8. Tasking with GtkAda

Note that Gtk+ under Windows does not interact properly with threads, so the only safe approach under this operating system is to perform all your Gtk+ calls in the same task.

On other platforms, the Glib library can be used in a task-safe mode by calling Gdk.Threads.G_Init and Gdk.Threads.Init before making any other Glib/Gdk calls. Gdk routines may then be called simultaneously by multiple tasks, thanks to task-safe construction of Gdk’s internal data structures. However, Gdk objects such as hash tables are not automatically protected, so it is the application’s responsibility to prevent simultaneous access to user-defined objects (e.g. by using protected objects).

When Gdk is initialized to be task-safe, GtkAda becomes task aware. There is a single global lock that you must acquire with Gdk.Threads.Enter before making any Gdk/Gtk call, and which you must release with Gdk.Threads.Leave afterwards.

Gtk.Main.Main should be called with the lock acquired (see example below), ensuring that all the functions executed in the task that started the main loop do not need to protect themselves again.

Beware that the GtkAda main loop (Gtk.Main.Main) can only be be run inside one specific task. In other words, you cannot call Gtk.Main.Main from any task other than the one that started the outer level main loop.

Note that Gdk.Threads assumes that you are using a tasking run time that maps Ada tasks to native threads.

A minimal main program for a tasking GtkAda application looks like:

with Gdk.Threads;
with Gtk.Main;
with Gtk.Enums; use Gtk.Enums;
with Gtk.Window; use Gtk.Window;

procedure GtkAda_With_Tasks is
   Window : Gtk_Window;

   Gtk_New (Window, Window_Toplevel);
   Show (Window);

end GtkAda_With_Tasks;

Callbacks require a bit of attention. Callbacks from GtkAda (signals) are made within the GtkAda lock. However, callbacks from Glib (timeouts, IO callbacks, and idle functions) are made outside of the GtkAda lock. So, within a signal handler you do not need to call Gdk.Threads.Enter, but within the other types of callbacks, you do.