4. Signal handling

In GtkAda, the interaction between the interface and the core application is done via signals. Most user actions on the graphical application trigger some signals to be emitted.

A signal is a message that an object wants to broadcast. It is identified by its name, and each one is associated with certain events which happen during the widget’s lifetime. For instance, when the user clicks on a Gtk_Button, a ‘clicked’ signal is emitted by that button. More examples of signals can be found in the GtkAda reference manual.

It is possible to cause the application to react to such events by connecting to a signal a special procedure called a handler or callback. This handler will be called every time that signal is emitted, giving the application a chance to do any processing it needs. More than one handler can be connected to the same signal on the same object; the handlers are invoked in the order they were connected.

4.1. Predefined signals

Widgets, depending on their type, may define zero or more different signals. The signals defined for the parent widget are also automatically inherited; thus every widget answers many signals.

The easiest way to find out which signals can be emitted by a widget is to look at the GtkAda reference manual. Every widget will be documented there. The GtkAda RM explains when particular signals are emitted, and the general form that their handlers should have (although you can always add a User_Data if you wish, see below).

In general, your handlers should have the exact same profile that is documented (the GtkAda RM is automatically generated, so you can in fact find the same documentation directly in GtkAda’s *.ads files).

However, if you connect to signals via the generic packages defined in Gtk.Handlers (see below), it is valid to pass a procedure that drops all arguments except the first one, i.e. the actual widget that emitted the signal. To get a better documented code, though, we recommend to always use the full profile for your handlers.

4.2. Connecting signals

There are currently two ways to connect widgets to signal handlers. One of them is much simpler to use, although it has some limited capabilities.

4.2.1. Connecting via the On_* procedures

Each widget has a number of primitive operations (including inherited ones) for all the signals it might emit. In fact, for each signal there are two On_<signal_name> procedures that can be used to easily connect to the corresponding signal:

procedure Handler (Button : access Gtk_Button_Record'Class) is
end Handler;

Button.On_Clicked (Handler'Access);

The code above ensures that the procedure Handler is called whenever the button is clicked.

The On_* procedures ensure that the profile of the handler is correct, and thus are type-safe.

The type of the first parameter to the handler is always the type where the signal is defined, not the type to which the handler is connected.

For instance, the “draw” signal is defined for a Gtk_Widget. But if you connect this signal to a Gtk_Button, the first paramter of the handler is always of type access Gtk_Widget_Record’Class.

There is a second version of the On_* procedures, which is used to pass a different object than the one the signal is connected to. In practice, this is the version that is used more often. For instance, clicking on a toolbar button will in general affect some other widget than the button itself, and you would typically pass the main window as a parameter to the handler. Here is an example, note how the type of the first parameter is different:

procedure Handler (Win : access GObject_Record'Class) is
end Handler;

Button.On_Clicked (Handler'Access, Slot => Main_Window);

This subprogram also ensures that the handler is automaticall disconnected if the second object is destroyed.

4.2.2. Connecting via the Gtk.Handlers package

All signal handling work is performed internally using services provided by the Gtk.Handlers package. But this package can also be used directly by user applications.

This file is collection of several generic packages that need to be instantiated before you can actually start connecting handlers to widgets. A number of predefined instantiations are provided in GtkAda.Handlers to make it slightly easier.

Compared to the previous approach based on the On_* procedures described above, this approach has a number of additional capabilities, at the cost of slightly more complex code:

  • It is possible to retrieve a handle on the Widget/Signal/Handle tuple, so that you can later on disconnect the handler, or temporarily block the signal for instance.

  • It is possible to pass additional user data to the handler. For instance, you could have a single handler connected to multiple check buttons. When you press any of the button, the handler is called and passes an additional integer to indicate which button was pressed. This is sometimes convenient, although it can often be avoided by creating new Ada tagged types derived from the standard GtkAda types.

  • You have full control over the type of the first parameter to the handler. As discussed earlier, the On_* subprograms force specific types (either a GObject_Record or the type on which the signal was defined). With the generic packages, you can avoid the often necessary type casts in the handler, although this approach does not guarantee more (or less) type safety.

  • A very limited number of signals do not have a corresponding On_* for circular dependency (or elaboration circularity) reasons. For those, you need to use the generic packages. However, we believe these signals are hardly ever used by user-level applications.

A short, annotated example of connecting signals follows; a complete example can be found in create_file_selection.adb (inside the testgtk/ directory). In our example, an application opens a file selector to allow the user to select a file. GtkAda provides a high-level widget called Gtk_File_Selection which can be used in this case:

   Window : Gtk_File_Selection;
   Gtk.File_Selection.Gtk_New (Window, Title => "Select a file");

When the ‘OK’ button is pressed, the application needs to retrieve the selected file and then close the dialog. The only information that the handler for the button press needs is which widget to operate upon. This can be achieved by the following handler:

procedure OK (Files : access Gtk_File_Selection_Record'Class) is
   --  Prints the name of the selected file.
   Ada.Text_IO.Put_Line ("Selected " & Get_Filename (Files));

   --  Destroys the file selector dialog
   Destroy (Files);
end Ok;

We now need to connect the object we created in the first part with the new callback we just defined. Gtk.Handlers defines four types of generic packages, depending on the arguments one expects in the callback and whether the callback returns a value or not. Note that you can not use an arbitrary list of arguments; these depend on the signal, as explained in the previous section.

In our example, since the callback does not return any value and does not handle any User_Data (that is, we don’t pass it extra data, which will be specified at connection time), the appropriate package to use is Gtk.Handlers.Callback. We thus instantiate that package.

Remember that generic package instantiations in GtkAda must be present in memory at all times, since they take care of freeing allocated memory when finished. GtkAda generic package instantiations must therefore always be performed at the library level, and not inside any inner block:

package Files_Cb is new Handlers.Callback (Gtk_File_Selection_Record);

The Files_Cb package now provides a set of Connect subprograms that can be used to establish a tie between a widget and a handler. It also provides a set of other subprograms which you can use to emit the signals manually, although most of the time, the signals are simply emitted internally by GtkAda. We will not discuss the Emit_By_Name subprograms here.

The general form of handler, as used in Gtk.Handlers, expects some handlers that take two or three arguments: the widget on which the signal was applied, an array of all the extra arguments sent internally by GtkAda, and possibly some user data given when the connection was made.

This is the most general form of handler and it covers all the possible cases. However, it also expects the user to manually extract the needed values from the array of arguments. This is not always the most convenient solution. This is why GtkAda provides a second package related to signals, Gtk.Marshallers.

The Gtk.Marshallers package provides a set of functions that can be used as callbacks directly for GtkAda, and that will call your application’s handlers after extracting the required values from the array of arguments. Although this might sound somewhat complicated, in practice it simplifies the task of connecting signals. In fact, the techniques employed are similar to what is done internally by gtk+ in C. Because of the similarity of techniques, there is no overhead involved in using Gtk.Marshallers with Ada over the C code in gtk+.

A set of functions To_Marshaller is found in every generic package in Gtk.Handlers. They each take a single argument, the name of the function you want to call, and return a handler that can be used directly in Connect.

The connection is then done with the following piece of code:

  (Get_Ok_Button (Window),  --  The object to connect to the handler
   "clicked",               --  The name of the signal
   Files_Cb.To_Marshaller (Ok'Access),  --  The signal handler
   Slot_Object => Window);

Note that this can be done just after creating the widget, in the same block. As soon as it is created, a widget is ready to accept connections (although no signals will be emitted before the widget is shown on the screen).

We use To_Marshaller since our handler does not accept the array of arguments as a parameter, and we use the special Object_Connect procedure. This means that the parameter to our callback (Files) will be the Slot_Object given in Object_Connect, instead of being the button itself.

Compare the above code to the approach described in the first section, in particular when using Ada05 notation:

Window.Get_Ok_Button.On_Clicked (Ok'Access, Window);

4.3. Handling user data

As described above, it is possible to define some data that is that passed to the callback when it is called. This data is called user_data, and is passed to the Connect or Object_Connect subprograms.

GtkAda will automatically free any memory it has allocated internally to store this user data. For instance, if you instantiated the generic package User_Callback with a String, it means that you want to be able to have a callback of the form:

procedure My_Callback (Widget : access Gtk_Widget_Record'Class;
                        User_Data : String);

and connect it with a call similar to:

Connect (Button, "Clicked", To_Marshaller (My_Callback'Access),
         User_Data => "any string");

GtkAda needs to allocate some memory to store the string (an unconstrained type). However, this memory is automatically freed when the callback is destroyed.

There are a few subtleties in the use of user_data, most importantly when the user data is itself a widget.

The following four examples do exactly the same thing: each creates two buttons, where clicking on the first one will destroy the second one. They all work fine the first time, while both buttons exist. However, some of them will fail if you press on the first button a second time.

Complete, compilable source code for these examples can be found in the distribution’s examples/user_data directory, from which the code samples below are excerpted.

4.3.1. First case: simple user data

This code will fail: even after Button2 is destroyed, the Ada pointer continues to reference memory that has been deallocated. The second call to Destroy will fail with a Storage_Error:

package User_Callback is new Gtk.Handlers.User_Callback
  (Gtk_Widget_Record, Gtk_Widget);

procedure My_Destroy2
  (Button : access Gtk_Widget_Record'Class; Data : Gtk_Widget) is
   Destroy (Data);
end My_Destroy2;

     (Button1, "clicked",
      User_Callback.To_Marshaller (My_Destroy2'Access),
      Gtk_Widget (Button2));

4.3.2. Second case: using Object_Connect instead

One of the solutions to fix the above problem is to use Object_Connect instead of Connect. In that case, GtkAda automatically takes care of disconnecting the callback when either of the two widgets is destroyed:

procedure My_Destroy (Button : access Gtk_Widget_Record'Class) is
   Destroy (Button);
end My_Destroy;

     (Button1, "clicked",
      Widget_Callback.To_Marshaller (My_Destroy'Access),

4.3.3. Third case: manually disconnecting the callback

Using Object_Connect is not always possible. In that case, one of the possibilities is to store the Id of the callback, and properly disconnect it when appropriate. This is the most complex method, and very often is not applicable, since you cannot know for sure when the callback is no longer needed:

type My_Data3 is record
   Button, Object : Gtk_Widget;
   Id             : Handler_Id;
end record;
type My_Data3_Access is access My_Data3;

package User_Callback3 is new Gtk.Handlers.User_Callback
  (Gtk_Widget_Record, My_Data3_Access);

procedure My_Destroy3
  (Button : access Gtk_Widget_Record'Class;
   Data   : My_Data3_Access) is
   Destroy (Data.Button);
   Disconnect (Data.Object, Data.Id);
end My_Destroy3;

   Id : Handler_Id;
   Data3 := new My_Data3' (Object => Gtk_Widget (Button1),
                           Button => Gtk_Widget (Button2),
                           Id     => (Null_Signal_Id, null));
   Id := User_Callback3.Connect
     (Button1, "clicked",
      User_Callback3.To_Marshaller (My_Destroy3'Access),
   Data3.Id := Id;

4.3.4. Fourth case: setting a watch on a specific widget

GtkAda provides a function Add_Watch, that will automatically disconnect a callback when a given widget is destroyed. This is the function used internally by Object_Connect. In the example below, the callback is automatically disconnected whenever Button2 is destroyed:

procedure My_Destroy2
  (Button : access Gtk_Widget_Record'Class; Data : Gtk_Widget) is
   Destroy (Data);
end My_Destroy2;

   Id : Handler_Id;
   Id := User_Callback.Connect
     (Button1, "clicked",
      User_Callback.To_Marshaller (My_Destroy2'Access),
      Gtk_Widget (Button2));
   Add_Watch (Id, Button2);