title: How to get the size of a window
body: <p>I want to allocate memory for the points array.</p>
  
  <p>And I want to depend on screen size, so that I can draw properly.</p>
  
  <p>But I don't know the size of the screen except in function <code>onPaint()</code> to call <code>GetClientSize()</code>.</p>
  
  <p>And I found that the result of <code>GetClientSize()</code> and <code>GetSize()</code> is so different that I can't know what's the connection between them.</p>
  
  <p>Is there anyway that I can know the size before I call draw function???</p>