state_machine

Dynamic Definition

There may be cases where the definition of a state machine is dynamic so you can't know the possible states or events for a machine until runtime. For example, users in your application may need to manage the state machine of a project or a task. Further, transitions (and their associated states / events) might need to be defined dynamically by an external source like admins.

To define dynamically-generated state machines:

class Vehicle
  attr_accessor :state

  # Make sure the machine gets initialized so the initial state gets set properly
  def initialize(*)
    super
    machine
  end

  # Replace this with an external source (like a db)
  def transitions
    [
      {:parked => :idling, :on => :ignite},
      {:idling => :first_gear, :first_gear => :second_gear, :on => :shift_up}
      # ...
    ]
  end

  # Create a state machine for this vehicle instance dynamically based on the
  # transitions defined from the source above
  def machine
    vehicle = self
    @machine ||= Machine.new(vehicle, :initial => :parked, :action => :save) do
      vehicle.transitions.each {|attrs| transition(attrs)}
    end
  end

  def save
    # Save the state change...
    true
  end
end

# Generic class for building machines
class Machine
  def self.new(object, *args, &block)
    machine_class = Class.new
    machine = machine_class.state_machine(*args, &block)
    attribute = machine.attribute
    action = machine.action

    # Delegate attributes
    machine_class.class_eval do
      define_method(:definition) { machine }
      define_method(attribute) { object.send(attribute) }
      define_method("#{attribute}=") {|value| object.send("#{attribute}=", value) }
      define_method(action) { object.send(action) } if action
    end

    machine_class.new
  end
end

vehicle = Vehicle.new                   # => #<Vehicle:0xb708412c @state="parked" ...>
vehicle.state                           # => "parked"
vehicle.machine.ignite                  # => true
vehicle.machine.state                   # => "idling
vehicle.state                           # => "idling"
vehicle.machine.state_transitions       # => [#<StateMachine::Transition ...>]
vehicle.machine.definition.states.keys  # => :first_gear, :second_gear, :parked, :idling