下面是实现代码：

NSString *blah = @"OOXXOOX";
NSSize size = [blah sizeWithAttributes: nil];
 
NSPoint startPoint;
startPoint.x = bounds.origin.x + bounds.size.width / 2 - size.width / 2;
startPoint.y = bounds.origin.y + bounds.size.height / 2 - size.height / 2;
 
[blah drawAtPoint: startPoint
   withAttributes: nil];