title: iOS: draw a UIImage of circle at a UIView's center
body: <p>I have a circle image with radius 50, and now I want to put the circyle's center the same as my View's center like this:</p>
  
  <pre><code>- (void)drawRect:(CGRect)rect {
      UIImage *arrow = [UIImage imageWithPDFNamed:@"circle" atSize:CGSizeMake(50, 50)];
      [arrow drawAtPoint:self.center];
  }
  </code></pre>
  
  <p>But drawAtPoint is not drawing at the center.
  I want to know how to locate the center of the image? so I can draw it?</p>