State Manager

This provides a controlled way of managing a state object/array, and being notified when parts of it have changed. It ensures that state updates are atomic, meaning change subscribers are only notified of changes when the state has been updated completely. Subscribers also get the latest state and changes whenver they are invoked.

It requires Proxy support. If you are running on an environment where this is unavailable I'd recommend GoogleChrome/proxy-polyfill. You can provide the implementation on the Proxy config option if you don't want to polyfill. Take note of the differences in behaviour to real Proxy's in the README.

Subscribers receive a hasChanged function which when called with a property path will return true if something at or below the provided path has changed since the subscriber was last invoked.

Subscribers are also able to update the state. Earlier subscribers see the changes from later subscribers. The last subsciber won't see the intermediary changes.

Changes to values on plain objects (created by the Object constructor) or arrays, and nested plain objects or arrays in the state are detected (as these are proxied). Values inside other types of objects are not watched.

Installation

npm install --save @tjenkinson/state-manager

or available on JSDelivr at "https://cdn.jsdelivr.net/npm/@tjenkinson/state-manager@4".

API

Constructor

Provide the initial state as the first argument. You must not mutate this state directly. You can get a reference to a mutable version using the getState() method.

The second argument is an optional object which can contain the following properties:

• beforeUpdate: This is called after the first update() call but before the callback. It receives the state as the first argument and you are allowed to update it.
• afterUpdate: This is called after an update occurs after the last subscriber has finished. It receives an object in the first argument with the following:
  • state: A Proxy to the current state.
  • exceptionOccurred: This is a boolean which is true if an exception occured in one or more of the subscribers.
  • retrieveExceptions: This returns an array of exceptions that occurred in one of more of the subscribers. If you do not call this function the exceptions will be thrown asynchronously when the current stack ends. If you do call this function the exceptions will not be thrown and it's up to you to handle them.

You can also provide the type of the state as a generic. E.g. new StateManager<StateType>.

const initialState = { a: 1, b: 'something' };
const stateManager = new StateManager(initialState, {
  // optional
  beforeUpdate: (state) => {},
  afterUpdate: ({ state, exceptionOccurred, retrieveException }) => {},
});

getState()

Returns a mutable version of the state. This is not a snapshot. The object you get back is a Proxy to the original state. Properties to plain objects or arrays are also Proxy's.

If you are updating multiple properties, you should use update instead to batch the updates together and only notify subscribers once all updates are done.

stateManager.getState();

hasChanged(...propertyPath)

Informs you if the thing at the given property path or below has changed. This can be useful when you are subscribing if you want to catch up with changes you missed.

stateManager.hasChanged(...propertyPath);

update(fn)

This is how you update the state.

The first argument takes a function that will be invoked synchronously and provided with a Proxy to the current state as the first argument. To make changes to the state just update the object. It is possible to have multiple nested update calls and the subscribers will only be invoked when all calls have completed.

stateManager.update((state) => {
  // update `state` here
});

It is possible to omit function, which is only useful if beforeUpdate makes changes to the state.

The return value is passed through.

subscribe(fn)

This is how you are notified of changes to the state.

The first argument taked a function which is invoked with 2 arguments:

• hasChanged: This is a function which takes a property path. It returns true if something at or below the provided path has changed since the subscriber was last invoked. E.g. hasChanged('a', 'b') for checking state.a.b.
• state: This is a mutable Proxy to the current state.

You are allowed to update the state again from your subscriber.

Subscribers are invoked in the order they were registered. If a subscriber changes the state the first subscriber will be invoked again. This means earlier subscribers see the changes from later subscribers. The last subscriber won't see the intermediary changes.

An object is returned that contains a remove function. Call this to unsubscribe.

If your subscriber throws an error it will not prevent other subscribers being invoked. The afterUpdate function will have an oppurtunity to handle the exception, or it will be rethrown asynchronously.

Example

import { StateManager } from '@tjenkinson/state-manager';

const stateManager = new StateManager({ a: 1, b: 2, c: { d: 3, e: 4 } });

stateManager.subscribe((hasChanged, state) => {
  if (hasChanged('a')) {
    console.log(`subscriber1 a=${state.a}`);
  }
});

stateManager.subscribe((hasChanged, { b }) => {
  if (hasChanged('b')) {
    console.log(`subscriber2 b=${b}`);
    if (b === 3) {
      stateManager.update((state) => (state.a = 1));
    }
  }
});

stateManager.subscribe((hasChanged, state) => {
  if (hasChanged('c', 'e')) {
    console.log(`subscriber3 c.e=${state.c.e}`);
  }
});

stateManager.subscribe((hasChanged, state) => {
  console.log(`subscriber4 state=${JSON.stringify(state)}`);
});

stateManager.update((state) => {
  state.a = 100;
  stateManager.update((state2) => {
    state2.a = 2;
  });
});

// just before reaching this point there the following would be logged
// - subscriber1 a=2
// - subscriber4 state={"a":2,"b":2",c:{"d":3,"e":4}}

stateManager.update((state) => {
  state.b = 3;
  state.c.e = 5;
});

// just before reaching this point there the following would be logged
// - subscriber2 b=3
// - subscriber1 a=1
// - subscriber3 c.e=5
// - subscriber4 state={"a":1,"b":3",c:{"d":3,"e":5}}