- (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.myTextField == textField)  //判断是否时我们想要限定的那个输入框
    { 
        if ([toBeString length] > 20) { //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] autorelease]; 
            [alert show]; 
            return NO; 
        } 
    } 
    return YES; 
}