iOS四大对象之UIApplication对象1. 获取2. 创建联网指示器3. 设置app右上角小图标的数字4. 打电话、发短信、上网5. 判断系统版本

//获取这个单例对象
UIApplication *app = [UIApplication sharedApplication];

//NS_EXTENSION_UNAVAILABLE_IOS("Use view controller based solutions where appropriate instead.");
@property(class, nonatomic, readonly) UIApplication *sharedApplication

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
//    获取应用程序对象
    UIApplication *app = [UIApplication sharedApplication];
//    设置联网指示器
    app.networkActivityIndicatorVisible = YES;
}

//    获取应用程序对象
    UIApplication *app = [UIApplication sharedApplication];
//    创建用户通知设置
    UIUserNotificationSettings *setting = [UIUserNotificationSettings settingsForTypes:UIUserNotificationTypeBadge categories:nil];
//    注册授权通知请求
    [app registerUserNotificationSettings:setting];
//    设置APP右上角的提示数字
    app.applicationIconBadgeNumber = 998;

    app.applicationIconBadgeNumber = 998;

// set to 0 to hide. default is 0. In iOS 8.0 and later, your application must register for user notifications using -[UIApplication registerUserNotificationSettings:] before being able to set the icon badge.
@property(nonatomic) NSInteger applicationIconBadgeNumber;  

//    打开百度
    NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
    [app openURL:url];
    
//    打电话
    NSURL *phone = [NSURL URLWithString:@"tel:10086"];
    [app openURL:phone];
    
    //    发短信
    NSURL *sms = [NSURL URLWithString:@"sms:10086"];
    [app openURL:sms];

    //    发邮件
    NSURL *email = [NSURL URLWithString:@"mailto:hah@22.com"];
    [app openURL:email];

//获取当前系统版本
[UIDevice currentDevice].systemVersion.floatValue