sect05.htm

Pythn design pattern

  <font color=#0000ff>def</font> __init__(self, name, value):
    self.name = name
    self.value = value
  <font color=#0000ff>def</font> __str__(self): <font color=#0000ff>return</font> self.name 
  <font color=#0000ff>def</font> getValue(self): <font color=#0000ff>return</font> self.value 

Money.quarter = Money(<font color=#004488>"Quarter"</font>, 25)
Money.dollar = Money(<font color=#004488>"Dollar"</font>, 100)

<font color=#0000ff>class</font> Quit:
  <font color=#0000ff>def</font> __str__(self): <font color=#0000ff>return</font> <font color=#004488>"Quit"</font> 

Quit.quit = Quit()

<font color=#0000ff>class</font> Digit:
  <font color=#0000ff>def</font> __init__(self, name, value):
    self.name = name
    self.value = value
  <font color=#0000ff>def</font> __str__(self): <font color=#0000ff>return</font> self.name 
  <font color=#0000ff>def</font> getValue(self): <font color=#0000ff>return</font> self.value 
  
<font color=#0000ff>class</font> FirstDigit(Digit): <font color=#0000ff>pass</font>
FirstDigit.A = FirstDigit(<font color=#004488>"A"</font>, 0)
FirstDigit.B = FirstDigit(<font color=#004488>"B"</font>, 1)
FirstDigit.C = FirstDigit(<font color=#004488>"C"</font>, 2)
FirstDigit.D = FirstDigit(<font color=#004488>"D"</font>, 3)

<font color=#0000ff>class</font> SecondDigit(Digit): <font color=#0000ff>pass</font>
SecondDigit.one = SecondDigit(<font color=#004488>"one"</font>, 0)
SecondDigit.two = SecondDigit(<font color=#004488>"two"</font>, 1)
SecondDigit.three = SecondDigit(<font color=#004488>"three"</font>, 2)
SecondDigit.four = SecondDigit(<font color=#004488>"four"</font>, 3)

<font color=#0000ff>class</font> ItemSlot:
  id = 0
  <font color=#0000ff>def</font> __init__(self, price, quantity):
    self.price = price
    self.quantity = quantity
  <font color=#0000ff>def</font> __str__(self): <font color=#0000ff>return</font> &#180;ItemSlot.id&#180;
  <font color=#0000ff>def</font> getPrice(self): <font color=#0000ff>return</font> self.price 
  <font color=#0000ff>def</font> getQuantity(self): <font color=#0000ff>return</font> self.quantity 
  <font color=#0000ff>def</font> decrQuantity(self): self.quantity -= 1

<font color=#0000ff>class</font> VendingMachine(StateMachine):
  changeAvailable = ChangeAvailable()
  amount = 0
  FirstDigit first = null
  ItemSlot[][] items = ItemSlot[4][4]

  # Conditions:
  <font color=#0000ff>def</font> notEnough(self, input):
    i1 = first.getValue()
    i2 = input.getValue()
    <font color=#0000ff>return</font> items[i1][i2].getPrice() &gt; amount

  <font color=#0000ff>def</font> itemAvailable(self, input):
    i1 = first.getValue()
    i2 = input.getValue()
    <font color=#0000ff>return</font> items[i1][i2].getQuantity() &gt; 0

  <font color=#0000ff>def</font> itemNotAvailable(self, input):
    <font color=#0000ff>return</font> !itemAvailable.condition(input)
    #i1 = first.getValue()
    #i2 = input.getValue()
    #<font color=#0000ff>return</font> items[i1][i2].getQuantity() == 0

  # Transitions:
  <font color=#0000ff>def</font> clearSelection(self, input):
    i1 = first.getValue()
    i2 = input.getValue()
    ItemSlot <font color=#0000ff>is</font> = items[i1][i2]
    <font color=#0000ff>print</font> (
      <font color=#004488>"Clearing selection: item "</font> + <font color=#0000ff>is</font> +
      <font color=#004488>" costs "</font> + <font color=#0000ff>is</font>.getPrice() +
      <font color=#004488>" and has quantity "</font> + <font color=#0000ff>is</font>.getQuantity())
    first = null

  <font color=#0000ff>def</font> dispense(self, input):
    i1 = first.getValue()
    i2 = input.getValue()
    ItemSlot <font color=#0000ff>is</font> = items[i1][i2]
    <font color=#0000ff>print</font> (<font color=#004488>"Dispensing item "</font> + 
      <font color=#0000ff>is</font> + <font color=#004488>" costs "</font> + <font color=#0000ff>is</font>.getPrice() +
      <font color=#004488>" and has quantity "</font> + <font color=#0000ff>is</font>.getQuantity())
    items[i1][i2].decrQuantity()
    <font color=#0000ff>print</font> (<font color=#004488>"Quantity "</font> + 
      <font color=#0000ff>is</font>.getQuantity())
    amount -= <font color=#0000ff>is</font>.getPrice()
    <font color=#0000ff>print</font>(<font color=#004488>"Amount remaining "</font> + 
      amount)

  <font color=#0000ff>def</font> showTotal(self, input):
    amount += ((Money)input).getValue()
    <font color=#0000ff>print</font> <font color=#004488>"Total amount = "</font> + amount

  <font color=#0000ff>def</font> returnChange(self, input):
    <font color=#0000ff>print</font> <font color=#004488>"Returning "</font> + amount
    amount = 0

  <font color=#0000ff>def</font> showDigit(self, input):
    first = (FirstDigit)input
    <font color=#0000ff>print</font> <font color=#004488>"First Digit= "</font>+ first

  <font color=#0000ff>def</font> __init__(self):
    StateMachine.__init__(self, State.quiescent)
    <font color=#0000ff>for</font>(int i = 0 i &lt; items.length i++)
      <font color=#0000ff>for</font>(int j = 0 j &lt; items[i].length j++)
        items[i][j] = ItemSlot((j+1)*25, 5)
    items[3][0] = ItemSlot(25, 0)
    buildTable(Object[][][]{
     ::State.quiescent, # Current state
        # Input, test, transition, next state:
       :Money.<font color=#0000ff>class</font>, null, 
         showTotal, State.collecting,
     ::State.collecting, # Current state
        # Input, test, transition, next state:
       :Quit.quit, null, 
         returnChange, State.quiescent,
       :Money.<font color=#0000ff>class</font>, null, 
         showTotal, State.collecting,
       :FirstDigit.<font color=#0000ff>class</font>, null, 
         showDigit, State.selecting,
     ::State.selecting, # Current state
        # Input, test, transition, next state:
       :Quit.quit, null, 
         returnChange, State.quiescent,
       :SecondDigit.<font color=#0000ff>class</font>, notEnough, 
         clearSelection, State.collecting,
       :SecondDigit.<font color=#0000ff>class</font>, itemNotAvailable, 
         clearSelection, State.unavailable,
       :SecondDigit.<font color=#0000ff>class</font>, itemAvailable, 
         dispense, State.wantMore,
     ::State.unavailable, # Current state
        # Input, test, transition, next state:
       :Quit.quit, null, 
         returnChange, State.quiescent,
       :FirstDigit.<font color=#0000ff>class</font>, null, 
         showDigit, State.selecting,
     ::State.wantMore, # Current state
        # Input, test, transition, next state:
       :Quit.quit, null, 
         returnChange, State.quiescent,
       :FirstDigit.<font color=#0000ff>class</font>, null, 
         showDigit, State.selecting,
    )

# :~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><A NAME="_Toc534420094"></A><BR></P></DIV>
<A NAME="Heading45"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H3 ALIGN="LEFT">
Testing the machine</H3></FONT>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE># c04:vendingmachine:VendingMachineTest.py
# Demonstrates use of StateMachine.py

vm = VendingMachine()
<font color=#0000ff>for</font> input <font color=#0000ff>in</font> [  
    Money.quarter,
    Money.quarter,
    Money.dollar,
    FirstDigit.A,
    SecondDigit.two,
    FirstDigit.A,
    SecondDigit.two,
    FirstDigit.C,
    SecondDigit.three,
    FirstDigit.D,
    SecondDigit.one,
    Quit.quit]:
  vm.nextState(input)

# :~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><A NAME="_Toc534420095"></A><BR></P></DIV>
<A NAME="Heading46"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Tools</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Another approach, as your state machine
gets bigger, is to use an automation tool whereby you configure a table and let
the tool generate the state machine code for you. This can be created yourself
using a language like Python, but there are also free, open-source tools such as
<I>Libero</I>, at <A HREF="http://www.imatix.com">http://www.imatix.com</A>.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_189">Add Comment</A></FONT><A NAME="_Toc534420096"></A><BR></P></DIV>
<A NAME="Heading47"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Exercises</H2></FONT>
<OL>
<LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create an example of the
&#147;virtual proxy.&#148;
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_190">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
an example of the &#147;Smart reference&#148; proxy where you keep count of
the number of method calls to a particular object.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_191">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
a program similar to certain DBMS systems that only allow a certain number of
connections at any time. To implement this, use a singleton-like system that
controls the number of &#147;connection&#148; objects that it creates. When a
user is finished with a connection, the system must be informed so that it can
check that connection back in to be reused. To guarantee this, provide a proxy
object instead of a reference to the actual connection, and design the proxy so
that it will cause the connection to be released back to the system.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_192">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Using
the <I>State</I>,<I> </I>make a class called <B>UnpredictablePerson</B> which
changes the kind of response to its <B>hello(&#160;)</B> method depending on
what kind of <B>Mood</B> it&#146;s in. Add an additional kind of <B>Mood</B>
called <B>Prozac</B>.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_193">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
a simple copy-on write implementation.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_194">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Apply
<B>TransitionTable.py</B> to the &#147;Washer&#148; problem.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_195">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
a <I>StateMachine</I> system whereby the current state along with input
information determines the next state that the system will be in. To do this,
each state must store a reference back to the proxy object (the state
controller) so that it can request the state change. Use a <B>HashMap</B> to
create a table of states, where the key is a <B>String</B> naming the new state
and the value is the new state object. Inside each state subclass override a
method <B>nextState(&#160;)</B> that has its own state-transition table. The
input to <B>nextState(&#160;)</B> should be a single word that comes from a text
file containing one word per line.<B>
</B><A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_196">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Modify
the previous exercise so that the state machine can be configured by
creating/modifying a single multi-dimensional array.<B>
</B><A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_197">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Modify
the &#147;mood&#148; exercise from the previous session so that it becomes a
state machine using StateMachine.java
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_198">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
an elevator state machine system using StateMachine.java
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_199">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create
a heating/air-conditioning system using StateMachine.java<B>
</B><A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_200">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">A
<I>generator</I> is an object that produces other objects, just like a factory,
except that the generator function doesn&#146;t require any arguments. Create a
<B>MouseMoveGenerator</B> which produces correct <B>MouseMove</B> actions as
outputs each time the generator function is called (that is, the mouse must move
in the proper sequence, thus the possible moves are based on the previous move
&#150; it&#146;s another state machine). Add a method <B>iterator(&#160;)
</B>to produce an iterator, but this method should take an <B>int</B> argument
that specifies the number of moves to produce before <B>hasNext(&#160;)</B>
returns <B>false</B>.<B>
</B><A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_201">Add Comment</A></FONT></OL>
<HR><DIV ALIGN="LEFT"><P><A NAME="fn13" HREF="#fnB13">[13]</A><FONT FACE="Georgia"> No
mice were harmed in the creation of this example.</FONT><BR></P></DIV>

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 Last Update:12/31/2001</P></DIV>

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