GObject的物件屬性

在glib物件系統中，一般的類都是從Gobject類繼承過來的。儘管glib是用C語言開發的，它卻幾乎支援所有的物件導向的特性。這裡不談glib的物件導向機制，只是說說關於GObject物件屬性的一些問題。

在OOP中，物件的屬性一般用成員變數來表示。程式設計高手常常告誡我們，把物件的屬性隱藏起來，用成員函式去訪問，可以減少與使用者之間的耦合，同時可以提高程式的健壯性。

實際上，僅僅是把物件的屬性封裝起來，有時是不夠的。比如，你無法通過基類的指標去訪問子類的屬性。要做到這一點，需要把這些setter/getter函式實現成虛擬函式，在子類中去過載它們。不過，這樣做會讓基類的介面顯示很龐雜，而且實際上你也無法預知子類中的會有哪些屬性。

另外一個更重要問題是，有時候外部物件可能會關心某個物件的屬性改變。比如，像對話方塊可能關心其上控制元件的屬性的變化，控制元件的屬性變化時，對話方塊需要做一些調整。如果能實現一種機制支援這種功能將有很大用處。

Gobject定義了兩個函式(get_property/set_propery)，用於存取物件的屬性，在子類中過載這兩個函式就行了，不用實現一大堆setter/getter函式，同時它提供了一種物件屬性監視機制，在物件屬性改變時通知相關關注者。

下面這個例子演示了以上所有功能：

/*

* fooobject.h

*

* This library is free software; you can redistribute it and/or

* modify it under the terms of the GNU Lesser General Public

* License as published by the Free Software Foundation; either

* version 2 of the License, or (at your option) any later version.

*

* This library is distributed in the hope that it will be useful,

* but WITHOUT ANY WARRANTY; without even the implied warranty of

* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU

* Lesser General Public License for more details.

*

* You should have received a copy of the GNU Lesser General Public

* License along with this library; if not, write to the

* Free Software Foundation, Inc., 59 Temple Place - Suite 330,

* Boston, MA 02111-1307, USA.

*/

#ifndef FOO_OBJECT_H

#define FOO_OBJECT_H

#include <glib.h>

#include <glib-object.h>

G_BEGIN_DECLS

#define FOO_TYPE_OBJECT (foo_object_get_type ())

#define FOO_OBJECT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), FOO_TYPE_OBJECT, FooObject))

#define FOO_OBJECT_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), FOO_TYPE_OBJECT, FooObjectClass))

#define FOO_IS_OBJECT(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), FOO_TYPE_OBJECT))

#define FOO_IS_OBJECT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), FOO_TYPE_OBJECT))

#define FOO_OBJECT_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), FOO_TYPE_OBJECT, FooObjectClass))

typedef struct _FooObject FooObject;

typedef struct _FooObjectClass FooObjectClass;

struct _FooObject

{

GObject gobject;

gint width;

gint height;

gboolean visible;

};

struct _FooObjectClass

{

GObjectClass parent_class;

};

GType foo_object_get_type(void) G_GNUC_CONST;

FooObject* foo_object_new(void);

G_END_DECLS

#endif /*FOO_OBJECT_H*/

/*

* fooobject.c

*

* This library is free software; you can redistribute it and/or

* modify it under the terms of the GNU Lesser General Public

* License as published by the Free Software Foundation; either

* version 2 of the License, or (at your option) any later version.

*

* This library is distributed in the hope that it will be useful,

* but WITHOUT ANY WARRANTY; without even the implied warranty of

* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU

* Lesser General Public License for more details.

*

* You should have received a copy of the GNU Lesser General Public

* License along with this library; if not, write to the

* Free Software Foundation, Inc., 59 Temple Place - Suite 330,

* Boston, MA 02111-1307, USA.

*/

#include "fooobject.h"

enum {

PROP_0,

PROP_VISIBLE,

PROP_WIDTH,

PROP_HEIGHT,

PROP_LAST

};

static GObjectClass* parent_class = NULL;

static void foo_object_class_init(FooObjectClass *class);

static void foo_object_init (FooObject *object);

static void foo_object_set_property (GObject *object,

guint property_id,

const GValue *value,

GParamSpec *pspec);

static void foo_object_get_property (GObject *object,

guint property_id,

GValue *value,

GParamSpec *pspec);

GType

foo_object_get_type (void)

{

static GType object_type = 0;

if (!object_type)

{

static const GTypeInfo object_info =

{

sizeof (FooObjectClass),

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc) foo_object_class_init,

NULL, /* class_finalize */

NULL, /* class_data */

sizeof (FooObject),

0, /* n_preallocs */

(GInstanceInitFunc) foo_object_init,

};

object_type = g_type_register_static (G_TYPE_OBJECT, "FooObject", &object_info, 0);

};

return object_type;

}

FooObject* foo_object_new(void)

{

return (FooObject*)g_object_new(FOO_TYPE_OBJECT, 0);

}

static void foo_object_class_init(FooObjectClass *class)

{

GObjectClass* gobject_class = G_OBJECT_CLASS(class);

gobject_class->set_property = foo_object_set_property;

gobject_class->get_property = foo_object_get_property;

g_object_class_install_property (gobject_class,

PROP_VISIBLE,

g_param_spec_boolean ("visible",

("Visible"),

("Whether the widget is visible"),

FALSE,

G_PARAM_READWRITE));

g_object_class_install_property (gobject_class,

PROP_HEIGHT,

g_param_spec_int ("height",

("Height"),

("Override for height request of the widget, or -1 if natural request should be used"),

-1,

G_MAXINT,

-1,

G_PARAM_READWRITE));

g_object_class_install_property (gobject_class,

PROP_WIDTH,

g_param_spec_int ("width",

("Width request"),

("Override for width request of the widget, or -1 if natural request should be used"),

-1,

G_MAXINT,

-1,

G_PARAM_READWRITE));

return;

}

static void foo_object_init (FooObject *object)

{

return;

}

static void foo_object_set_property (GObject *object,

guint property_id,

const GValue *value,

GParamSpec *pspec)

{

FooObject* foo_object = FOO_OBJECT(object);

switch(property_id)

{

case PROP_VISIBLE:

{

foo_object->visible = g_value_get_boolean(value);

break;

}

case PROP_WIDTH:

{

foo_object->width = g_value_get_int(value);

break;

}

case PROP_HEIGHT:

{

foo_object->height = g_value_get_int(value);

break;

}

default:

{

G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);

break;

}

return;

}

static void foo_object_get_property (GObject *object,

guint property_id,

GValue *value,

GParamSpec *pspec)

{

FooObject* foo_object = FOO_OBJECT(object);

switch(property_id)

{

case PROP_VISIBLE:

{

g_value_set_boolean(value, foo_object->visible);

break;

}

case PROP_WIDTH:

{

g_value_set_int(value, foo_object->width);

break;

}

case PROP_HEIGHT:

{

g_value_set_int(value, foo_object->height);

break;

}

default:

{

G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);

break;

}

return;

}

static void property_change (gpointer data1,

gpointer arg_1,

gpointer data2)

{

GParamSpec *pspec = (GParamSpec*) arg_1;

g_printf("property %s change/n", g_param_spec_get_name(pspec));

return;

}

int main(int argc, char* argv[])

{

GValue value = {0};

FooObject* object = NULL;

g_type_init();

object = foo_object_new();

g_signal_connect(G_OBJECT(object), "notify", G_CALLBACK(property_change), object);

g_value_init(&value, G_TYPE_BOOLEAN);

g_value_set_boolean(&value, TRUE);

g_object_set_property(G_OBJECT(object), "visible", &value);

g_value_unset(&value);

g_value_init(&value, G_TYPE_INT);

g_value_set_int(&value, 400);

g_object_set_property(G_OBJECT(object), "width", &value);

g_value_set_int(&value, 300);

g_object_set_property(G_OBJECT(object), "height", &value);

g_object_unref(object);

return 0;

}

GObject的物件屬性

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