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CHAPTER 2 YOUR FIRST GTK+ APPLICATIONS
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Listing 2-3. The GtkButton Widget (buttons.c) #include <gtk/gtk.h> static void destroy (GtkWidget*, gpointer); int main (int argc, char *argv[]) { GtkWidget *window, *button; gtk_init (&argc, &argv); window = gtk_window_new (GTK_WINDOW_TOPLEVEL); gtk_window_set_title (GTK_WINDOW (window), "Buttons"); gtk_container_set_border_width (GTK_CONTAINER (window), 25); gtk_widget_set_size_request (window, 200, 100); g_signal_connect (G_OBJECT (window), "destroy", G_CALLBACK (destroy), NULL); /* Create a new button that has a mnemonic key of Alt+C. */ button = gtk_button_new_with_mnemonic ("_Close"); gtk_button_set_relief (GTK_BUTTON (button), GTK_RELIEF_NONE); /* Connect the button to the clicked signal. The callback function recieves the * window followed by the button because the arguments are swapped. */ g_signal_connect_swapped (G_OBJECT (button), "clicked", G_CALLBACK (gtk_widget_destroy), (gpointer) window); gtk_container_add (GTK_CONTAINER (window), button); gtk_widget_show_all (window); gtk_main (); return 0; } /* Stop the GTK+ main loop function. */ static void destroy (GtkWidget *window, gpointer data) { gtk_main_quit (); } In Listing 2-3, gtk_widget_destroy() is called on the main window when the button is clicked. This is a very simple example, but it has a practical use in most applications.
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CHAPTER 2 YOUR FIRST GTK+ APPLICATIONS
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The GNOME Human Interface Guidelines, which can be viewed or downloaded at http://developer.gnome.org/projects/gup/hig, state that preferences dialogs should apply settings immediately after a setting is changed. Therefore, if you create a preferences dialog, there is a good chance that you will only need one button. The purpose of the button would be to destroy the window that contains the button and save the changes. After creating the button, gtk_button_set_relief() can be used to add a certain magnitude of relief around the GtkButton. Relief is a type of 3-D border that distinguishes the button from surrounding widgets. Values of the GtkReliefStyle enumeration follow: GTK_RELIEF_NORMAL: Add relief around all edges of the button. GTK_RELIEF_HALF: Add relief around only half of the button. GTK_RELIEF_NONE: Add no relief around the button. Listing 2-3 introduces g_signal_connect_swapped(), a new signal connection function. This function swaps the position of the object on which the signal is being emitted and the data parameter when running the callback function. g_signal_connect_swapped (G_OBJECT (button), "clicked", G_CALLBACK (gtk_widget_destroy), (gpointer) window); This allows you to use gtk_widget_destroy() on the callback function, which will call gtk_widget_destroy (window). If the callback function only receives one parameter, the object will be ignored.
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Widget Properties
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GObject provides a property system, which allows you to customize how widgets interact with the user and how they are drawn on the screen. In this section, you will learn how to use styles, resource files and GObject s property system. Every class derived from the GObject class can install any number of properties. In GTK+, these properties store information about how the widget will act. For example, GtkButton has a property called relief that defines the relief style used by the button. In the following code, g_object_get() is used to retrieve the current value stored by the button s relief property. This function accepts a NULL-terminated list of properties and variables to store the returned value. g_object_get (button, "relief", &value, NULL); Each object can have many properties, so a full list will not be found in this book. For more information on properties available for a specific widget, you should reference the API documentation.
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CHAPTER 2 YOUR FIRST GTK+ APPLICATIONS
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Setting a new value for a property is easily done with g_object_set(). In this example, the relief property of the button was set to GTK_RELIEF_NORMAL: g_object_set (button, "relief", GTK_RELIEF_NORMAL, NULL); Functions are provided to set and retrieve many of the properties of each widget. However, not every property has that option. These functions will become extremely important when you learn about the GtkTreeView widget in 8, because many objects used in that chapter do not provide get or set functions for any properties. It is also possible to monitor a specific property with GObject s notify signal. You can monitor a property by connecting to the notify::property-name signal. The example in Listing 2-4 calls property_changed() when the relief property is changed. Listing 2-4. Using the Notify Property g_signal_connect (G_OBJECT (button), "notify::relief", G_CALLBACK (property_changed), NULL); ... static void property_changed (GObject *button, GParamSpec *property, gpointer data) { /* Handle the property change ... */ }
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Caution While it is acceptable to use either a dash or an underscore when typing signal names, you must always use dashes when using the notify signal. For example, if you need to monitor GtkWidget s can-focus property, notify::can_focus is not acceptable! Remember that notify is the signal name, and can-focus is the name of the widget property.
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The callback function receives a new type of object called GParamSpec, which holds information about the property that was changed. For now, all you need to know is that you can retrieve the name of the property that was changed with property->name. You will learn more about the GParamSpec structure in 11 when you learn how to add properties to your own custom widgets.
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