Simple State Machine

A lightweight and flexible state management library designed from scratch in Typescript, for modern JavaScript and TypeScript applications. The simple-state-machine follows a command-based architecture, making it easy to manage and manipulate application state in a structured manner.

Features

• Command Pattern: Encapsulate state-changing logic in isolated, testable command objects. This allows you to separate business logic from UI code.
• Singleton State Management: Centralize application state with a singleton pattern for easy access and management.
• Type-Safe API: Leverages TypeScript for strong typing and compile-time safety.
• Observable State: Integrates with observables for state subscription and reactivity.
• Framework-Agnostic: Works seamlessly with React, Angular, Node.js, or plain JavaScript.

Installation

Install the library via npm:

npm install @state-management/simple-state-machine

OR

yarn add @state-management/simple-state-machine

For Angular Applications

For Angular applications use the Angular wrapper which provides it as an injectable module:

npm install @state-management/ngx-state-machine

OR

yarn add @state-management/ngx-state-machine

For ReactJs and React Native Applications

For react and react native applications use the react wrapper which provides it react hooks:

npm install @state-management/state-machine-react

OR

yarn add @state-management/state-machine-react

Usage

StateKeys.constants.ts

A sample constants file for all state keys,

import { StateKey } from '@state-management/simple-state-machine';

# NOTE: the generics in the StateKey defines the data type of the value stored against this key.
export const CounterKey = new StateKey<number>('Counter');

IncrementCounterCommand.ts

A sample Command class, contains application logic, and it updates the state with output.

import { Command } from '@state-management/simple-state-machine';
import { CounterKey } from './StateKeys.constants';

# NOTE: the generics "<number>" here defines the data type of the execution context, that is the parameter passed to the "execute" method.
export class IncrementCounterCommand extends Command<number> {
  execute(incrementBy:number): void {
    const currentValue = this.getLatest(CounterKey) || 0;
    
    # NOTE: ONLY command can call "putState" to modify the state.
    # The data type of the value must match the generics of the Key.       
    this.putState(CounterKey, currentValue + incrementBy);
  }
}

Usage in application - Dispatch Command and Change State

import { StateMachine } from '@state-management/simple-state-machine';

const stateMachine = StateMachine.getInstance();

# dispatch the command object along with the parameter object it operates on, 
# in this case its a number.  The datatype of the parameter must match the generics of the Command class.
stateMachine.dispatch(new IncrementCounterCommand(1));

Usage in application - Observe state change

import { StateMachine } from '@state-management/simple-state-machine';
import { CounterKey } from './pathTo/StateKeys.constants';

const stateMachine = StateMachine.getInstance();

stateMachine.onChange(CounterKey, value => {
  console.log(value);
});

Usage in application - Quick state change

Quickly update state without creating a new Command Object.

import { StateMachine, UpdateStateCommand} from '@state-management/simple-state-machine';
import { CounterKey } from './pathTo/StateKeys.constants';

const stateMachine = StateMachine.getInstance();

# set the initial value of the counter.
stateMachine.dispatch(new UpdateStateCommand({stateKey: CounterKey, value: 0}));

# Please Note:  For easy tracing and debugging it is recommended, NOT to re-use a command class.
# For example the initial value of the "CounterKey" in this example can be set from, say 
#   a. application load
#   b. click of a reset button.
# It is recommended that, for both scenarios, use a different command object, which can call 
# the same "service" class containing the logic to set the initial value

API Documentation

This section provides detailed documentation for the core classes in the @state-management/simple-state-machine library: Command and StateMachine.

Command<P> Class

The Command class is an abstract base class that encapsulates business logic to interact with and modify the global state. It uses the Command Pattern to separate application logic from state management.
You will be extending this class to create multiple commands to be dispatched using StateMachine.
The generics <P> defines the data type of the execution context, the parameter to the command's "execute" method.

Constructor

constructor(executionContext: P)

Initializes a new Command instance with the provided execution context.

| Parameter | Type | Description | |--------------------|--------|-----------------------------------------------------------------------------------------------------------------------| | executionContext | P | The parameter required for executing the command. The type of parameter is generic, <P> is defined at class level. |

Methods

protected putState<T>(key: StateKey<T>, value: T): void

Stores a value in the global state and makes it observable.

| Parameter | Type | Description | |-----------|-----------------|-----------------------------------------------------| | key | StateKey<T> | The key associated with the state value. | | value | T | The value to store in the state. |

getLatest<T>(key: StateKey<T>): T | undefined

Retrieves the latest value associated with the given key from the global state.

| Parameter | Type | Description | |-----------|-----------------|------------------------------------| | key | StateKey<T> | The key associated with the state. |

| Returns | Type | Description | |---------|-----------------|----------------------------------------------| | T | T \| undefined| The latest state value or undefined. |

// you can write the following inside a command class.
const value = this.getLatest(someKey);
console.log('Latest value:', value);

abstract execute(executionContext: P): void

Defines the application logic for the command. This method must be implemented in subclasses. This method is called by the StateMachine when you dispatch a command.
It would perform the application logic and will set/change the state.

| Parameter | Type | Description | |--------------------|--------|----------------------------------------------------------------------------------------------------------------------| | executionContext | P | The parameter required for executing the command. The type of parameter is generic, <P> is defined at class level. |

StateMachine Class

The StateMachine class provides the core API for managing global state. It follows the Singleton Pattern and offers methods to dispatch commands and observe or retrieve state values.

Static Methods

static getInstance(): StateMachine

Retrieves the singleton instance of the StateMachine. You can use this to dispatch commands and to observe state changes.

| Returns | Type | Description | |---------|--------------|----------------------------------| | StateMachine | The singleton instance of StateMachine. |

Methods

dispatch<T>(command: Command<T>): void

Executes the Command, invoking its execute method. This is the only way to modify the global state.

| Parameter | Type | Description | |-----------|--------------|--------------------------------------------------------------------| | command | Command<T> | The command to be executed, encapsulating the application logic. |

Example:

import { StateMachine } from '@state-management/simple-state-machinee';
import { IncrementCounterCommand } from './commands/incrementCounterCommand';

const stateMachine = StateMachine.getInstance();
stateMachine.dispatch(new IncrementCounterCommand(1));

onChange<T>(key: StateKey<T>, onChange: (value: T) => void): Subscription

Convenience method to subscribe to changes for a specific state key. It triggers the onChange callback whenever the value associated with the state key changes.
This method can be used even if the key does not yet exist in the state.

| Parameter | Type | Description | |------------|-----------------------|--------------------------------------------------------------| | key | StateKey<T> | The key associated with the state. | | onChange | (value: T) => void | Callback function that gets triggered when the state changes.|

| Returns | Type | Description | |------------|-----------------------|--------------------------------------------------------------| | Subscription | A subscription to manage the observer lifecycle. Use unsubscribe() to stop observing. |

Example:

import { StateMachine, StateKey } from '@state-management/simple-state-machine';

const CounterKey = new StateKey<number>('counter');
const stateMachine = StateMachine.getInstance();

const subscription = stateMachine.onChange(CounterKey, (newValue) => {
    console.log('Counter updated:', newValue);
});

// to stop observing
subscription.unsubscribe();

observe<T>(key: StateKey<T>): Observable<T>

Returns an Observable to observe changes to the value associated with the given key. This method can be used even if the key does not yet exist in the state.

| Parameter | Type | Description | |-----------|----------------|--------------------------------------| | key | StateKey<T> | The key associated with the state. |

| Returns | Type | Description | |---------|---------------|------------------------------------| | Observable<T> | Emits state updates for the given key. |

Example:

const subscription = stateMachine.observe(someKey).subscribe((value) => {
    console.log('State updated:', value);
});
subscription.unsubscribe();

UpdateStateCommand Class

UpdateStateCommand<T> extends Command<UpdateStateParam<T>>

• Convenience class to quickly update the state without creating a new command object.
• It can be used for one-off initialization of state or a one-off state change.

Example:

stateMachine.dispatch(new UpdateStateCommand({stateKey: CounterKey, value: 0}));

Note: For easy tracing and debugging, do not re-use the same command class to make state changes from different parts of the application.

In this example the initial value of the "CounterKey" in this example can be set from, say

• Application load
• Click of a reset button.

It is recommended that, for both scenarios, use a different command object, which can call the same "service" class containing the logic to set the initial value.

Contributing

We welcome contributions! Please open an issue or submit a pull request if you’d like to improve the library.

How to Contribute

1. Fork the Repository:

• Visit the simple-state-machine GitHub repository.
• Click the "Fork" button to create a copy of the repository under your GitHub account.

2. Clone the Fork:

git clone https://github.com/state-management/simple-state-machine.git
cd simple-state-machine

3. Create a Feature Branch:

git checkout -b feature/add-simple-state-machine-feature

4. Make Your Changes:

Add or update code, write tests, and ensure the changes are well-documented. Run tests locally, ensure all existing and new tests pass

npm install
npm test

5. Commit and Push Your Changes:

Write a clear and concise commit message:

git add .
git commit -m "Add new feature to simple-state-machine"
git push origin feature/add-simple-state-machine-feature

6. Create a Pull Request:

Go to your fork on GitHub and click the “New Pull Request” button. Provide a description of your changes and any additional context.