title: How to not clear context before drawing
body: <p>I need to draw incrementally in a subclassed UIView, but the view is cleared every time I call <code>[self setNeedsDisplay]</code>.</p>
  
  <p>I'm doing this:</p>
  
  <pre><code>- (id)initWithCoder:(NSCoder *)aDecoder {
      if ((self = [super initWithCoder:aDecoder])) {
          self.clearsContextBeforeDrawing = FALSE;
      }
      return self;
  }
  </code></pre>
  
  <p>Am I missing something? How do I stop <code>UIView</code> from clearing?</p>