State

The intent of the State design pattern is to "allow an object to alter its behavior when its internal state changes. The object will appear to change its class". The State pattern can be used, for instance, to implement a Finite State Machine efficiently and elegantly. This approach can be useful when implementing business processes or workflows.

Example: Door

Consider a class Door that represents a door. A door can be in one of three states: open, closed, locked. When a Door object receives messages (such as open(), close(), lock(), or unlock()) from other objects, it responds differently depending on its current state. For example, the effect of an open() message depends on whether the door is in its closed state or not (a locked door has to be unlocked before it can be opened, for instance). The State pattern describes how a Door object can exhibit different behavior in each state. The key idea in this pattern is to introduce classes to represent the states of the door.

DoorState

The DoorState interface (source) declares an interface common to all classes that represent different states.

<?php
interface DoorState
{
    public function open();
    public function close();
    public function lock();
    public function unlock();
}

AbstractDoorState

The AbstractDoorState class (source) implements the operations required by the DoorState interface in such a way that all methods raise an IllegalStateTransitionException by default.

<?php
abstract class AbstractDoorState implements DoorState
{
    public function open()
    {
        throw new IllegalStateTransitionException;
    }

    public function close()
    {
        throw new IllegalStateTransitionException;
    }

    public function lock()
    {
        throw new IllegalStateTransitionException;
    }

    public function unlock()
    {
        throw new IllegalStateTransitionException;
    }
}

OpenDoorState, ClosedDoorState, and LockedDoorState

OpenDoorState (source), ClosedDoorState (source), and LockedDoorState (source) are child classes of AbstractDoorState that overwrite the open(), close(), lock(), and unlock() methods appropriately to return the object that represents the new state. OpenDoorState::close() returns an instance of ClosedDoorState, for instance:

<?php
class OpenDoorState extends AbstractDoorState
{
    public function close()
    {
        return new ClosedDoorState;
    }
}

Door

The Door class (source) maintains a state object (an instance of a subclass of AbstractDoorState) that represents the current state of the door:

<?php
class Door
{
    private $state;

    public function __construct(DoorState $state)
    {
        $this->setState($state);
    }

    public function open()
    {
        $this->setState($this->state->open());
    }

    public function close()
    {
        $this->setState($this->state->close());
    }

    public function lock()
    {
        $this->setState($this->state->lock());
    }

    public function unlock()
    {
        $this->setState($this->state->unlock());
    }

    private function setState(DoorState $state)
    {
        $this->state = $state;
    }
}

The Door class forwards all state-specific messages to this state object. Whenever the door changes state, the Door object changes the state object it uses.

Usage Example

<?php
require __DIR__ . '/src/autoload.php';

$door = new Door(new OpenDoorState);
var_dump($door->isOpen());

$door->close();
var_dump($door->isClosed());

$door->lock();
var_dump($door->isLocked());

$door->lock();

The example script above yields the output below:

bool(true)
bool(true)
bool(true)

Fatal error: Uncaught exception 'IllegalStateTransitionException' in AbstractDoorState.php:25
Stack trace:
#0 Door.php(35): AbstractDoorState->lock()
#1 example.php(13): Door->lock()
#2 {main}
  thrown in AbstractDoorState.php on line 25

Generating the State Machine

Using a code generator, the code shown above can be automatically generated from an XML specification such as the one shown below:

<?xml version="1.0" encoding="UTF-8"?>
<specification>
 <configuration>
  <class         name="Door"/>
  <interface     name="DoorState"/>
  <abstractClass name="AbstractDoorState"/>
 </configuration>
 <states>
  <state name="OpenDoorState"   query="isOpen"/>
  <state name="ClosedDoorState" query="isClosed"/>
  <state name="LockedDoorState" query="isLocked"/>
 </states>
 <transitions>
  <transition from="ClosedDoorState" to="OpenDoorState"   operation="open"/>
  <transition from="OpenDoorState"   to="ClosedDoorState" operation="close"/>
  <transition from="ClosedDoorState" to="LockedDoorState" operation="lock"/>
  <transition from="LockedDoorState" to="ClosedDoorState" operation="unlock"/>
 </transitions>
 <operations>
  <operation name="open"   allowed="canBeOpened"   disallowed="cannotBeOpened"/>
  <operation name="close"  allowed="canBeClosed"   disallowed="cannotBeClosed"/>
  <operation name="lock"   allowed="canBeLocked"   disallowed="cannotBeLocked"/>
  <operation name="unlock" allowed="canBeUnlocked" disallowed="cannotBeUnlocked"/>
 </operations>
</specification>

Visualizing the State Machine

Using static code analysis we can automatically find all child classes of the AbstractDoorState class. By looking at the @return annotations of open(), close(), lock(), and unlock() methods of these classes we can figure the transitions that are allowed from the state represented by these classes.

The visualize.php (source) script implements this approach to generate a representation of the state machine as a directed graph in Dot markup.

Static code analysis is, of course, not required to visualize the state machine as the visualization could also be generated based on the state machine's XML specification (see above). However, generating the visualization based on the actual code has the advantage of also working in the absence of an XML specification when the code is not generated automatically.

Documenting the State Machine through Tests

Using PHPUnit's TestDox functionality we can automatically generate documentation for the state machine based on its tests:

OpenDoor
 [x] Can be closed
 [x] Cannot be opened
 [x] Cannot be locked
 [x] Cannot be unlocked

ClosedDoor
 [x] Cannot be closed
 [x] Can be opened
 [x] Can be locked
 [x] Cannot be unlocked

LockedDoor
 [x] Cannot be closed
 [x] Cannot be opened
 [x] Cannot be locked
 [x] Can be unlocked

This automatically generated checklist makes it clear which transitions are allowed between the three states of the state machine.

sebastianbergmann/state