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Setting and Retrieving Object Properties
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In the class initialization function, the default set_property() and get_property() functions were overridden in GObjectClass. These two functions must be overridden if your new GObject has one or more properties. Listing 11-8 is the implementation of the function that will be called for every property sent to g_object_set().
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CHAPTER 11 CREATING CUSTOM WIDGETS
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Listing 11-8. Setting Object Properties static void my_ip_address_set_property (GObject *object, guint prop_id, const GValue *value, GParamSpec *pspec) { MyIPAddress *ipaddress = MY_IP_ADDRESS (object); gint address[4] = { -1, -1, -1, -1 }; switch (prop_id) { case PROP_IP1: address[0] = g_value_get_int (value); my_ip_address_set_address (ipaddress, address); break; case PROP_IP2: address[1] = g_value_get_int (value); my_ip_address_set_address (ipaddress, address); break; case PROP_IP3: address[2] = g_value_get_int (value); my_ip_address_set_address (ipaddress, address); break; case PROP_IP4: address[3] = g_value_get_int (value); my_ip_address_set_address (ipaddress, address); break; default: G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec); break; } } When the property arrives, it is stored as a GValue object, which is a generic container used to store any type of object. The g_value_get_int() function is used to retrieve the integer value stored by GValue. There are functions available for converting between all fundamental data types and GValue objects available, defined in the previous section. The next step is to store the new value of the property if it is a valid property. The property identifier is stored in prop_id, which can be compared to the installed property to find the one that is being altered. The function my_ip_address_set_address() was used to apply the changes. You can view the implementation of this function later in this section.
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CHAPTER 11 CREATING CUSTOM WIDGETS
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It would also be possible to implement the same functionality provided by this function with the method shown in the following code snippet. However, it was given to you in the expanded form, because it is a rare case when every property of a widget is the same type and is stored in an array. address[prop_id-1] = g_value_get_int (value); my_ip_address_set_address (ipaddress, address); The MyIPAddress example in this chapter is one of the simplest examples possible and could be greatly expanded. If you were implementing this widget for use in an application, you would want to provide further properties and signals, as well as provide further functionality. Keep this in mind as you continue to examine this example. The default get_property() function of the object class was overridden. Therefore, when g_object_set() is called on a property of MyIPAddress, my_ip_address_get_property() will be called as shown in Listing 11-9. Listing 11-9. Retrieving Object Properties static void my_ip_address_get_property (GObject *object, guint prop_id, GValue *value, GParamSpec *pspec) { MyIPAddress *ipaddress = MY_IP_ADDRESS (object); MyIPAddressPrivate *priv = MY_IP_ADDRESS_GET_PRIVATE (ipaddress); switch (prop_id) { case PROP_IP1: g_value_set_int (value, priv->address[0]); break; case PROP_IP2: g_value_set_int (value, priv->address[1]); break; case PROP_IP3: g_value_set_int (value, priv->address[2]); break; case PROP_IP4: g_value_set_int (value, priv->address[3]); break; default: G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec); break; } }
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CHAPTER 11 CREATING CUSTOM WIDGETS
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The my_ip_address_get_property() function takes the appropriate property from the MyIPAddressPrivate structure and converts it to a GValue. The new value is then applied to the user s variable and cast to the correct variable type. The private structure is retrieved by using the MY_IP_ADDRESS_GET_PRIVATE() function that was defined at the top of the source file.
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Instantiating the Widget
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The other initialization function that needs to be implemented is my_ip_address_init(), which is called every time a new MyIPAddress widget is created. This differs from the class initialization function, which is only called in order to set up the object class, not every time the object is instantiated. This instance initialization function, displayed in Listing 11-10, sets the initial IP address values to zero, performs initial rendering, and connects the necessary signals. Listing 11-10. Instantiating a MyIPAddress Object static void my_ip_address_init (MyIPAddress *ipaddress) { MyIPAddressPrivate *priv = MY_IP_ADDRESS_GET_PRIVATE (ipaddress); PangoFontDescription *fd; guint i; for (i = 0; i < 4; i++) priv->address[i] = 0; fd = pango_font_description_from_string ("Monospace"); gtk_widget_modify_font (GTK_WIDGET (ipaddress), fd); my_ip_address_render (ipaddress); pango_font_description_free (fd); /* The key-press-event signal will be used to filter out certain keys. We will * also monitor the cursor-position property so it can be moved correctly. */ g_signal_connect (G_OBJECT (ipaddress), "key-press-event", G_CALLBACK (my_ip_address_key_pressed), NULL); g_signal_connect (G_OBJECT (ipaddress), "notify::cursor-position", G_CALLBACK (my_ip_address_move_cursor), NULL); } The my_ip_address_init()function accepts a MyIPAddress object that has already been created and cast. Your task is to do any further processing that needs to be performed on the widget before it is returned to the programmer and displayed to the user. In this example, the function first initializes the four IP address values to zero. Then, the font of the widget is set to Monospace. Notice that the size is not specified, which allows the user s theme to dictate the size. This is done so that users with large fonts will still be able to read the content of the widget.
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