/**
 *  系统在第一次使用这个类的时候调用(1个类只会调用一次)
 */
+ (void)initialize
{
    // 设置导航栏主题
    UINavigationBar *navBar = [UINavigationBar appearance];
    // 设置背景图片
    NSString *bgName = nil;
    if (iOS7) { // 至少是iOS 7.0
        bgName = @"NavBar64";
    } else { // 非iOS7
        bgName = @"NavBar";
    }
    [navBar setBackgroundImage:[UIImage imageNamed:bgName] forBarMetrics:UIBarMetricsDefault];
    
    // 设置标题文字颜色
    NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
    attrs[NSForegroundColorAttributeName] = [UIColor whiteColor];
    attrs[NSFontAttributeName] = [UIFont systemFontOfSize:16];
    [navBar setTitleTextAttributes:attrs];
    
    //设置BarButtonItem的主题
    UIBarButtonItem *item=[UIBarButtonItem appearance];
    //设置文字颜色
    NSMutableDictionary *itemAttrs=[NSMutableDictionary dictionary];
    itemAttrs[NSFontAttributeName]=[UIFont systemFontOfSize:14];
    itemAttrs[NSForegroundColorAttributeName]=[UIColor whiteColor];
    [item setTitleTextAttributes:itemAttrs forState:UIControlStateNormal];
    if (!iOS7) {
        //设置按钮背景
        
        [item setBackgroundImage:[UIImage imageNamed:@"NavButton"] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
        [item setBackgroundImage:[UIImage imageNamed:@"NavButtonPRessed"] forState:UIControlStateHighlighted barMetrics:UIBarMetricsDefault];
        
        //设置返回按钮的背景
        [item setBackButtonBackgroundImage:[UIImage imageNamed:@"NavButton"] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
        [item setBackButtonBackgroundImage:[UIImage imageNamed:@"NavButtonPressed"] forState:UIControlStateHighlighted barMetrics:UIBarMetricsDefault];

    }
    else{
        navBar.tintColor=[UIColor whiteColor];
    }
   }

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