====== Handling Events ======

Handling events in FLTK is done via [[http://en.wikipedia.org/wiki/Callback_(computer_science)|callbacks]]. Unlike other alternatives, probably more suitable for C++ object oriented approach, like [[http://en.wikipedia.org/wiki/Signals_and_slots|signals and slots]], implemented in Qt, FLTK callback mechanism is fairly simple, but no less powerfull.

FLTK callbacks are one of the reasons why FLTK is so fast and has small core so edelib explores this property extensively. For example, when you receive a D-BUS message via [[http://equinox-project.org/api/edelib/classedelib_1_1EdbusConnection.html|EdbusConnection]], it will be reported via callback. The same applies when something was changed in directory, monitored with [[http://equinox-project.org/api/edelib/classedelib_1_1DirWatch.html|DirWatch]].

===== Example Application =====
To demonstrate FLTK callbacks, here will be presented a simple application with ''Close'' button, where pressing the button will close the window. Application will use ''Window'' class from edelib.

<code cpp>
/* event.cpp */

#include <FL/FL.H>
#include <FL/Fl_Button.H>
#include <edelib/Window.h>

EDELIB_NS_USING_AS(Window, MainWindow)

/* called when button is pressed */
static void close_cb(Fl_Widget*, void *w) {
  MainWindow *win = (MainWindow*)w;
  win->hide();
}

int main(int argc, char **argv) {
  MainWindow *win = new MainWindow(115, 60, "Demo");
  Fl_Button *button = new Fl_Button(15, 15, 90, 25, "&Close");

  /* register our callback */
  button->callback(close_cb, win);

  win->end();
  w->show(argc, argv);
  return Fl::run();
}
</code>
As you can see, ''callback()'' function, besides a pointer to function, accepts additional parameter called ''user data''; in this case is our window object, so we can call it's memeber ''hide()'' that will close the window.

FLTK uses callbacks in many places; you can get events when window is closed, when text is modified, etc. [[http://www.fltk.org/doc-1.1/editor.html|Designing a Simple Text Editor]] chapter in FLTK documentation gives some example how to monitor changes when text is changed in editor widget.

===== More from callbacks =====
Sometimes you want to send more data to callback, other than single object. For example, callback function needs to access object who owns callbacks with additional data; you can use struct or class to pack them, but FLTK already solves named problem.

If you look carefully at above example, '''callback()''' receives two parameters: some'' Fl_Widget*'' and ''void*''; ''void*'' is used  to send win object, but ''Fl_Widget*'' can be used to (implicitly) send button object. Here is how it works:

<code cpp>
/* event.cpp */
...
/* called when button is pressed */
static void close_cb(Fl_Button *button, void *w) {
  /* we can access button object too */
}

int main(int argc, char **argv) {
...
  /* register our callback */
  button->callback((Fl_Callback*)close_cb, win);
...
}
</code>
The main difference from original example is how we explicitly cast-ed callback function to '''Fl_Callback*''' and our callback function ''close_cb'' is now able to accept any Fl_Widget inherited object. If you try without that, the compiler will complain.