title: Why on iPad, self.clearsContextBeforeDrawing = NO; won't make the drawRect not clear itself before drawing
body: <p>I have a <code>TestView</code> inherited from <code>UIView</code>, and everything is drawn on the screen using its <code>drawRect</code> method.</p>
  
  <p>But if I don't want <code>drawRect</code> to clear the view before drawing, I used</p>
  
  <pre><code>self.clearsContextBeforeDrawing = NO;  
  self.opaque = NO;   // also added because the doc says the rect 
                      //   passed to drawRect will be filled with 
                      //   background color first if it is set to YES
  </code></pre>
  
  <p>in the <code>initWithFrame</code> method.</p>
  
  <p>The <code>drawRect</code> method is invoked by using</p>
  
  <pre><code>[self.view setNeedsDisplay];
  </code></pre>
  
  <p>in the <code>ViewController</code> event handler. (for touchesMoved events)</p>
  
  <p>But still, everything is cleared before anything is drawn?  How to make it work?</p>