海神

代码：
@implementation UINavigationBar (CustomImage)

- (UIImage*)barBackground { 
  return[UIImage imageNamed:@"bg_header.png"];
}
- (void)didMoveToSuperview { 
  //iOS5 only 
  if ([self respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)]) { 
    [self setBackgroundImage:[self barBackground] forBarMetrics:UIBarMetricsDefault]; 
  }
}
//this doesn't work on iOS5 but is needed for iOS4 and earlier
- (void)drawRect:(CGRect)rect { 
  //draw image 
  [[self barBackground] drawInRect:rect];
}

@end

对于iOS4之前的版本需要覆盖drawRect方法，iOS5之后增加了setBackgroundImage:forBarMetrics:方法。所以需要判断是否支持这个方法。