I believe it's a bit more complicated than that. IME is often more a stateful build-and-commit process than a collection of individual insertable keys.

If you want to handle individual composition changes, you can look at CompositionChangeEvent. Note that these events are delivered to a control's UI, not the control itself. So, for example, the following would dump out the current value under composition in a TextFieldUI:

{ui.add-event-handler
    {on e:CompositionChangeEvent at tfui:TextFieldUI do
        {dump e, e.string}
    }
}