2.2. Theory of Signals and Callbacks
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## 2.2. Theory of Signals and Callbacks

### Note

In GTK+ version 2.0, the signal system has been moved from GTK to GLib. We won't go into details about the extensions which the GLib 2.0 signal system has relative to the GTK+ 1.2 signal system. The differences should not be apparent to PyGTK users.

Before we look in detail at helloworld.py, we'll discuss signals and callbacks. GTK+ is an event driven toolkit, which means it will sleep in `gtk.main`() until an event occurs and control is passed to the appropriate function.

This passing of control is done using the idea of "signals". (Note that these signals are not the same as the Unix system signals, and are not implemented using them, although the terminology is almost identical.) When an event occurs, such as the press of a mouse button, the appropriate signal will be "emitted" by the widget that was pressed. This is how GTK+ does most of its useful work. There are signals that all widgets inherit, such as "destroy", and there are signals that are widget specific, such as "toggled" on a toggle button.

To make a button perform an action, we set up a signal handler to catch these signals and call the appropriate function. This is done by using a `GtkWidget` (from the `GObject` class) method such as:

      handler_id = object.connect(name, func, func_data)

where object is the `GtkWidget` instance which will be emitting the signal, and the first argument `name` is a string containing the name of the signal you wish to catch. The second argument, `func`, is the function you wish to be called when it is caught, and the third, `func_data`, the data you wish to pass to this function. The method returns a handler_id that can be used to disconnect or block the handler.

The function specified in the second argument is called a "callback function", and should generally be of the form:

      def callback_func(widget, callback_data):

where the first argument will be a pointer to the `widget` that emitted the signal, and the second (`callback_data`) a pointer to the data given as the last argument to the `connect`() method as shown above.

If the callback function is an object method then it will have the general form:

      def callback_meth(self, widget, callback_data):

where `self` is the object instance invoking the method. This is the form used in the helloworld.py example program.

### Note

The above form for a signal callback function declaration is only a general guide, as some widget specific signals generate different calling parameters.

Another call used in the helloworld.py example is:

      handler_id = object.connect_object(name, func, slot_object)

`connect_object`() is the same as `connect`() except a callback function only uses one argument and a callback method, two arguments:

      def callback_func(object)
      def callback_meth(self, object)

where `object` is usually a widget. `connect_object`() allows the PyGTK widget methods that only take a single argument (`self`) to be used as signal handlers.

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