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# 35. Dirty Tricks

This chapter describes some of the routines that may be used in special situations where more power or flexibility from Forms Library is needed. These routines are classified as "dirty tricks" either because they can easily mess up the normal operation of Forms Library or they depend on internal information that might change in the future, or they rely too much on the underlying window systems. Thus whenever possible, try not to use these routines.

## 35.1 Interaction

### 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.

### 35.1.2 Object Events

Just as you can by-pass the internal event processing for a particular form, you can also do so for an object. Unlike in raw callbacks, you can not select individual events.

The mechanism provided is via the registration of a pre-handler for an object. The pre-handler will be called before the built-in object handler. By electing to handle some of the events, a pre-handler can, in effect, replace part of the built-in handler.

In the chapter about pre-emptive handlers the API was already discussed in detail, so here we just repeat the discussion for completeness as any use of pre-emptive handler is considered "dirty tricks".

To register a pre-handler, use the following routine

    typedef int (*FL_HANDLEPTR)(FL_OBJECT *obj, int event,
                                FL_Coord mx, FL_Coord my,
                                int key, void *raw_event);

    void fl_set_object_prehandler(FL_OBJECT *, FL_HANDLEPTR prehandler);

where `event` is the generic event in the Forms Library, that is, `FL DRAW`, `FL ENTER` etc. The arguments `mx` and `my` are the mouse position and `key` is the key pressed. The last parameter, `raw_event` is a pointer to the XEvent that caused the invocation of the pre-handler. cast to a void pointer.

Notice that the pre-handler has the same function prototype as the built-in handler. Actually they are called with the exact same parameters by the event dispatcher. The prehandler should return `0` if the processing by the built-in handler should continue. A return value of `FL PREEMPT` will prevent the dispatcher from calling the built-in handler.

See demo program ``preemptive.c`' for an example.

A similar mechanism exists for registering a post-handler, i.e., a handler invoked after the built-in handler is finished, by using

    void fl_set_object_posthandler(FL_OBJECT *, FL_HANDLEPTR prehandler);

Whenever possible a post-handler should be used instead of a pre-handler.

## 35.2 Other

As stated earlier, `fl_set_defaults()` can be used to modify the Forms Library's defaults prior to calling `fl_initialize()`. Actually, this routine can also be used after `fl_initialize()` to override the values set on the command line or in the application databases. However, overriding users' preferences should be done with discretion. Further, setting `privateColormap` after `fl_initialize()` has no effect.

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This document was generated by Jens Thoms Toerring on January 5, 2014 using texi2html 1.82.