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The theory behind how GObject interfaces work is given in the section called “Non-instantiable classed types: interfaces”; this section covers how to define and implement an interface.
The first step is to get the header right. This interface defines two methods:
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/* * Copyright/Licensing information. */ #ifndef __VIEWER_EDITABLE_H__ #define __VIEWER_EDITABLE_H__ #include <glib-object.h> G_BEGIN_DECLS #define VIEWER_TYPE_EDITABLE viewer_editable_get_type () G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject) struct _ViewerEditableInterface { GTypeInterface parent_iface; void (*save) (ViewerEditable *self, GError **error); void (*undo) (ViewerEditable *self, guint n_steps); void (*redo) (ViewerEditable *self, guint n_steps); }; void viewer_editable_save (ViewerEditable *self, GError **error); void viewer_editable_undo (ViewerEditable *self, guint n_steps); void viewer_editable_redo (ViewerEditable *self, guint n_steps); G_END_DECLS #endif /* __VIEWER_EDITABLE_H__ */ |
This code is the same as the code for a normal GType which derives from a GObject except for a few details:
The _GET_CLASS function is called
_GET_IFACE (and is defined by
G_DECLARE_INTERFACE).
The instance type, ViewerEditable, is not fully defined: it is used merely as an abstract type which represents an instance of whatever object which implements the interface.
The parent of the ViewerEditableInterface is GTypeInterface, not GObjectClass.
The implementation of the ViewerEditable type itself is trivial:
G_DEFINE_INTERFACE
creates a viewer_editable_get_type function which registers the
type in the type system. The third argument is used to define a
prerequisite interface
(which we'll talk about more later). Just pass 0 for this
argument when an interface has no prerequisite.
viewer_editable_default_init is expected
to register the interface's signals if there are any (we will see a bit
later how to use them).
The interface methods viewer_editable_save,
viewer_editable_undo and viewer_editable_redo dereference the interface
structure to access its associated interface function and call it.
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G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT) static void viewer_editable_default_init (ViewerEditableInterface *iface) { /* add properties and signals to the interface here */ } void viewer_editable_save (ViewerEditable *self, GError **error) { ViewerEditableInterface *iface; g_return_if_fail (VIEWER_IS_EDITABLE (self)); g_return_if_fail (error == NULL || *error == NULL); iface = VIEWER_EDITABLE_GET_IFACE (self); g_return_if_fail (iface->save != NULL); iface->save (self, error); } void viewer_editable_undo (ViewerEditable *self, guint n_steps) { ViewerEditableInterface *iface; g_return_if_fail (VIEWER_IS_EDITABLE (self)); iface = VIEWER_EDITABLE_GET_IFACE (self); g_return_if_fail (iface->undo != NULL); iface->undo (self, n_steps); } void viewer_editable_redo (ViewerEditable *self, guint n_steps) { ViewerEditableInterface *iface; g_return_if_fail (VIEWER_IS_EDITABLE (self)); iface = VIEWER_EDITABLE_GET_IFACE (self); g_return_if_fail (iface->redo != NULL); iface->redo (self, n_steps); } |