Ios 如何验证Textfield输入

我有一个模块处理数据库调用功能，即我有一个文本字段，用户可以在其中输入他们的手机号码。当用户输入手机号码的前4位时，另一个字段称为Operator，它会根据该文本字段的4位输入值自动更改。但是，当用户输入有效的电话号码时，我的核心功能运行良好。当用户在文本字段中输入无效数字时，我的应用程序崩溃。当用户在文本字段中输入无效输入时，我不希望我的应用程序崩溃

我的示例代码：

 if(MobileNum.text.length ==4)
    {
        NSLog(@"MobileNum==>%@" ,MobileNum.text);

        Database *obj = [[Database alloc]init];
        NSMutableArray *dbArrray = [[NSMutableArray alloc]init];

        dbArrray =[obj getallRecords:MobileNum.text];

        NSString *value1 = [[dbArrray valueForKey:@"Operator"]objectAtIndex:0];

        if ([value1 isEqualToString:@""]) {
            NSString *data = @"Select Operator";
            NSString *data1 = @"Select Circle";

            operatorStr = [NSString stringWithFormat:@"%@", data];
            [operatorButton setTitle:operatorStr forState:UIControlStateNormal];
            circleStr = [NSString stringWithFormat:@"%@",data1];
            [circleButton setTitle:circleStr forState:UIControlStateNormal];
        }
        else
        {
            operatorStr = [[dbArrray valueForKey:@"Operator"]objectAtIndex:0];
            opString = operatorStr;
            NSLog(@"Operator Field==>%@",opString);
            [operatorButton setTitle:operatorStr forState:UIControlStateNormal];
            circleStr = [[dbArrray valueForKey:@"Circle"]objectAtIndex:0];
            cirString = circleStr;
            NSLog(@"Circle Field==>%@",cirString);
            [circleButton setTitle:circleStr forState:UIControlStateNormal];
        }
    }

而我的文本字段只接受起始数字，如7、8、9，所有输入都会被拒绝，应用程序会崩溃

dbArrray =[obj getallRecords:MobileNum.text];

 if (dbArrray.count==0)
{
  // do nothing , like select the another user

  [operatorButton setTitle:@"Select Operator" forState:UIControlStateNormal];           
  [circleButton setTitle:@"Select Circle" forState:UIControlStateNormal];
 }
else
 {

   [operatorButton setTitle:[[dbArrray valueForKey:@"Operator"]objectAtIndex:0] forState:UIControlStateNormal];
   [circleButton setTitle:[[dbArrray valueForKey:@"Circle"]objectAtIndex:0] forState:UIControlStateNormal];
 }

---

检查dbarray是否包含值A如果数组计数大于零，请始终执行此操作-添加值，否则不执行任何操作