Form Events (XForms (Forms Library) - Version 1.2 (Rev. 4) A Graphical User Interface Toolkit for X)

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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

- `KeyPressMask` and `KeyReleaseMask`
- `ButtonPressMask` and `ButtonReleaseMask`
- `EnterWindowMask` and `LeaveWindowMask`
- `ButtonMotionMask` and `PointerMotionMask`
- `FL ALL EVENT` (see below)

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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