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GObject interfaces can also have
properties. Declaration of the interface properties is similar to
declaring the properties of ordinary GObject types as explained in
the section called “Object properties”, except that
g_object_interface_install_property
is used to declare the properties instead of
g_object_class_install_property.
To include a property named 'autosave-frequency' of type gdouble in the
ViewerEditable interface example code above, we only need to
add one call in viewer_editable_default_init as shown
below:
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static void viewer_editable_default_init (ViewerEditableInterface *iface) { g_object_interface_install_property (iface, g_param_spec_double ("autosave-frequency", "Autosave frequency", "Frequency (in per-seconds) to autosave backups of the editable content at. " "Or zero to disable autosaves.", 0.0, /* minimum */ G_MAXDOUBLE, /* maximum */ 0.0, /* default */ G_PARAM_READWRITE)); } |
One point worth noting is that the declared property wasn't assigned an
integer ID. The reason being that integer IDs of properties are used
only inside the get_property and
set_property virtual methods. Since interfaces
declare but do not implement properties, there is no
need to assign integer IDs to them.
An implementation declares and defines its properties in the usual
way as explained in the section called “Object properties”, except for one
small change: it can declare the properties of the interface it
implements using g_object_class_override_property
instead of g_object_class_install_property.
The following code snippet shows the modifications needed in the
ViewerFile declaration and implementation above:
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struct _ViewerFile { GObject parent_instance; gdouble autosave_frequency; }; enum { PROP_AUTOSAVE_FREQUENCY = 1, N_PROPERTIES }; static void viewer_file_set_property (GObject *object, guint prop_id, const GValue *value, GParamSpec *pspec) { ViewerFile *file = VIEWER_FILE (object); switch (prop_id) { case PROP_AUTOSAVE_FREQUENCY: file->autosave_frequency = g_value_get_double (value); break; default: G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec); break; } } static void viewer_file_get_property (GObject *object, guint prop_id, GValue *value, GParamSpec *pspec) { ViewerFile *file = VIEWER_FILE (object); switch (prop_id) { case PROP_AUTOSAVE_FREQUENCY: g_value_set_double (value, file->autosave_frequency); break; default: G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec); break; } } static void viewer_file_class_init (ViewerFileClass *klass) { GObjectClass *object_class = G_OBJECT_CLASS (klass); object_class->set_property = viewer_file_set_property; object_class->get_property = viewer_file_get_property; g_object_class_override_property (object_class, PROP_AUTOSAVE_FREQUENCY, "autosave-frequency"); } |