title: difference between addonGlobalLayoutListener and addOnPreDrawListener
body: <p>We know <a href="http://developer.android.com/reference/android/view/ViewTreeObserver.html" rel="noreferrer">ViewTreeObserver</a> is used to register listeners that can be notified of global changes in the view tree. There are two method defined in this class are 
  <code>addOnGlobalLayoutListener(ViewTreeObserver.OnGlobalLayoutListener</code> listener) and <code>addOnPreDrawListener(ViewTreeObserver.OnPreDrawListener listener)</code>. But the the purpose of these methods are not clearly understandable to me which described in the developer site. I tried to understand it and also searched in google but was unable to find clear picture about this topic. Any help is greatly handful  for me.</p>