iOS textfield实现一行的数字限制，超出进行弹框

@interface LsGeXingQianMingVC ()<UITextFieldDelegate>

 _GeXingQianMingTiefield.delegate = self;

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;
{  //string就是此时输入的那个字符textField就是此时正在输入的那个输入框返回YES就是可以改变输入框的值NO相反
    
    if ([string isEqualToString:@"\n"])  //按会车可以改变
    {
        return YES;
    }
    
    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到输入框的内容
    
    if (self.GeXingQianMingTiefield == textField)  //判断是否时我们想要限定的那个输入框
    {
        if ([toBeString length] > 40) { //如果输入框内容大于20则弹出警告
            textField.text = [toBeString substringToIndex:20];
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"超过最大字数不能输入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil];
            [alert show];
            return NO;
        }
    }
    return YES;
}