ash_state_machine/lib/ash_state_machine.ex

defmodule AshStateMachine do
  defmodule Transition do
    @moduledoc """
    The configuration for an transition.
    """
    @type t :: %__MODULE__{
            action: atom,
            from: [atom],
            to: [atom]
          }

    defstruct [:action, :from, :to]
  end

  @transition %Spark.Dsl.Entity{
    name: :transition,
    target: Transition,
    args: [:action],
    schema: [
      action: [
        type: :atom,
        doc:
          "The corresponding action that is invoked for the transition. Use `:*` to allow any update action to perform this transition."
      ],
      from: [
        type: {:or, [{:list, :atom}, :atom]},
        required: true,
        doc:
          "The states in which this action may be called. If not specified, then any state is accepted."
      ],
      to: [
        type: {:or, [{:list, :atom}, :atom]},
        required: true,
        doc:
          "The states that this action may move to. If not specified, then any state is accepted."
      ]
    ]
  }

  @transitions %Spark.Dsl.Section{
    name: :transitions,
    entities: [
      @transition
    ]
  }

  @state_machine %Spark.Dsl.Section{
    name: :state_machine,
    schema: [
      deprecated_states: [
        type: {:list, :atom},
        default: [],
        doc: """
        A list of states that have been deprecated.
        The list of states is derived from the transitions normally.
        Use this option to express that certain types should still
        be included even though no transitions go to/from that state anymore.
        """
      ],
      state_attribute: [
        type: :atom,
        doc: "The attribute to store the state in.",
        default: :state
      ],
      initial_states: [
        type: {:list, :atom},
        required: true,
        doc: "The allowed starting states of this state machine."
      ],
      default_initial_state: [
        type: :atom,
        doc: "The default initial state"
      ]
    ],
    sections: [
      @transitions
    ]
  }

  @sections [@state_machine]

  @moduledoc """
  Functions for working with AshStateMachine.
  <!--- ash-hq-hide-start --> <!--- -->

  ## DSL Documentation

  ### Index

  #{Spark.Dsl.Extension.doc_index(@sections)}

  ### Docs

  #{Spark.Dsl.Extension.doc(@sections)}
  <!--- ash-hq-hide-stop --> <!--- -->
  """

  use Spark.Dsl.Extension,
    sections: @sections,
    transformers: [
      AshStateMachine.Transformers.FillInTransitionDefaults,
      AshStateMachine.Transformers.AddState,
      AshStateMachine.Transformers.EnsureStateSelected
    ],
    verifiers: [
      AshStateMachine.Verifiers.VerifyTransitionActions,
      AshStateMachine.Verifiers.VerifyDefaultInitialState
    ],
    imports: [
      AshStateMachine.BuiltinChanges
    ]

  @doc """
  A utility to transition the state of a changeset, honoring the rules of the resource.
  """
  def transition_state(%{action_type: :update} = changeset, target) do
    transitions =
      AshStateMachine.Info.state_machine_transitions(changeset.resource, changeset.action.name)

    attribute = AshStateMachine.Info.state_machine_state_attribute!(changeset.resource)
    old_state = Map.get(changeset.data, attribute)

    case Enum.find(transitions, fn transition ->
           old_state in List.wrap(transition.from) and target in List.wrap(transition.to)
         end) do
      nil ->
        Ash.Changeset.add_error(
          changeset,
          AshStateMachine.Errors.NoMatchingTransition.exception(
            old_state: old_state,
            target: target,
            action: changeset.action.name
          )
        )

      _transition ->
        Ash.Changeset.force_change_attribute(changeset, attribute, target)
    end
  end

  def transition_state(%{action_type: :create} = changeset, target) do
    attribute = AshStateMachine.Info.state_machine_state_attribute!(changeset.resource)

    if target in AshStateMachine.Info.state_machine_initial_states!(changeset.resource) do
      Ash.Changeset.force_change_attribute(changeset, attribute, target)
    else
      Ash.Changeset.add_error(
        changeset,
        AshStateMachine.Errors.InvalidInitialState.exception(
          target: target,
          action: changeset.action.name
        )
      )
    end
  end

  def transition_state(other, _target) do
    Ash.Changeset.add_error(other, "Can't transition states on destroy actions")
  end
end