10.3 GObject metaclass

See MOP for information what metaclass is and why is it useful.

GObject metaclass gobject-class bridges two object systems: GObject and CLOS.

Classes that correspond to GObject classes are instances of this class. Each CLOS class of gobject-class metaclass is mapped to one GObject class. Two or more CLOS classes may map into one GObject class. GObject and CLOS inheritance must be consistent: if class X is a subclass or the same class as Y in CLOS, then this relation must hold for X' and Y', where X' is a GObject class to which X class maps to.

For each instance of GObject-related CLOS class there is a corresponding instance of GObject class (of a GObject class to which the CLOS class maps). Whenever the GObject class instance reference enters the Lisp memory (by creating instance with make-instance, as the return value of foreign function or as a slot value of GObject class instance), an instance of CLOS class is created.

Defining the class with metaclass gobject-class registers the type :g-type-name for conversions using GValue and CFFI foreign type g-object.

This class has the following slots:

• g-type-name (accessor gobject-class-g-type-name, initarg :g-type-name)

  Specifies the name of corresponding GObject class. String or NIL is allowed. If the name is NIL, then the same GObject class as its parent. Only one class may have specified a given :g-type-name.

• g-type-initializer (accessor gobject-class-g-type-initializer, initarg :g-type-initializer)

  Name of foreign type initializer function. This function initializes the class and returns its GType. Typically it is named class_get_type. String or NIL is allowed.

• interface-p (accessor gobject-class-interface-p, initarg :interface-p)

  A boolean specifying whether this CLOS class corresponds to GInterface. It is NIL by default.

This metaclass defines the GObject classes.

Slots which have :allocation of :gobject-property are mapped to GObject properties. Such slots have following attributes:

• :g-property-type

  A string naming GType of property

• :g-property-name

  A name of a property

Slots which have :allocation of :gobject-fn are mapped to a pair of accessor functions (usually named class_get_property and class_set_property). This is included because some properties are not exposed as GObject properties. Such slots have following attributes:

• :g-property-type A CFFI foreign type of property
• :g-getter A string naming foreign getter function of a property or a symbol designating Lisp getter function. Foreign getter function should have signature type class_get_property(object *). Lisp function should be of type (function (class) type).
• :g-setter A string naming foreign setter function of a property or a symbol designating Lisp setter function. Foreign setter function should have signature void class_set_property(object *, type). Lisp function should be of type (function (class type)).

Initargs of a slot are used to construct the GObject class.

Example:

     (defclass container (widget atk-implementor-iface buildable)
         ((border-width :allocation :gobject-property
                        :g-property-type "guint"
                        :accessor container-border-width
                        :initarg :border-width
                        :g-property-name "border-width")
          (resize-mode :allocation :gobject-property
                       :g-property-type "GtkResizeMode"
                       :accessor container-resize-mode
                       :initarg :resize-mode
                       :g-property-name "resize-mode")
          (child :allocation :gobject-property
                 :g-property-type "GtkWidget"
                 :accessor container-child
                 :initarg :child
                 :g-property-name "child")
          (focus-child :allocation :gobject-fn
                       :g-property-type g-object
                       :accessor container-focus-child
                       :initarg :focus-child
                       :g-getter "gtk_container_get_focus_child"
                       :g-setter "gtk_container_set_focus_child")
          (focus-vadjustment :allocation :gobject-fn
                             :g-property-type (g-object adjustment)
                             :accessor container-focus-vadjustment
                             :initarg :focus-vadjustment
                             :g-getter "gtk_container_get_focus_vadjustment"
                             :g-setter "gtk_container_set_focus_vadjustment")
          (focus-hadjustment :allocation :gobject-fn
                             :g-property-type (g-object adjustment)
                             :accessor container-focus-hadjustment
                             :initarg :focus-hadjustment
                             :g-getter "gtk_container_get_focus_hadjustment"
                             :g-setter "gtk_container_set_focus_hadjustment"))
         (:metaclass gobject-class)
         (:g-type-name . "GtkContainer")
         (:g-type-initializer . "gtk_container_get_type"))

(note the dot in (:g-type-name . "GtkContainer") and in (:g-type-initializer . "gtk_container_get_type"). It should be present)