Aasm stands for "Acts As State Machine" which means that some abstract object can act as a finite state machine and can be in one of a finite number of states at a time; can change one state to another when initiated by a triggering event.

Adding a state machine to a Crystal class is as simple as including AASM module and writing act_as_state_machine method where you can define states and events with their transitions:

class Transaction
  include AASM

  def act_as_state_machine
    aasm.state :pending, initial: true
    aasm.state :active,  enter: -> { puts "Just got activated" }
    aasm.state :completed

    aasm.event :activate do |e|
      e.transitions from: :pending, to: :active
    end

    aasm.event :complete do |e|
      e.transitions from: :active, to: :completed
    end
  end
end

t = Transaction.new.tap &.act_as_state_machine
t.state          #=> :pending
t.next_state     #=> :active
t.fire :activate # Just got activated
t.state          #=> :active
t.next_state     #=> :completed

State can be defined using aasm.state method passing the name and options:

aasm.state :passive, initial: true

Currently state supports the following options:

• initial : Bool optional - indicates whether this state is initial or not. If initial state not specified, first one will be considered as initial
• guard : (-> Bool) optional - a callback, that gets evaluated once state is getting entered. State will not enter if guard returns false
• enter : (->) optional - a hook, that gets evaluated once state entered.

Event can be defined using aasm.state method passing the name and a block with transitions:

aasm.event :delete do |e|
  e.transitions from: :active, to: :deleted
end

Event has to be defined after state definition. In other case NoSuchStateException will be raise.

Currently event supports the following options:

• before : (->) optional - a callback, that gets evaluated once before changing a state
• after : (->) optional - a callback, that gets evaluated once after changing a state.

Transition can be defined on event with transitions method passing options:

aasm.event :complete do |e|
  e.transitions from: [:pending, :active], to: :deleted
end

Currently transition supports the following options:

• from : (Symbol | Array(Symbol)) required - state (or states) from which state of state machine can be changed when event fired
• to : Symbol required - state to which state of state machine will change when event fired.

class CircularStateMachine
  include AASM

  def act_as_state_machine
    aasm.state :started

    aasm.event :restart do |e|
      e.transitions from: :started, to: :started
    end
  end
end

class TwoStateMachine
  include AASM

  def act_as_state_machine
    assm.state :active
    aasm.state :deleted

    aasm.event :delete do |e|
      e.transitions from: :active, to: :deleted
    end
  end
end

class ThreeStatesMachine
  include AASM

  def act_as_state_machine
    aasm.state :pending, initial: true
    aasm.state :active
    aasm.state :completed

    aasm.event :activate do |e|
      e.transitions from: :pending, to: :active
    end
    aasm.event :complete do |e|
      e.before { puts "completing..." }
      e.after  { puts "completed" }
      e.transitions from: [:active, :pending], to: :completed
    end
  end
end