Description

Finite State Machine module for Nest.

Getting started

Install package:

$ npm i --save @depthlabs/nestjs-state-machine

For example, we will map the following state machine:

After installation, import StateMachineModule into your root module with state machine graph configurations:

// app.module.ts
import { StateMachineModule } from '@depthlabs/nestjs-state-machine';

@Module({
  imports: [
    // .forRoot takes array of graph configurations
    StateMachineModule.forRoot([
        {
            // Name of graph
            name: 'project-graph',
            // Initial state of graph
            initialState: 'new',
            // Available states in graph
            states: [
                'new',
                'in-progress',
                'done'
            ],
            // Available transitions in graph
            transitions: [
                {
                    // Name of transistion
                    name: 'start',
                    // Source states
                    from: ['new'],
                    // Target state of transition
                    to: 'in-progress',
                },
                {
                    name: 'finish',
                    from: ['in-progress'],
                    to: 'done',
                }
            ],
        },
    ]),
  ]
})
export class AppModule {}

It's good idea to define graph, state and transition names in one place e.g.:

// constants.ts
export const PROJECT_GRAPH = 'project-graph'

export enum ProjectState {
    NEW = 'new',
    IN_PROGRESS = 'in-progress',
    DONE = 'done'
}

export enum ProjectTransition {
    START = 'start',
    FINISH = 'finish'
}

and then:

import { StateMachineModule } from '@depthlabs/nestjs-state-machine';
import { PROJECT_GRAPH, ProjectState, ProjectTransition } from './constants';

// ...

StateMachineModule.forRoot([
    {
        name: PROJECT_GRAPH,
        initialState: ProjectState.NEW,
        states: [
            ProjectState.NEW,
            ProjectState.IN_PROGRESS,
            ProjectState.DONE
        ],
        transitions: [
            {
                name: ProjectTransition.START,
                from: [ProjectState.NEW],
                to: ProjectState.IN_PROGRESS,
            },
            {
                name: ProjectTransition.FINISH,
                from: [ProjectState.IN_PROGRESS],
                to: ProjectState.DONE,
            }
        ],
    }
]);

Next, create model or use your exisiting model and decorate property which will be responsible for storing state of model:

// project.model.ts
import { StateStore } from '@depthlabs/nestjs-state-machine';
import { PROJECT_GRAPH, ProjectState } from './constants';

export class Project {
    name: string;

    @StateStore(PROJECT_GRAPH)
    state: string = ProjectState.NEW;

}

@StateStore takes one argument with graph name (string). Thanks to this one model can handle more then many graphs:

@StateStore(PROJECT_GRAPH)
state: string = ProjectState.NEW;

@StateStore(PROJECT_STATUS_GRAPH)
status: string = ProjectStatusState.ACTIVE;

At this point we can create our state machine. First inject StateMachineFactory using standard constructor injection:

import { StateMachineFactory } from '@depthlabs/nestjs-state-machine';

// ...

constructor(
    private readonly stateMachineFactory: StateMachineFactory,
) {}

Create state machine with instance of Project model as subject in first argument of factory and with graph name in second.

const projectStateMachine = this.stateMachineFactory.create<Project>(project, PROJECT_GRAPH)

State Machine methods

Apply transition:

// takes transition name in argument
await projectStateMachine.apply(ProjectTransition.START)
// return void but project.state is now equal ProjectState.IN_PROGRESS

Check if transition is possible:

// takes transition name in argument
await projectStateMachine.can(ProjectTransition.START)
// return true or false, can() don't throw Errors

Get all possible transitions:

await projectStateMachine.getAvailableTransitions()
// return TransitionInterface[];

Guards

You can create guards to block transitions.
To declare an Guard, decorate a method with the @OnGuard() decorator:

import { GuardEvent, OnGuard } from '@depthlabs/nestjs-state-machine';
import { ProjectTransition, PROJECT_GRAPH } from '../constance';
import { Project } from '../project.model';

export class ProjectCantBeNamedBlockmeGuard {

    // Graph name in first argument, transition name in second
    @OnGuard(PROJECT_GRAPH, ProjectTransition.START)
    handle(event: GuardEvent<Project>) {
        // event.subject is our Project instance
        if (event.subject.name == 'blockme') { // if name isn't allowed for some reasons
            event.setBlocked('transition-blocked'); // block transition using setBlocked() method
        }
    }

}

Than you need to register ProjectCantBeNamedBlockmeGuard in module as provider:

@Module({
    imports: [
        StateMachineModule.forRoot([
            // ...
        ])
    ],
    providers: [
        ProjectCantBeNamedBlockmeGuard
    ],
})
export class AppModule {}

Now, if you create StateMachine instance and try to apply START transition you will get:

const project = new Project()
project.name = 'blockme'

const projectStateMachine = this.stateMachineFactory.create<Project>(project, PROJECT_GRAPH);

await projectStateMachine.can(ProjectTransition.START)
// false

await projectStateMachine.apply(ProjectTransition.START)
// Throw TransitionBlockedByGuardException with .blockingReasons property that contain ['transition-blocked']

projectStateMachine.getAvailableTransitions()
// [] - empty

Transition Events

You can create transition event listeners to do additional actions when a state machine operation happened (e.g. sending emails, recalculate)

When a state transition is initiated, the events are dispatched in the following order:

Order	Event	Decorator	Decorator second argument
1	LeaveState (The subject is about to leave a place).	OnLeaveState	State name
2	BeginTransition (The subject is going through this transition.)	OnBeginTransition	Transition name
3	EnterState (The subject is about to enter a new place. This event is triggered right before the subject places are updated.)	OnEnterState	State name
4	-> Change of state
5	EnteredState (The subject has entered in the places and the marking is updated.)	OnEnteredState	State name
6	CompletedTransition (The object has completed this transition.)	OnCompletedTransition	Transition name
7	AnnounceTransitions (Triggered for each transition that now is accessible for the subject.)	OnAnnounceTransitions	State name

Example:

import { OnCompletedTransition, CompletedTransitionEvent } from '@depthlabs/nestjs-state-machine';
import { ProjectTransition, PROJECT_GRAPH } from '../constance';
import { Project } from '../project.model';

export class NotifyTeamAboutFinishedTask {

    // Graph name in first argument, transition name in second. Third if truem method is async.
    @OnCompletedTransition(PROJECT_GRAPH, ProjectTransition.FINISH, true)
    async handle(event: CompletedTransitionEvent<Project>) {
        // Send emails to all users in team
    }

}

and of course:

@Module({
    // ...
    providers: [
        NotifyTeamAboutFinishedTask
    ],
    // ...
})
export class AppModule {}

License

Nestjs State Machine is MIT licensed.