<s>
A	O
widget	B-Library
toolkit	I-Library
,	O
widget	B-Library
library	I-Library
,	O
GUI	B-Library
toolkit	I-Library
,	O
or	O
UX	O
library	B-Library
is	O
a	O
library	B-Library
or	O
a	O
collection	O
of	O
libraries	O
containing	O
a	O
set	O
of	O
graphical	O
control	O
elements	O
(	O
called	O
widgets	O
)	O
used	O
to	O
construct	O
the	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
of	O
programs	O
.	O
</s>
<s>
Most	O
widget	B-Library
toolkits	I-Library
additionally	O
include	O
their	O
own	O
rendering	O
engine	O
.	O
</s>
<s>
This	O
engine	O
can	O
be	O
specific	O
to	O
a	O
certain	O
operating	B-General_Concept
system	I-General_Concept
or	O
windowing	B-Application
system	I-Application
or	O
contain	O
back-ends	O
to	O
interface	O
with	O
more	O
multiple	O
ones	O
and	O
also	O
with	O
rendering	O
APIs	O
such	O
as	O
OpenGL	B-Application
,	O
OpenVG	B-Library
,	O
or	O
EGL	B-Application
.	O
</s>
<s>
The	O
look	B-Application
and	I-Application
feel	I-Application
of	O
the	O
graphical	O
control	O
elements	O
can	O
be	O
hard-coded	O
or	O
decoupled	O
,	O
allowing	O
the	O
graphical	O
control	O
elements	O
to	O
be	O
themed/skinned	O
.	O
</s>
<s>
Some	O
toolkits	O
may	O
be	O
used	O
from	O
other	O
languages	O
by	O
employing	O
language	B-Application
bindings	I-Application
.	O
</s>
<s>
Graphical	B-Application
user	I-Application
interface	I-Application
builders	O
such	O
as	O
e.g.	O
</s>
<s>
Glade	B-Language
Interface	I-Language
Designer	I-Language
facilitate	O
the	O
authoring	O
of	O
GUIs	B-Application
in	O
a	O
WYSIWYG	B-Application
manner	O
employing	O
a	O
user	B-Language
interface	I-Language
markup	I-Language
language	I-Language
such	O
as	O
in	O
this	O
case	O
GtkBuilder	B-Language
.	O
</s>
<s>
The	O
GUI	B-Application
of	O
a	O
program	O
is	O
commonly	O
constructed	O
in	O
a	O
cascading	O
manner	O
,	O
with	O
graphical	O
control	O
elements	O
being	O
added	O
directly	O
to	O
on	O
top	O
of	O
one	O
another	O
.	O
</s>
<s>
Most	O
widget	B-Library
toolkits	I-Library
use	O
event-driven	B-Application
programming	I-Application
as	O
a	O
model	O
for	O
interaction	O
.	O
</s>
<s>
Finite	B-Architecture
state	I-Architecture
machines	I-Architecture
and	O
hierarchical	B-Application
state	I-Application
machines	I-Application
have	O
been	O
proposed	O
as	O
high-level	O
models	O
to	O
represent	O
the	O
interactive	O
state	O
changes	O
for	O
reactive	O
programs	O
.	O
</s>
<s>
In	O
some	O
windowing	B-Application
systems	I-Application
,	O
windows	O
are	O
added	O
directly	O
to	O
the	O
scene	O
graph	O
(	O
canvas	O
)	O
by	O
the	O
window	B-Application
manager	I-Application
,	O
and	O
can	O
be	O
stacked	O
and	O
layered	O
on	O
top	O
of	O
each	O
other	O
through	O
various	O
means	O
.	O
</s>
