Problem with defadvice for evil-find-char

Hi developers, I'm tring to add some <Input Method Engine> support/compatibility for evil-mode recently. Now I'm having trouble with advising the evil-find-char (evil-find-char &optional COUNT CHAR). I want to automatically switch to an IME when typing _f_ in normal-mode, and switch back to English after entring the CHAR. To do so, I add a before-advice to evil-find-char, but finding that the advice will actually work after entering the CHAR. Is there any mechanism which would enable me to exec some code right after typing f ? Redefine the function and kbd macro are no good if there is a better way. Regards