MBProgressHUD

 //判断用户名密码是否正确定，只有正确的情况下，才能进行下一个界面
    NSString *account = self.accountField.text;
    NSString *password = self.passwordField.text;
    
    
    //不添加toView参数，提示框是添加在window上
    [MBProgressHUD showMessage:@"正在登录中。。。"];
   
    //toView代表提示框添加到哪个view上
    //[MBProgressHUD showMessage:@"正在登录中。。。" toView:self.view];
    //[MBProgressHUD showMessage:@"正在登录中。。。" toView:self.navigationController.view];
    
    
    //模拟登录有一个等待过程
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        //登录完成 隐藏提示框
        [MBProgressHUD hideHUD];
        //[MBProgressHUD hideHUDForView:self.view];
        
        if ([account isEqualToString:@"zhangsan"] && [password isEqualToString:@"123"]) {//帐号与密码正确
            NSLog(@"帐号与密码正确");
            
            //执行一个segue，就会进入segue所指的控制器
            [self performSegueWithIdentifier:@"toContactsSegue" sender:nil];
        }else{
            NSLog(@"帐号或者密码不正确");
            
            //给一个错误的提示
            [MBProgressHUD showError:@"帐号或者密码不正确"];
        }
    });