title: GridView's "Num colunms" change with screen resolution
body: <p>I use a GridView to show list of pictures.
  I want to set the num columns change with screen resolution.
  Such as below:
  If screen width > 512, then set num columns to 5.
  Else if screen width > 256, then set num columns to 3.
  Else, then set num columns to 2.
  How to arrive it?</p>