import <UIKit/UIKit.h>

@interface LoginViewController : UIViewController

@end

+(UIAlertController *)showWithInfo:(NSString *)title and:(NSString *)message;

import "YDAlert.h"

@implementation YDAlert

+(UIAlertController *)showWithInfo:(NSString *)title and:(NSString *)message{

UIAlertController * alert = [UIAlertController alertControllerWithTitle:title message:message preferredStyle:UIAlertControllerStyleAlert];

UIAlertAction *actionOk = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:nil];

[alert addAction:actionOk];

[[YDAlert currentViewController] presentViewController:alert animated:NO completion:^{
    
}];

return alert;

}

• (UIViewController *)currentViewController {
  UIWindow *window = [[UIApplication sharedApplication].delegate window];
  UIViewController *presentedVC = [[window rootViewController] presentedViewController];
  if (presentedVC) {
  return presentedVC;

  } else {
  return window.rootViewController;
  }
  }

一句代码调用
[YDAlert showWithInfo:@"账号和密码不能为空" and:@"请输入账号密码"];