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;  
}