1.
Andy Bulka
Technical Director
Austhink Software
www.austhink.com

2.
State
 Alter an object's behavior when its state changes. Put
the “state” in a class and delegate to it.
Intent
 Allow an object to alter its behavior when its internal
state changes. The object will appear to change its
class.

3.
State pattern - UML
 Client code talks to the “context.Request()” which
behaves differently

4.
 So what if
you want
to add the
new
"Orange"
state?
You
usually
have to
modify
code in a
number
of places.

5.
State instances as Singletons
 You often want to have singleton states rather than
recreating each state time and time again, and having
to copy across any state attributes each time. Of
course by keeping most of the attributes in the
context, there is not much data in state to copy
between state instance when the state switches.

6.
What States instances can access
 One main issue in state pattern is how do state
instances access attributes in the context.
Simplest solution is to have a pointer back from each
state object back to the context. Another solution is to
completely decouple the state from the context and
simply pass the state instance what it needs e.g. as
parameters to state methods (as defined in the the
state interface)

7.
Who changes the state
 Sometimes client code drives the changing of the state
 Sometimes client code holds the state instances too,
though I prefer to map these to enums and look them
up in a dictionary (see singleton issue above)
 Often the states themselves or the context (e.g. car)
will take on this responsibility. If the states are given
the power to set the next state – they will need need
access to the context (via a backpointer) so that they
can call a method called e.g. SetState().

8.
Flex 3 example

9.
Flex 3 classes
 Implement dumb
graphic in either flash
or mxml
 Deep state pattern
switching logic is in
pure actionscript code
 Hook up the gui to the
pattern using various
techniques

10.
Flash – dumb graphic
 Graphics
reified as
symbols
 Symbols given
names
 Symbols
marked as
inheriting
from Flex’s
mx.flash.UIMovieClip
Should
actually
be
com.Traf
ficLightC
ontroller

11.
UML 1
 Simple version – flash component subclasses “context”

12.
UML vers2

13.
Re the behaviour - display()
Three versions of
implementing the
graphic part is shown.
 pure mxml (code
embedded inside the
mxml)
 mxml + actionscript
class
 flash component
(dumb)

14.
Flex 3 MXML graphics

15.
End.
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