//- (void)setBackgroundImage:(UIImage *)backgroundImage forBarMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;
-(id)init
{
    self=[super init];
    if(self)
    {
        UINavigationBar *navBar=[self navigationBar];
        if([navBar respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)])
        {
//如果是IOS5
            [[UINavigationBar appearance]setBackgroundImage:[UIImage imageNamed:@"NavBar.png"] forBarMetrics:UIBarMetricsDefault];
        }
        else
        {
            UIImageView *imgView=(UIImageView*)[navBar viewWithTag:NAVBARIMAGETAG];
            if(imgView==nil)
            {
                imgView=[[UIImageView alloc]initWithFrame:CGRectMake(0, 0,navBar.frame.size.width, navBar.frame.size.height)];
                [imgView setImage:[UIImage imageNamed:@"NavBar.png"]];
                [navBar insertSubview:imgView atIndex:0];
                [imgView release];
            }
        }
    }
    return self;
}