[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];
這樣就可以更好地限制輸入長度：
- (void)textFieldDidChange:(UITextField *)textField
{
if (textField == self.titleField) {
if (textField.text.length > 20) {
textField.text = [textField.text substringToIndex:20];
}
}
}
