<s>
AppKit	B-Operating_System
(	O
formally	O
Application	B-Operating_System
Kit	I-Operating_System
)	O
is	O
a	O
graphical	B-Library
user	I-Library
interface	I-Library
toolkit	I-Library
.	O
</s>
<s>
It	O
initially	O
served	O
as	O
the	O
UI	O
framework	O
for	O
NeXTSTEP	B-Application
.	O
</s>
<s>
Along	O
with	O
Foundation	B-Operating_System
and	O
Display	O
PostScript	O
,	O
it	O
became	O
one	O
of	O
the	O
core	O
parts	O
of	O
the	O
OpenStep	B-Operating_System
specification	O
of	O
APIs	O
.	O
</s>
<s>
Later	O
,	O
AppKit	B-Operating_System
and	O
Foundation	B-Operating_System
became	O
part	O
of	O
Cocoa	B-Operating_System
,	O
the	O
Objective-C	B-Language
API	O
framework	O
of	O
macOS	B-Application
.	O
</s>
<s>
GNUstep	B-Application
,	O
GNU	B-Operating_System
's	O
implementation	O
of	O
the	O
OpenStep/Cocoa	O
API	O
,	O
also	O
contains	O
an	O
implementation	O
of	O
the	O
AppKit	B-Operating_System
API	O
.	O
</s>
<s>
AppKit	B-Operating_System
comprises	O
a	O
collection	O
of	O
Objective-C	B-Language
classes	O
and	O
protocols	B-Application
that	O
can	O
be	O
used	O
to	O
build	O
an	O
application	O
in	O
OpenStep/Cocoa	O
.	O
</s>
<s>
These	O
classes	O
can	O
also	O
be	O
used	O
in	O
Swift	B-Application
through	O
its	O
Objective-C	B-Language
bridge	O
.	O
</s>
<s>
Xcode	B-Operating_System
has	O
built-in	O
functionality	O
for	O
developing	O
a	O
Cocoa	B-Operating_System
application	O
using	O
AppKit	B-Operating_System
,	O
including	O
the	O
ability	O
to	O
visually	O
design	O
user	O
interfaces	O
with	O
Interface	O
Builder	O
.	O
</s>
<s>
It	O
relies	O
heavily	O
on	O
patterns	O
like	O
reference	B-Language
types	I-Language
,	O
delegation	B-Application
,	O
notifications	B-Operating_System
,	O
target	B-Architecture
–	I-Architecture
action	I-Architecture
,	O
and	O
model	O
–	O
view	O
–	O
controller	O
.	O
</s>
<s>
A	O
sign	O
of	O
the	O
NeXTSTEP	B-Application
heritage	O
,	O
AppKit	B-Operating_System
's	O
classes	O
and	O
protocols	B-Application
still	O
use	O
the	O
"	O
NS	O
"	O
prefix	O
.	O
</s>
<s>
Most	O
of	O
the	O
applications	O
bundled	O
with	O
macOS	B-Application
—	O
for	O
example	O
,	O
the	O
Finder	B-Application
,	O
TextEdit	B-Application
,	O
Calendar	B-Operating_System
,	O
and	O
Preview	B-Language
–	O
use	O
AppKit	B-Operating_System
to	O
provide	O
their	O
user	O
interface	O
.	O
</s>
<s>
macOS	B-Application
,	O
iOS	B-Application
,	O
iPadOS	B-Operating_System
,	O
and	O
tvOS	B-Operating_System
also	O
support	O
other	O
UI	O
frameworks	O
,	O
including	O
UIKit	B-Operating_System
,	O
which	O
is	O
derived	O
from	O
AppKit	B-Operating_System
and	O
uses	O
many	O
similar	O
structures	O
,	O
and	O
SwiftUI	O
,	O
a	O
Swift-only	O
declarative	O
UI	O
framework	O
.	O
</s>
<s>
Prior	O
to	O
macOS	B-Device
Catalina	I-Device
,	O
macOS	B-Application
also	O
supported	O
Carbon	B-Operating_System
,	O
a	O
UI	O
framework	O
derived	O
from	O
the	O
Macintosh	B-Device
Toolbox	I-Device
.	O
</s>
<s>
Of	O
the	O
more	O
than	O
170	O
classes	O
included	O
in	O
the	O
Application	B-Operating_System
Kit	I-Operating_System
,	O
the	O
following	O
classes	O
form	O
the	O
core	O
:	O
</s>
