XLIB Programming Manual, Rel. 5, Third Edition

11.9 Filtering Events

An input method needs to receive X events other than keystrokes. It must receive expose events when its Preedit or Status areas need refreshing, it needs mouse button events if it is to support full-featured editing of pre-edit text, and it needs mouse motion events if it implements popup menus. The input method needs to get first crack at these events, but will not always be able to intercept them directly from the server, so the application is responsible for passing all events to the input method before processing them itself. This is done with the function XFilterEvent(). It should be called from the event loop of all internationalized applications, generally right after XNextEvent(). XFilterEvent() takes two arguments, the event to filter, and the window to which the event is directed. If the application (or a toolkit used by the application) performs event redirection, this window may not be the same as the window in which the event occurred. If the window argument is None, the window of the event will be used. An application cannot know in advance which events the IM will need to filter; it must pass all events to XFilterEvent(). If XFilterEvent() returns True, it filtered the event the application should dispatch the event no further.

Remember that an input method may be interested in different types of events than the application is. If the application is to pass events to the input method through XFilterEvent(), the application must have registered ...