UINaviagtionBar设置背景图

- (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。