Before looking at the "Hello World" program in details, we should study a bit how GTK handles signals and callbacks.
In the same way as your system delivers a signal to processes upon shutdown *1, GTK sends a signal to the main loop (Gtk.main) when a special event occured. The main loop will therefore call back the appropriate function of the widget.
Until a signal is received, the main loop will sleep.
In order to tell a GTK widget that it has to catch a specific signal, and then execute appropriate code, we need to set a signal handler.
This can be done with the GLib::Instantiatable#signal_connect method, which is part of the Ruby/GLib library (do not forget that GTK is built on GLib):
GLib::Instantiatable#signal_connect("signal name") do # Code to execute when "signal name" is caught. end
GLib::Instantiatable#signal_connect needs 2 things:
- the name of the signal which will be caught;
- a block code that will be executed upon reception of the given signal.
The block code can take an optional parameter as follows:
GLib::Instantiatable#signal_connect("signal name") do |w| # ... end
The widget which issues the signal will be therefore substituted by the parameter w.
*1Note that GTK signals are not related in any way with UNIX signals. The GTK toolkit is independent of the underlying system.