如何限制一个UITextField的输入长度

[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];

- (void)textFieldDidChange:(UITextField *)textField
{
    if (textField == self.titleField) {
        if (textField.text.length > 20) {
            textField.text = [textField.text substringToIndex:20];
        }
    }
}