<s>
The	O
term	O
target	B-Architecture
–	I-Architecture
action	I-Architecture
design	O
paradigm	O
refers	O
to	O
a	O
kind	O
of	O
software	B-Architecture
architecture	I-Architecture
,	O
where	O
a	O
computer	B-Application
program	I-Application
is	O
divided	O
into	O
objects	O
which	O
dynamically	O
establish	O
relationships	O
by	O
telling	O
each	O
other	O
which	O
object	O
they	O
should	O
target	O
and	O
what	O
action	O
or	O
message	O
to	O
send	O
to	O
that	O
target	O
when	O
an	O
event	O
occurs	O
.	O
</s>
<s>
This	O
is	O
especially	O
useful	O
when	O
implementing	O
graphical	B-Application
user	I-Application
interfaces	I-Application
,	O
which	O
are	O
by	O
nature	O
event-driven	B-Application
.	O
</s>
<s>
The	O
target	B-Architecture
–	I-Architecture
action	I-Architecture
approach	O
to	O
event-driven	B-Application
systems	O
allows	O
far	O
more	O
dynamism	O
when	O
compared	O
to	O
other	O
,	O
more	O
static	O
approaches	O
,	O
such	O
as	O
by	O
subclassing	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
target-action	B-Architecture
based	O
programming	O
can	O
change	O
these	O
completely	O
at	O
run-time	B-Library
,	O
thus	O
allowing	O
the	O
program	O
to	O
create	O
new	O
interrelationships	O
and	O
novel	O
behavior	O
by	O
itself	O
.	O
</s>
<s>
A	O
prime	O
example	O
of	O
this	O
approach	O
is	O
the	O
OpenStep	B-Operating_System
API	B-General_Concept
,	O
which	O
partly	O
thanks	O
to	O
being	O
based	O
on	O
the	O
dynamic	O
Objective-C	B-Language
language	O
,	O
has	O
much	O
of	O
its	O
graphical	B-Application
user	I-Application
interface	I-Application
implemented	O
by	O
using	O
the	O
target-action	B-Architecture
paradigm	O
.	O
</s>
<s>
Consider	O
the	O
following	O
example	O
,	O
written	O
in	O
Objective-C	B-Language
:	O
</s>
<s>
Now	O
when	O
the	O
button	O
identified	O
by	O
the	O
variable	O
is	O
pressed	O
,	O
the	O
runtime	B-Library
system	O
will	O
try	O
to	O
send	O
a	O
message	O
named	O
to	O
the	O
object	O
in	O
which	O
this	O
code	O
has	O
been	O
invoked	O
.	O
</s>
<s>
It	O
is	O
also	O
very	O
well	O
possible	O
to	O
determine	O
the	O
message	O
to	O
be	O
sent	O
at	O
run-time	B-Library
:	O
</s>
<s>
This	O
is	O
possible	O
because	O
,	O
under	O
Objective-C	B-Language
,	O
methods	O
are	O
represented	O
by	O
a	O
selector	O
,	O
a	O
simple	O
string	O
describing	O
the	O
method	O
to	O
be	O
called	O
.	O
</s>
<s>
When	O
a	O
message	O
is	O
sent	O
,	O
the	O
selector	O
is	O
sent	O
into	O
the	O
ObjC	B-Language
runtime	B-Library
,	O
matched	O
against	O
a	O
list	O
of	O
available	O
methods	O
,	O
and	O
the	O
method	O
's	O
implementation	O
is	O
called	O
.	O
</s>
<s>
The	O
implementation	O
of	O
the	O
method	O
is	O
looked	O
up	O
at	B-Library
runtime	I-Library
,	O
not	O
compile	B-Application
time	I-Application
.	O
</s>
<s>
Because	O
of	O
the	O
extreme	O
dynamism	O
and	O
freedom	O
of	O
behavior	O
given	O
to	O
programs	O
designed	O
with	O
the	O
target-action	B-Architecture
paradigm	O
,	O
it	O
can	O
happen	O
that	O
the	O
program	O
designer	O
incorrectly	O
implements	O
a	O
part	O
of	O
the	O
interconnection	O
logic	O
and	O
this	O
can	O
lead	O
to	O
sometimes	O
hard	O
to	O
trace	O
bugs	B-Error_Name
.	O
</s>
<s>
This	O
is	O
due	O
to	O
the	O
lack	O
of	O
compile-time	B-Application
control	O
provided	O
by	O
the	O
compiler	B-Language
which	O
cannot	O
see	O
the	O
interconnections	O
.	O
</s>
<s>
The	O
result	O
of	O
an	O
incorrectly	O
connected	O
target-action	B-Architecture
binding	O
can	O
differ	O
based	O
on	O
how	O
the	O
particular	O
system	O
in	O
which	O
the	O
program	O
is	O
implemented	O
regards	O
this	O
:	O
</s>
