画GtkDrawingArea的焦点边框

1.  需要加GDK_EXPOSURE_MASK和GDK_BUTTON_PRESS_MASK
2. 把GtkDrawingArea设为可接受焦点
    gtk_widget_set_can_focus(videowidget, TRUE);
   下面是为videowidget设置grab-focus事件并做相关动作
    g_signal_connect(G_OBJECT(videowidget), "grab-focus" ,
              G_CALLBACK(videoarea_grab_focus_cb) , NULL);
3.在button-press-event事件中grab focus
static gint 
cb_videoarea_button_event_handler (GtkWidget *widget, GdkEventButton *event, gpointer data)
{
    gtk_widget_grab_focus (widget);
}
4.在videowidget的expose-event中画焦点边框
static gboolean
videoarea_expose_event_cb (GtkWidget *widget, GdkEventExpose *event, gpointer user_data)
{
    if (gtk_widget_has_focus(widget) == TRUE)
    {
        GtkStyle *style;
        gtk_paint_focus(widget->style, widget->window,
                        GTK_STATE_SELECTED,
                        NULL, NULL, NULL, 0, 0,
                        widget->allocation.width-1,
                        widget->allocation.height-1);
    }