Handle Text Change in Compobox

I have the requirement to handle any text changes. Combobox does not support the TextChangeListener. I want to know as soon as any character is typed in combo box. Assume combobox allows new entries which I did using
this.setNewItemsAllowed(true);
this.setNewItemHandler(this);

addNewItem is called one you type some character and press enter or focus out.

My requirement is to know as soon as somebody types a character in combo box.

I tried to have ComboBox implementing TextChangeListener & TextChangeNotifier, it works except it does not work when first char is typed. It works on second character onwards. Also is this the correct approach?