限制textfield的文字长度

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

    NSString *toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string];
    if ([toBeString length] > 25) {
        textField.text = [toBeString substringToIndex:25];
        return NO;
    }

    return YES;
}