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35.1.1 Form Events

It is possible to by-pass the form event processing entirely by setting a "raw callback" that sits between the event reading and dispatching stage, thus a sneak preview can be implemented and optionally the event can even be consumed before the libraries internal form processing machinery gets to it.

Use the following routines to register such a preemptive processing routine

typedef int (*FL_RAW_CALLBACK)(FL_FORM *, void *xevent);
FL_RAW_CALL_BACK fl_register_raw_callback(FL_FORM *form,
                                          unsigned long mask,
                                          FL_RAW_CALLBACK callback);

where mask is the event mask you are interested in (same as the XEvent mask). The function returns the old handler for the event.

Currently only handlers for the following events are supported

Further, there is only one handler for each event pair, (e.g., ButtonPress and ButtonRelease), thus you can’t have two separate handlers for each pair although it is possible to register a handler only for one of them (but almost always a mistake) if you know what you’re doing. If you register a single handler for more than one pair of events, e.g., setting mask to KeyPressMask|ButtonPressMask, the returned old handler is random.

A special constant, FL_ALL_EVENT, is defined so that the handler registered will received all events that are selected. To select events, use fl_addto_selected_xevent().

Once an event handler is registered and the event is detected, then instead of doing the default processing by the dispatcher, the registered handler function is invoked. The handler function must return either FL_PREEMPT if the event is consumed) and 0 otherwise so that the internal processing of the event can continue. See the demo program minput2.c for an example.

Since these kind of handlers work on a rather low level there’s a chance that they interfere with some mechanisms of the library. Consider the case of setting a raw callback handler for mouse press and release events, in which the handler returns 0 for mouse press events but FL_PREEMPT on relese events. In that case the mouse press event results in the normal processing and e.g., a button below the mouse will receive it (and be drawn correspondingly). To be drawn again in its normal way it also needs to receive the release event (even if the mouse isn’t on top of it anymore when the mouse button is released). But when the handler function doesn’t also let the release event propagate to the normal handling of events then the button will never receive the expected release event and will stay drawn in the way as if the release event never happened. Thus one should avoid having different return values from the handler for pairs of related events.


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