title: How can i know that a imageview has been set a drawable that is or is not a specific one?
body: <p>the question is about imageview in android:</p>
  
  <p>There is an imageview, after I call method like </p>
  
  <pre><code>imageview.setImageDrawable(drawable) 
  </code></pre>
  
  <p>or</p>
  
  <pre><code>imageview.setImageBitmap(bitmap)
  </code></pre>
  
  <p>then a drawable will be drawned on the imageview, I want to know that before I call another setXXXDrawable/Bitmap method to the same imageview , the drawable that had been drawned on this imageview has or has not been changed by other operation? or the image showned on the imageview has been changed since my last call?</p>
  
  <p>Any help will be appreciated~ thks!</p>