Commit 535723ce authored by Alexander Larsson's avatar Alexander Larsson Committed by Matthias Clasen

Add GtkGLArea widget

parent 7eae4985
......@@ -242,6 +242,7 @@
<xi:include href="xml/gtkadjustment.xml" />
<xi:include href="xml/gtkcalendar.xml" />
<xi:include href="xml/gtkdrawingarea.xml" />
<xi:include href="xml/gtkglarea.xml" />
<xi:include href="xml/gtkeventbox.xml" />
<xi:include href="xml/gtkhandlebox.xml" />
<xi:include href="xml/gtkimcontextsimple.xml" />
......
......@@ -8166,7 +8166,6 @@ GTK_GESTURE_ZOOM_GET_CLASS
gtk_gesture_zoom_get_type
</SECTION>
<SECTION>
<FILE>gtksidebar</FILE>
GtkSidebar
......@@ -8185,3 +8184,25 @@ GTK_SIDEBAR_GET_CLASS
GtkSidebarPrivate
gtk_sidebar_get_type
</SECTION>
<SECTION>
<FILE>gtkglarea</FILE>
GtkGLArea
GtkGLAreaClass
gtk_gl_area_new
gtk_gl_area_get_context
<SUBSECTION>
gtk_gl_area_set_has_alpha
gtk_gl_area_get_has_alpha
gtk_gl_area_set_has_depth_buffer
gtk_gl_area_get_has_depth_buffer
gtk_gl_area_make_current
<SUBSECTION Standard>
GTK_TYPE_GL_AREA
GTK_GL_AREA
GTK_GL_AREA_CLASS
GTK_IS_GL_AREA
GTK_IS_GL_AREA_CLASS
<SUBSECTION Private>
gtk_gl_area_get_type
</SECTION>
......@@ -363,6 +363,7 @@ gtk_public_h_sources = \
gtkgesturesingle.h \
gtkgestureswipe.h \
gtkgesturezoom.h \
gtkglarea.h \
gtkgrid.h \
gtkheaderbar.h \
gtkicontheme.h \
......@@ -937,6 +938,7 @@ gtk_base_c_sources = \
gtkgesturesingle.c \
gtkgestureswipe.c \
gtkgesturezoom.c \
gtkglarea.c \
gtkgrid.c \
gtkheaderbar.c \
gtkhsla.c \
......
......@@ -114,6 +114,7 @@
#include <gtk/gtkgesturesingle.h>
#include <gtk/gtkgestureswipe.h>
#include <gtk/gtkgesturezoom.h>
#include <gtk/gtkglarea.h>
#include <gtk/gtkgrid.h>
#include <gtk/gtkheaderbar.h>
#include <gtk/gtkicontheme.h>
......
/* GTK - The GIMP Toolkit
*
* gtkglarea.c: A GL drawing area
*
* Copyright © 2014 Emmanuele Bassi
*
* 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, see <http://www.gnu.org/licenses/>.
*/
#include "config.h"
#include "config.h"
#include "gtkglarea.h"
#include "gtkintl.h"
#include "gtkstylecontext.h"
#include "gtkmarshalers.h"
#include "gtkprivate.h"
#include <epoxy/gl.h>
/**
* SECTION:gtkglarea
* @Title: GtkGLArea
* @Short_description: A widget for custom drawing with OpenGL
*
* #GtkGLArea is a widget that allows drawing with OpenGL.
*
* #GtkGLArea sets up its own #GdkGLContext for the window it creates, and
* creates a custom GL framebuffer that the widget will do GL rendering onto.
* It also ensures that this framebuffer is the default GL rendering target
* when rendering.
*
* In order to draw, you have to connect to the #GtkGLArea::render signal,
* or subclass #GtkGLArea and override the @GtkGLAreaClass.render() virtual
* function.
*
* The #GtkGLArea widget ensures that the #GdkGLContext is associated with
* the widget's drawing area, and it is kept updated when the size and
* position of the drawing area changes.
*
* ## Drawing with GtkGLArea ##
*
* The simplest way to draw using OpenGL commands in a #GtkGLArea is to
* create a widget instance and connect to the #GtkGLArea::render signal:
*
* |[<!-- language="C" -->
* // create a GtkGLArea instance
* GtkWidget *gl_area = gtk_gl_area_new ();
*
* // connect to the "render" signal
* g_signal_connect (gl_area, "render", G_CALLBACK (render), NULL);
* ]|
*
* The `render()` function will be called when the #GtkGLArea is ready
* for you to draw its content:
*
* |[<!-- language="C" -->
* static gboolean
* render (GtkGLArea *area, GdkGLContext *context)
* {
* // inside this function it's safe to use GL; the given
* // #GdkGLContext has been made current to the drawable
* // surface used by the #GtkGLArea and the viewport has
* // already been set to be the size of the allocation
*
* // we can start by clearing the buffer
* glClearColor (0, 0, 0, 0);
* glClear (GL_COLOR_BUFFER_BIT);
*
* // draw your object
* draw_an_object ();
*
* // we completed our drawing; the draw commands will be
* // flushed at the end of the signal emission chain, and
* // the buffers will be drawn on the window
* return TRUE;
* }
* ]|
*
* The `draw_an_object()` function draws a 2D, gold-colored
* triangle:
*
* |[<!-- language="C" -->
* static void
* draw_an_object (void)
* {
* // set the color
* glColor3f (1.0f, 0.85f, 0.35f);
*
* // draw our triangle
* glBegin (GL_TRIANGLES);
* {
* glVertex3f ( 0.0f, 0.6f, 0.0f);
* glVertex3f (-0.2f, -0.3f, 0.0f);
* glVertex3f ( 0.2f, -0.3f, 0.0f);
* }
* glEnd ();
* }
* ]|
*
* This is an extremely simple example; in a real-world application you
* would probably replace the immediate mode drawing with persistent
* geometry primitives, like a Vertex Buffer Object, and only redraw what
* changed in your scene.
*
*/
typedef struct {
GdkGLContext *context;
GLuint framebuffer;
gboolean has_alpha;
gboolean has_depth_buffer;
} GtkGLAreaPrivate;
enum {
PROP_0,
PROP_CONTEXT,
PROP_HAS_ALPHA,
PROP_HAS_DEPTH_BUFFER,
LAST_PROP
};
static GParamSpec *obj_props[LAST_PROP] = { NULL, };
enum {
RENDER,
LAST_SIGNAL
};
static guint area_signals[LAST_SIGNAL] = { 0, };
G_DEFINE_TYPE_WITH_PRIVATE (GtkGLArea, gtk_gl_area, GTK_TYPE_WIDGET)
static void
gtk_gl_area_dispose (GObject *gobject)
{
GtkGLArea *self = GTK_GL_AREA (gobject);
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (self);
g_clear_object (&priv->context);
G_OBJECT_CLASS (gtk_gl_area_parent_class)->dispose (gobject);
}
static void
gtk_gl_area_set_property (GObject *gobject,
guint prop_id,
const GValue *value,
GParamSpec *pspec)
{
switch (prop_id)
{
case PROP_HAS_ALPHA:
gtk_gl_area_set_has_alpha (GTK_GL_AREA(gobject),
g_value_get_boolean (value));
break;
case PROP_HAS_DEPTH_BUFFER:
gtk_gl_area_set_has_depth_buffer (GTK_GL_AREA(gobject),
g_value_get_boolean (value));
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
}
}
static void
gtk_gl_area_get_property (GObject *gobject,
guint prop_id,
GValue *value,
GParamSpec *pspec)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (GTK_GL_AREA (gobject));
switch (prop_id)
{
case PROP_HAS_ALPHA:
g_value_set_boolean (value, priv->has_alpha);
break;
case PROP_HAS_DEPTH_BUFFER:
g_value_set_boolean (value, priv->has_depth_buffer);
break;
case PROP_CONTEXT:
g_value_set_object (value, priv->context);
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
}
}
static void
gtk_gl_area_realize (GtkWidget *widget)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private ((GtkGLArea *) widget);
GdkWindow *window;
GTK_WIDGET_CLASS (gtk_gl_area_parent_class)->realize (widget);
window = gtk_widget_get_window (widget);
priv->context = gdk_window_create_gl_context (window,
GDK_GL_PROFILE_DEFAULT,
NULL);
if (priv->context != NULL)
{
if (gdk_gl_context_make_current (priv->context))
{
glGenFramebuffersEXT (1, &priv->framebuffer);
glBindFramebufferEXT (GL_FRAMEBUFFER_EXT, priv->framebuffer);
}
else
{
g_warning ("Unable to make new context current");
}
}
}
static void
gtk_gl_area_unrealize (GtkWidget *widget)
{
GtkGLArea *self = GTK_GL_AREA (widget);
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (self);
if (priv->context != NULL)
{
if (priv->framebuffer != 0 && gtk_gl_area_make_current (self))
{
/* Bind 0, which means render to back buffer, as a result, fb is unbound */
glBindFramebufferEXT (GL_FRAMEBUFFER_EXT, 0);
glDeleteFramebuffersEXT (1, &priv->framebuffer);
priv->framebuffer = 0;
}
else
g_warning ("can't free framebuffer");
gdk_gl_context_clear_current ();
}
GTK_WIDGET_CLASS (gtk_gl_area_parent_class)->unrealize (widget);
}
static void
gtk_gl_area_size_allocate (GtkWidget *widget,
GtkAllocation *allocation)
{
GTK_WIDGET_CLASS (gtk_gl_area_parent_class)->size_allocate (widget, allocation);
}
static gboolean
gtk_gl_area_draw (GtkWidget *widget,
cairo_t *cr)
{
GtkGLArea *self = GTK_GL_AREA (widget);
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (self);
gboolean unused;
int w, h, scale;
GLuint color_rb = 0, depth_rb = 0, color_tex = 0;
GLenum status;
if (priv->context == NULL)
return FALSE;
if (!gtk_gl_area_make_current (self))
return FALSE;
scale = gtk_widget_get_scale_factor (widget);
w = gtk_widget_get_allocated_width (widget) * scale;
h = gtk_widget_get_allocated_height (widget) * scale;
if (priv->has_alpha)
{
/* For alpha we use textures as that is required for blending to work */
glGenTextures (1, &color_tex);
glBindTexture (GL_TEXTURE_2D, color_tex);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D (GL_TEXTURE_2D, 0, GL_RGBA8, w, h, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
glFramebufferTexture2DEXT (GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
GL_TEXTURE_2D, color_tex, 0);
}
else
{
/* For non-alpha we use render buffers so we can blit instead of texture the result */
glGenRenderbuffersEXT (1, &color_rb);
glBindRenderbufferEXT (GL_RENDERBUFFER_EXT, color_rb);
glRenderbufferStorageEXT (GL_RENDERBUFFER_EXT, GL_RGB8, w, h);
glFramebufferRenderbufferEXT (GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
GL_RENDERBUFFER_EXT, color_rb);
}
if (priv->has_depth_buffer)
{
glGenRenderbuffersEXT (1, &depth_rb);
glBindRenderbufferEXT (GL_RENDERBUFFER_EXT, depth_rb);
/* TODO: Pick actual requested depth */
glRenderbufferStorageEXT (GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24, w, h);
glFramebufferRenderbufferEXT (GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT,
GL_RENDERBUFFER_EXT, depth_rb);
glEnable (GL_DEPTH_TEST);
}
else
glDisable (GL_DEPTH_TEST);
status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if (status == GL_FRAMEBUFFER_COMPLETE_EXT)
{
glViewport(0, 0, w, h);
g_signal_emit (self, area_signals[RENDER], 0, priv->context, &unused);
gdk_cairo_draw_from_gl (cr,
gtk_widget_get_window (widget),
color_tex ? color_tex : color_rb,
color_tex ? GL_TEXTURE : GL_RENDERBUFFER,
scale, 0, 0, w, h);
if (!gtk_gl_area_make_current (self))
g_error ("can't make old context current again");
}
else
{
g_print ("fb setup not supported\n");
}
if (color_tex != 0)
glDeleteTextures(1, &color_tex);
if (color_rb != 0)
glDeleteRenderbuffersEXT(1, &color_rb);
if (depth_rb != 0)
glDeleteRenderbuffersEXT(1, &depth_rb);
return TRUE;
}
static void
gtk_gl_area_screen_changed (GtkWidget *widget,
GdkScreen *old_screen)
{
GtkGLArea *self = GTK_GL_AREA (widget);
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (self);
/* this will cause the context to be recreated on realize */
g_clear_object (&priv->context);
}
static void
gtk_gl_area_class_init (GtkGLAreaClass *klass)
{
GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (klass);
widget_class->screen_changed = gtk_gl_area_screen_changed;
widget_class->realize = gtk_gl_area_realize;
widget_class->unrealize = gtk_gl_area_unrealize;
widget_class->size_allocate = gtk_gl_area_size_allocate;
widget_class->draw = gtk_gl_area_draw;
gtk_widget_class_set_accessible_role (widget_class, ATK_ROLE_DRAWING_AREA);
/**
* GtkGLArea:context:
*
* The #GdkGLContext used by the #GtkGLArea widget.
*
* The #GtkGLArea widget is responsible for creating the #GdkGLContext
* instance. See the #GtkGLArea::create-context signal on how to
* override the default behavior.
*
* Since: 3.16
*/
obj_props[PROP_CONTEXT] =
g_param_spec_object ("context",
P_("Context"),
P_("The GL context"),
GDK_TYPE_GL_CONTEXT,
G_PARAM_READABLE |
G_PARAM_STATIC_STRINGS);
/**
* GtkGLArea:has-alpha:
*
* If set to #TRUE the buffer allocated by the widget will have an alpha channel component,
* and when rendering to the window the result will be composited over whatever is below
* the widget.
*
* If set to #FALSE there will be no alpha channel, and the buffer will fully replace anything
* below the widget.
*
* Since: 3.16
*/
obj_props[PROP_HAS_ALPHA] =
g_param_spec_boolean ("has-alpha",
P_("Has alpha"),
P_("Whether the gl area color buffer has an alpha component"),
FALSE,
GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY);
/**
* GtkGLArea:has-depth-buffer:
*
* If set to #TRUE the widget will allocate and enable a depth buffer for the target
* framebuffer.
*
* Since: 3.16
*/
obj_props[PROP_HAS_DEPTH_BUFFER] =
g_param_spec_boolean ("has-depth-buffer",
P_("Has depth buffer"),
P_("Whether a depth buffer is allocated"),
FALSE,
GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY);
gobject_class->set_property = gtk_gl_area_set_property;
gobject_class->get_property = gtk_gl_area_get_property;
gobject_class->dispose = gtk_gl_area_dispose;
g_object_class_install_properties (gobject_class, LAST_PROP, obj_props);
/**
* GtkGLArea::render:
* @area: the #GtkGLArea that emitted the signal
* @context: the #GdkGLContext used by @area
*
* The ::render signal is emitted every time the contents
* of the #GtkGLArea should be redrawn.
*
* The @context is bound to the @area prior to emitting this function,
* and the buffers are painted to the window once the emission terminates.
*
* Returns: %TRUE to stop other handlers from being invoked for the event.
* %FALSE to propagate the event further.
*
* Since: 3.16
*/
area_signals[RENDER] =
g_signal_new (I_("render"),
G_TYPE_FROM_CLASS (gobject_class),
G_SIGNAL_RUN_LAST,
G_STRUCT_OFFSET (GtkGLAreaClass, render),
_gtk_boolean_handled_accumulator, NULL,
NULL,
G_TYPE_BOOLEAN, 1,
GDK_TYPE_GL_CONTEXT);
}
static void
gtk_gl_area_init (GtkGLArea *self)
{
gtk_widget_set_has_window (GTK_WIDGET (self), FALSE);
gtk_widget_set_app_paintable (GTK_WIDGET (self), TRUE);
}
/**
* gtk_gl_area_new:
*
* Creates a new #GtkGLArea widget.
*
* Returns: (transfer full): the newly created #GtkGLArea
*
* Since: 3.16
*/
GtkWidget *
gtk_gl_area_new (void)
{
return g_object_new (GTK_TYPE_GL_AREA,
NULL);
}
/**
* gtk_gl_area_get_has_alpha:
* @area: a #GtkGLArea
*
* Returns: @true if the @area has an alpha component, @false otherwise
*
* Since: 3.16
*/
gboolean
gtk_gl_area_get_has_alpha (GtkGLArea *area)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
g_return_val_if_fail (GTK_IS_GL_AREA (area), FALSE);
return priv->has_alpha;
}
/**
* gtk_gl_area_set_has_alpha:
* @area: a #GtkGLArea
* @has_alpha: a boolean
*
* If @has_alpha is #TRUE the buffer allocated by the widget will have an alpha channel component,
* and when rendering to the window the result will be composited over whatever is below
* the widget.
*
* If @has_alpha is #FALSE there will be no alpha channel, and the buffer will fully replace anything
* below the widget.
*
* Since: 3.16
*/
void
gtk_gl_area_set_has_alpha (GtkGLArea *area,
gboolean has_alpha)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
g_return_if_fail (GTK_IS_GL_AREA (area));
has_alpha = !!has_alpha;
if (priv->has_alpha != has_alpha)
{
priv->has_alpha = has_alpha;
g_object_notify (G_OBJECT (area), "has-alpha");
}
}
/**
* gtk_gl_area_get_has_depth_buffer:
* @area: a #GtkGLArea
*
* Returns: @true if the @area has an depth buffer, @false otherwise
*
* Since: 3.16
*/
gboolean
gtk_gl_area_get_has_depth_buffer (GtkGLArea *area)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
g_return_val_if_fail (GTK_IS_GL_AREA (area), FALSE);
return priv->has_depth_buffer;
}
/**
* gtk_gl_area_set_has_depth_buffer:
* @area: a #GtkGLArea
* @has_depth_buffer: a boolean
*
* If @has_depth_buffer is #TRUE the widget will allocate and enable a depth buffer for the target
* framebuffer. Otherwise there will be none.
*
* Since: 3.16
*/
void
gtk_gl_area_set_has_depth_buffer (GtkGLArea *area,
gboolean has_depth_buffer)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
g_return_if_fail (GTK_IS_GL_AREA (area));
has_depth_buffer = !!has_depth_buffer;
if (priv->has_depth_buffer != has_depth_buffer)
{
priv->has_depth_buffer = has_depth_buffer;
g_object_notify (G_OBJECT (area), "has-depth-buffer");
}
}
/**
* gtk_gl_area_get_context:
* @area: a #GtkGLArea
*
* Retrieves the #GdkGLContext used by @area.
*
* Returns: (transfer none): the #GdkGLContext
*
* Since: 3.16
*/
GdkGLContext *
gtk_gl_area_get_context (GtkGLArea *area)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
g_return_val_if_fail (GTK_IS_GL_AREA (area), NULL);
return priv->context;
}
/**
* gtk_gl_area_make_current:
* @area: a #GtkGLArea
*
* Ensures that the #GdkGLContext used by @area is associated with
* the #GtkGLArea.
*
* This function is automatically called before emitting the
* #GtkGLArea::render signal, and should not be called by
* application code.
*
* Returns: %TRUE if the context was associated successfully with
* the widget
*
* Since: 3.16
*/
gboolean
gtk_gl_area_make_current (GtkGLArea *area)
{
GtkGLAreaPrivate *priv = gtk_gl_area_get_instance_private (area);
GtkWidget *widget;
g_return_val_if_fail (GTK_IS_GL_AREA (area), FALSE);
widget = GTK_WIDGET (area);
g_return_val_if_fail (gtk_widget_get_realized (widget), FALSE);
if (priv->context == NULL)
return FALSE;
return gdk_gl_context_make_current (priv->context);
}
/* GTK - The GIMP Toolkit
*
* gtkglarea.h: A GL drawing area
*
* Copyright © 2014 Emmanuele Bassi
*
* 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, see <http://www.gnu.org/licenses/>.
*/
#ifndef __GTK_GL_AREA_H__
#define __GTK_GL_AREA_H__
#if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
#error "Only <gtk/gtk.h> can be included directly."
#endif
#include <gtk/gtkwidget.h>
G_BEGIN_DECLS
#define GTK_TYPE_GL_AREA (gtk_gl_area_get_type ())
#define GTK_GL_AREA(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), GTK_TYPE_GL_AREA, GtkGLArea))
#define GTK_IS_GL_AREA(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GTK_TYPE_GL_AREA))
#define GTK_GL_AREA_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), GTK_TYPE_GL_AREA, GtkGLAreaClass))
#define GTK_IS_GL_AREA_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GTK_TYPE_GL_AREA))
#define GTK_GL_AREA_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), GTK_TYPE_GL_AREA, GtkGLAreaClass))
typedef struct _GtkGLArea GtkGLArea;
typedef struct _GtkGLAreaClass GtkGLAreaClass;
/**
* GtkGLArea:
*
* A #GtkWidget used for drawing with OpenGL.
*
* Since: 3.16
*/
struct _GtkGLArea
{
/*< private >*/
GtkWidget parent_instance;
};
/**
* GtkGLAreaClass:
* @render: class closure for the #GtkGLArea::render signal
*
* The `GtkGLAreaClass` structure contains only private data.
*
* Since: 3.16
*/
struct _GtkGLAreaClass
{
/*< private >*/
GtkWidgetClass parent_class;