UITextField限制字数的方法

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;  

{  

    if ([string isEqualToString:@"\n"])   

    {  

        return YES;  

    }  

    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string];  

      

    if (self.myTextField == textField)   

    {  

        if ([toBeString length] > 20) {  

            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;  

}