someone explain in detail what is happening here?</p>