Source

dictionary_switches / src / StateMachine.rst

StateMachine

While State has a way to allow the client programmer to change the implementation, StateMachine imposes a structure to automatically change the implementation from one object to the next. The current implementation represents the state that a system is in, and the system behaves differently from one state to the next (because it uses State). Basically, this is a "state machine" using objects.

The code that moves the system from one state to the next is often a Template Method, as seen in the following framework for a basic state machine.

Each state can be run( ) to perform its behavior, and (in this design) you can also pass it an "input" object so it can tell you what new state to move to based on that "input". The key distinction between this design and the next is that here, each State object decides what other states it can move to, based on the "input", whereas in the subsequent design all of the state transitions are held in a single table. Another way to put it is that here, each State object has its own little State table, and in the subsequent design there is a single master state transition table for the whole system:

# stateMachine/State.py
# A State has an operation, and can be moved
# into the next State given an Input:

class State:
    def run(self):
        assert 0, "run not implemented"
    def next(self, input):
        assert 0, "next not implemented"

This class is clearly unnecessary, but it allows us to say that something is a State object in code, and provide a slightly different error message when all the methods are not implemented. We could have gotten basically the same effect by saying:

class State: pass

because we would still get exceptions if run( ) or next( ) were called for a derived type, and they hadn't been implemented.

The StateMachine keeps track of the current state, which is initialized by the constructor. The runAll( ) method takes a list of Input objects. This method not only moves to the next state, but it also calls run( ) for each state object - thus you can see it's an expansion of the idea of the State pattern, since run( ) does something different depending on the state that the system is in:

# stateMachine/StateMachine.py
# Takes a list of Inputs to move from State to
# State using a template method.

class StateMachine:
    def __init__(self, initialState):
        self.currentState = initialState
        self.currentState.run()
    # Template method:
    def runAll(self, inputs):
        for i in inputs:
            print(i)
            self.currentState = self.currentState.next(i)
            self.currentState.run()

I've also treated runAll( ) as a template method. This is typical, but certainly not required - you could concievably want to override it, but typically the behavior change will occur in State's run( ) instead.

At this point the basic framework for this style of StateMachine (where each state decides the next states) is complete. As an example, I'll use a fancy mousetrap that can move through several states in the process of trapping a mouse [1]. The mouse classes and information are stored in the mouse package, including a class representing all the possible moves that a mouse can make, which will be the inputs to the state machine:

# stateMachine/mouse/MouseAction.py

class MouseAction:
    def __init__(self, action):
        self.action = action
    def __str__(self): return self.action
    def __cmp__(self, other):
        return cmp(self.action, other.action)
    # Necessary when __cmp__ or __eq__ is defined
    # in order to make this class usable as a
    # dictionary key:
    def __hash__(self):
        return hash(self.action)

# Static fields; an enumeration of instances:
MouseAction.appears = MouseAction("mouse appears")
MouseAction.runsAway = MouseAction("mouse runs away")
MouseAction.enters = MouseAction("mouse enters trap")
MouseAction.escapes = MouseAction("mouse escapes")
MouseAction.trapped = MouseAction("mouse trapped")
MouseAction.removed = MouseAction("mouse removed")

You'll note that __cmp__( ) has been overidden to implement a comparison between action values. Also, each possible move by a mouse is enumerated as a MouseAction object, all of which are static fields in MouseAction.

For creating test code, a sequence of mouse inputs is provided from a text file:

# stateMachine/mouse/MouseMoves.txt
mouse appears
mouse runs away
mouse appears
mouse enters trap
mouse escapes
mouse appears
mouse enters trap
mouse trapped
mouse removed
mouse appears
mouse runs away
mouse appears
mouse enters trap
mouse trapped
mouse removed

With these tools in place, it's now possible to create the first version of the mousetrap program. Each State subclass defines its run( ) behavior, and also establishes its next state with an if-else clause:

# stateMachine/mousetrap1/MouseTrapTest.py
# State Machine pattern using 'if' statements
# to determine the next state.
import string, sys
sys.path += ['../stateMachine', '../mouse']
from State import State
from StateMachine import StateMachine
from MouseAction import MouseAction
# A different subclass for each state:

class Waiting(State):
    def run(self):
        print("Waiting: Broadcasting cheese smell")

    def next(self, input):
        if input == MouseAction.appears:
            return MouseTrap.luring
        return MouseTrap.waiting

class Luring(State):
    def run(self):
        print("Luring: Presenting Cheese, door open")

    def next(self, input):
        if input == MouseAction.runsAway:
            return MouseTrap.waiting
        if input == MouseAction.enters:
            return MouseTrap.trapping
        return MouseTrap.luring

class Trapping(State):
    def run(self):
        print("Trapping: Closing door")

    def next(self, input):
        if input == MouseAction.escapes:
            return MouseTrap.waiting
        if input == MouseAction.trapped:
            return MouseTrap.holding
        return MouseTrap.trapping

class Holding(State):
    def run(self):
        print("Holding: Mouse caught")

    def next(self, input):
        if input == MouseAction.removed:
            return MouseTrap.waiting
        return MouseTrap.holding

class MouseTrap(StateMachine):
    def __init__(self):
        # Initial state
        StateMachine.__init__(self, MouseTrap.waiting)

# Static variable initialization:
MouseTrap.waiting = Waiting()
MouseTrap.luring = Luring()
MouseTrap.trapping = Trapping()
MouseTrap.holding = Holding()

moves = map(string.strip,
  open("../mouse/MouseMoves.txt").readlines())
MouseTrap().runAll(map(MouseAction, moves))

The StateMachine class simply defines all the possible states as static objects, and also sets up the initial state. The UnitTest creates a MouseTrap and then tests it with all the inputs from a MouseMoveList.

While the use of if statements inside the next( ) methods is perfectly reasonable, managing a large number of these could become difficult. Another approach is to create tables inside each State object defining the various next states based on the input.

Initially, this seems like it ought to be quite simple. You should be able to define a static table in each State subclass that defines the transitions in terms of the other State objects. However, it turns out that this approach generates cyclic initialization dependencies. To solve the problem, I've had to delay the initialization of the tables until the first time that the next( ) method is called for a particular State object. Initially, the next( ) methods can appear a little strange because of this.

The StateT class is an implementation of State (so that the same StateMachine class can be used from the previous example) that adds a Map and a method to initialize the map from a two-dimensional array. The next( ) method has a base-class implementation which must be called from the overridden derived class next( ) methods after they test for a null Map (and initialize it if it's null):

# stateMachine/mousetrap2/MouseTrap2Test.py
# A better mousetrap using tables
import string, sys
sys.path += ['../stateMachine', '../mouse']
from State import State
from StateMachine import StateMachine
from MouseAction import MouseAction

class StateT(State):
    def __init__(self):
        self.transitions = None
    def next(self, input):
        if self.transitions.has_key(input):
            return self.transitions[input]
        else:
            raise "Input not supported for current state"

class Waiting(StateT):
    def run(self):
        print("Waiting: Broadcasting cheese smell")
    def next(self, input):
        # Lazy initialization:
        if not self.transitions:
            self.transitions = {
              MouseAction.appears : MouseTrap.luring
            }
        return StateT.next(self, input)

class Luring(StateT):
    def run(self):
        print("Luring: Presenting Cheese, door open")
    def next(self, input):
        # Lazy initialization:
        if not self.transitions:
            self.transitions = {
              MouseAction.enters : MouseTrap.trapping,
              MouseAction.runsAway : MouseTrap.waiting
            }
        return StateT.next(self, input)

class Trapping(StateT):
    def run(self):
        print("Trapping: Closing door")
    def next(self, input):
        # Lazy initialization:
        if not self.transitions:
            self.transitions = {
              MouseAction.escapes : MouseTrap.waiting,
              MouseAction.trapped : MouseTrap.holding
            }
        return StateT.next(self, input)

class Holding(StateT):
    def run(self):
        print("Holding: Mouse caught")
    def next(self, input):
        # Lazy initialization:
        if not self.transitions:
            self.transitions = {
              MouseAction.removed : MouseTrap.waiting
            }
        return StateT.next(self, input)

class MouseTrap(StateMachine):
    def __init__(self):
        # Initial state
        StateMachine.__init__(self, MouseTrap.waiting)

# Static variable initialization:
MouseTrap.waiting = Waiting()
MouseTrap.luring = Luring()
MouseTrap.trapping = Trapping()
MouseTrap.holding = Holding()

moves = map(string.strip,
  open("../mouse/MouseMoves.txt").readlines())
mouseMoves = map(MouseAction, moves)
MouseTrap().runAll(mouseMoves)

The rest of the code is identical - the difference is in the next( ) methods and the StateT class.

If you have to create and maintain a lot of State classes, this approach is an improvement, since it's easier to quickly read and understand the state transitions from looking at the table.

Table-Driven State Machine

The advantage of the previous design is that all the information about a state, including the state transition information, is located within the state class itself. This is generally a good design principle.

However, in a pure state machine, the machine can be completely represented by a single state-transition table. This has the advantage of locating all the information about the state machine in a single place, which means that you can more easily create and maintain the table based on a classic state-transition diagram.

The classic state-transition diagram uses a circle to represent each state, and lines from the state pointing to all states that state can transition into. Each transition line is annotated with conditions for transition and an action during transition. Here's what it looks like:

(Simple State Machine Diagram)

Goals:

  • Direct translation of state diagram
  • Vector of change: the state diagram representation
  • Reasonable implementation
  • No excess of states (you could represent every single change with a new state)
  • Simplicity and flexibility

Observations:

  • States are trivial - no information or functions/data, just an identity
  • Not like the State pattern!
  • The machine governs the move from state to state
  • Similar to flyweight
  • Each state may move to many others
  • Condition & action functions must also be external to states
  • Centralize description in a single table containing all variations, for ease of configuration

Example:

  • State Machine & Table-Driven Code
  • Implements a vending machine
  • Uses several other patterns
  • Separates common state-machine code from specific application (like template method)
  • Each input causes a seek for appropriate solution (like chain of responsibility)
  • Tests and transitions are encapsulated in function objects (objects that hold functions)
  • Java constraint: methods are not first-class objects
_images/stateMachine.*

The State Class

The State class is distinctly different from before, since it is really just a placeholder with a name. Thus it is not inherited from previous State classes:

# stateMachine/stateMachine2/State.py

class State:
    def __init__(self, name): self.name = name
    def __str__(self): return self.name

Conditions for Transition

In the state transition diagram, an input is tested to see if it meets the condition necessary to transfer to the state under question. As before, the Input is just a tagging interface:

# stateMachine/stateMachine2/Input.py
# Inputs to a state machine

class Input: pass

The Condition evaluates the Input to decide whether this row in the table is the correct transition:

# stateMachine/stateMachine2/Condition.py
# Condition function object for state machine

class Condition:
    boolean condition(input) :
        assert 0, "condition() not implemented"

Transition Actions

If the Condition returns true, then the transition to a new state is made, and as that transition is made some kind of action occurs (in the previous state machine design, this was the run( ) method):

# stateMachine/stateMachine2/Transition.py
# Transition function object for state machine

class Transition:
    def transition(self, input):
        assert 0, "transition() not implemented"

The Table

With these classes in place, we can set up a 3-dimensional table where each row completely describes a state. The first element in the row is the current state, and the rest of the elements are each a row indicating what the type of the input can be, the condition that must be satisfied in order for this state change to be the correct one, the action that happens during transition, and the new state to move into. Note that the Input object is not just used for its type, it is also a Messenger object that carries information to the Condition and Transition objects:

{(CurrentState, InputA) : (ConditionA, TransitionA, NextA),
 (CurrentState, InputB) : (ConditionB, TransitionB, NextB),
 (CurrentState, InputC) : (ConditionC, TransitionC, NextC),
 ...
}

The Basic Machine

Here's the basic machine, (code only roughly converted):

# stateMachine/stateMachine2/StateMachine.py
# A table-driven state machine

class StateMachine:
    def __init__(self, initialState, tranTable):
        self.state = initialState
        self.transitionTable = tranTable

    def nextState(self, input):

        Iterator it=((List)map.get(state)).iterator()
        while(it.hasNext()):
            Object[] tran = (Object[])it.next()
            if(input == tran[0] ||
               input.getClass() == tran[0]):
                if(tran[1] != null):
                    Condition c = (Condition)tran[1]
                    if(!c.condition(input))
                        continue # Failed test

                if(tran[2] != null)
                    ((Transition)tran[2]).transition(input)
                state = (State)tran[3]
                return


        throw RuntimeException(
          "Input not supported for current state")

Simple Vending Machine

Here's the simple vending machine, (code only roughly converted):

# stateMachine/vendingmachine/VendingMachine.py
# Demonstrates use of StateMachine.py
import sys
sys.path += ['../stateMachine2']
import StateMachine

class State:
    def __init__(self, name): self.name = name
    def __str__(self): return self.name

State.quiescent = State("Quiesecent")
State.collecting = State("Collecting")
State.selecting = State("Selecting")
State.unavailable = State("Unavailable")
State.wantMore = State("Want More?")
State.noChange = State("Use Exact Change Only")
State.makesChange = State("Machine makes change")

class HasChange:
    def __init__(self, name): self.name = name
    def __str__(self): return self.name

HasChange.yes = HasChange("Has change")
HasChange.no = HasChange("Cannot make change")

class ChangeAvailable(StateMachine):
    def __init__(self):
        StateMachine.__init__(State.makesChange, {
          # Current state, input
          (State.makesChange, HasChange.no) :
            # test, transition, next state:
            (null, null, State.noChange),
          (State.noChange, HasChange.yes) :
            (null, null, State.noChange)
        })

class Money:
    def __init__(self, name, value):
        self.name = name
        self.value = value
    def __str__(self): return self.name
    def getValue(self): return self.value

Money.quarter = Money("Quarter", 25)
Money.dollar = Money("Dollar", 100)

class Quit:
    def __str__(self): return "Quit"

Quit.quit = Quit()

class Digit:
    def __init__(self, name, value):
        self.name = name
        self.value = value
    def __str__(self): return self.name
    def getValue(self): return self.value

class FirstDigit(Digit): pass
FirstDigit.A = FirstDigit("A", 0)
FirstDigit.B = FirstDigit("B", 1)
FirstDigit.C = FirstDigit("C", 2)
FirstDigit.D = FirstDigit("D", 3)

class SecondDigit(Digit): pass
SecondDigit.one = SecondDigit("one", 0)
SecondDigit.two = SecondDigit("two", 1)
SecondDigit.three = SecondDigit("three", 2)
SecondDigit.four = SecondDigit("four", 3)

class ItemSlot:
    id = 0
    def __init__(self, price, quantity):
        self.price = price
        self.quantity = quantity
    def __str__(self): return `ItemSlot.id`
    def getPrice(self): return self.price
    def getQuantity(self): return self.quantity
    def decrQuantity(self): self.quantity -= 1

class VendingMachine(StateMachine):
    changeAvailable = ChangeAvailable()
    amount = 0
    FirstDigit first = null
    ItemSlot[][] items = ItemSlot[4][4]

    # Conditions:
    def notEnough(self, input):
        i1 = first.getValue()
        i2 = input.getValue()
        return items[i1][i2].getPrice() > amount

    def itemAvailable(self, input):
        i1 = first.getValue()
        i2 = input.getValue()
        return items[i1][i2].getQuantity() > 0

    def itemNotAvailable(self, input):
        return !itemAvailable.condition(input)
        #i1 = first.getValue()
        #i2 = input.getValue()
        #return items[i1][i2].getQuantity() == 0

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

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

    def showTotal(self, input):
        amount += ((Money)input).getValue()
        print("Total amount = " + amount)

    def returnChange(self, input):
        print("Returning " + amount)
        amount = 0

    def showDigit(self, input):
        first = (FirstDigit)input
        print("First Digit= "+ first)

    def __init__(self):
        StateMachine.__init__(self, State.quiescent)
        for(int i = 0 i < items.length i++)
            for(int j = 0 j < 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.class, null,
             showTotal, State.collecting,
         ::State.collecting, # Current state
            # Input, test, transition, next state:
           :Quit.quit, null,
             returnChange, State.quiescent,
           :Money.class, null,
             showTotal, State.collecting,
           :FirstDigit.class, null,
             showDigit, State.selecting,
         ::State.selecting, # Current state
            # Input, test, transition, next state:
           :Quit.quit, null,
             returnChange, State.quiescent,
           :SecondDigit.class, notEnough,
             clearSelection, State.collecting,
           :SecondDigit.class, itemNotAvailable,
             clearSelection, State.unavailable,
           :SecondDigit.class, itemAvailable,
             dispense, State.wantMore,
         ::State.unavailable, # Current state
            # Input, test, transition, next state:
           :Quit.quit, null,
             returnChange, State.quiescent,
           :FirstDigit.class, null,
             showDigit, State.selecting,
         ::State.wantMore, # Current state
            # Input, test, transition, next state:
           :Quit.quit, null,
             returnChange, State.quiescent,
           :FirstDigit.class, null,
             showDigit, State.selecting,
        )
        """

Testing the Machine

Here's a test of the machine, (code only roughly converted):

# stateMachine/vendingmachine/VendingMachineTest.py
# Demonstrates use of StateMachine.py

vm = VendingMachine()
for input in [
    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)

Tools

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 Libero, at http://www.imatix.com.

Exercises

  1. Create an example of the "virtual proxy."
  2. Create an example of the "Smart reference" proxy where you keep count of the number of method calls to a particular object.
  3. 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 "connection" 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.
  4. Using the State, make a class called UnpredictablePerson which changes the kind of response to its hello( ) method depending on what kind of Mood it's in. Add an additional kind of Mood called Prozac.
  5. Create a simple copy-on write implementation.
  6. Apply TransitionTable.py to the "Washer" problem.
  7. Create a StateMachine 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 HashMap to create a table of states, where the key is a String naming the new state and the value is the new state object. Inside each state subclass override a method nextState( ) that has its own state-transition table. The input to nextState( ) should be a single word that comes from a text file containing one word per line.
  8. Modify the previous exercise so that the state machine can be configured by creating/modifying a single multi-dimensional array.
  9. Modify the "mood" exercise from the previous session so that it becomes a state machine using StateMachine.py
  10. Create an elevator state machine system using StateMachine.py
  11. Create a heating/air-conditioning system using StateMachine.py
  12. A generator is an object that produces other objects, just like a factory, except that the generator function doesn't require any arguments. Create a MouseMoveGenerator which produces correct MouseMove 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 - it's another state machine). Add a method to produce an iterator, but this method should take an int argument that specifies the number of moves to produce before hasNext() returns false.

Footnotes

[1]No mice were harmed in the creation of this example.