Glib学习笔记（四）

你将学到什么

使用GObject模拟实现接口

使用接口

• 首先按照学习笔记（一）定义一个普通的GObject类
• 使用G_DEFINE_TYPE_WITH_CODE和G_IMPLEMENT_INTERFACE替代G_DEFINE_TYPE来实现类定义

static void viewer_file_editable_interface_init (ViewerEditableInterface *iface);

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init))

• 实现接口初始化函数viewer_file_editable_interface_init，接口内声明的每个虚函数指针都要被赋予实现

static void
viewer_file_editable_save (ViewerFile  *self,
                           GError     **error)
{
  g_print ("File implementation of editable interface save method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_undo (ViewerFile *self,
                           guint       n_steps)
{
  g_print ("File implementation of editable interface undo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_redo (ViewerFile *self,
                           guint       n_steps)
{
  g_print ("File implementation of editable interface redo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_editable_save;
  iface->undo = viewer_file_editable_undo;
  iface->redo = viewer_file_editable_redo;
}

static void
viewer_file_init (ViewerFile *self)
{
  /* Instance variable initialisation code. */
}

扩展接口

如果一个接口的实现依赖另一个接口的实现，有点类似继承，那么首先按照上面的方式实现两个接口的定义，然后在定义GType的时候依次使用G_IMPLEMENT_INTERFACE添加两个接口的实现。

/* Make the ViewerEditableLossy interface require ViewerEditable interface. */
G_DEFINE_INTERFACE (ViewerEditableLossy, viewer_editable_lossy, VIEWER_TYPE_EDITABLE);

static void
viewer_file_editable_lossy_compress (ViewerEditableLossy *editable)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of lossy editable interface compress method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_lossy_interface_init (ViewerEditableLossyInterface *iface)
{
  iface->compress = viewer_file_editable_lossy_compress;
}

static void
viewer_file_editable_save (ViewerEditable  *editable,
                           GError         **error)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface save method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_undo (ViewerEditable *editable,
                           guint           n_steps)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface undo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_redo (ViewerEditable *editable,
                           guint           n_steps)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface redo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_editable_save;
  iface->undo = viewer_file_editable_undo;
  iface->redo = viewer_file_editable_redo;
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_file_init (ViewerFile *self)
{
  /* Instance variable initialisation code. */
}

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                         viewer_file_editable_interface_init)
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE_LOSSY,
                         viewer_file_editable_lossy_interface_init))

接口属性

接口同样也可以拥有属性，不过是使用g_object_interface_install_property函数而不是像定义GType一样使用g_object_class_install_property函数来安装属性，如下所示：

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));
}

需要值得注意的一点是

重写接口方法

ViewerAudioFile继承自ViewerFile，两者都实现了ViewerEditable接口。ViewerAudioFile仅实现了ViewerEditable接口的其中一个方法，其他接口方法使用基类ViewerFile的实现，如下所示：

static void
viewer_audio_file_editable_save (ViewerEditable  *editable,
                                 GError         **error)
{
  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);

  g_print ("Audio file implementation of editable interface save method.\n");
}

static void
viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
{
  /* Override the implementation of save(). */
  iface->save = viewer_audio_file_editable_save;

  /*
   * Leave iface->undo and ->redo alone, they are already set to the
   * base class implementation.
   */
}

G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_audio_file_editable_interface_init))

static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_audio_file_init (ViewerAudioFile *self)
{
  /* Nothing here. */
}

在接口的default_init函数里面使用g_type_interface_peek_parent函数可以获取基类的接口实现，可将其保存到全局变量中供其他函数使用。下面的例子中，ViewerAudioFile重写了接口的save函数，并在重写函数中直接调用基类ViewerFile接口的save函数实现。

static ViewerEditableInterface *viewer_editable_parent_interface = NULL;

static void
viewer_audio_file_editable_save (ViewerEditable  *editable,
                                 GError         **error)
{
  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);

  g_print ("Audio file implementation of editable interface save method.\n");

  /* Now call the base implementation */
  viewer_editable_parent_interface->save (editable, error);
}

static void
viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
{
  viewer_editable_parent_interface = g_type_interface_peek_parent (iface);

  iface->save = viewer_audio_file_editable_save;
}

G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_audio_file_editable_interface_init))

static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_audio_file_init (ViewerAudioFile *self)
{
  /* Nothing here. */
}