react-state-object

react-state-object is a small library for building class-based React state that is designed to work primarily with MobX.

It gives you:

• a base class (ReactStateObject) with mount/unmount lifecycle hooks
• automatic lifecycle propagation through nested child state objects
• a way to use React hooks inside class accessors (@fromHook(...))
• a simple class-instance injection system for React trees
• a convenience decorator for injected state accessors (@injectInstance(...))

If you are new to MobX, this README teaches the basics first and then shows how react-state-object fits on top.

Who This Is For

This library is a good fit when you:

• like class-based state models
• use MobX for observability/computed values/actions
• want React lifecycle ownership (mount/unmount) for those classes
• want to inject shared state instances without wiring dozens of props

This library is not a replacement for MobX. It is a small layer that helps MobX state objects live inside a React app in a structured way.

Install

npm install react-state-object mobx react

You will usually also want:

npm install mobx-react-lite

mobx-react-lite is not a peer dependency of this package, but it is the most common way to make React components re-render when MobX observables change.

Peer Dependencies

• react >=18
• mobx >=6.0.0 <7

What MobX Does (Beginner-Friendly)

MobX is a state management library built around a simple idea:

• mark values as observable
• derive values with computed getters
• change values in action methods
• make React components observer(...) so they re-render when observables they read change

Tiny MobX example (without react-state-object yet)

import { action, computed, observable } from 'mobx';
import { observer } from 'mobx-react-lite';

class CounterStore {
  @observable accessor count = 0;

  @computed get doubled(): number {
    return this.count * 2;
  }

  @action increment(): void {
    this.count += 1;
  }
}

const counter = new CounterStore();

export const Counter = observer(() => {
  return (
    <div>
      <p>Count: {counter.count}</p>
      <p>Doubled: {counter.doubled}</p>
      <button onClick={() => counter.increment()}>
        Increment
      </button>
    </div>
  );
});

This works, but it leaves open questions:

• When should the store set up subscriptions?
• When should it clean them up?
• How do we structure nested state objects?
• How do we use React hooks in class state?
• How do we avoid prop-drilling shared state?

That is where react-state-object helps.

Mental Model: MobX + react-state-object

Think of a ReactStateObject as:

• a MobX-powered class state model
• whose lifecycle is controlled by a React component
• and which may contain child state objects that mount / unmount automatically

Typical flow:

1. React component creates the class via useMountStateObject(() => new MyState())
2. React owns the instance lifetime
3. The state object receives mount() on component mount
4. The state object receives unmount() on component unmount
5. Nested child ReactStateObjects are mounted/unmounted automatically

This pattern is common in production apps for app layout state, environment observers (window size / element size), page-level state, and feature-specific state trees.

Quick Start (Recommended First Example)

This example shows the full intended pattern with MobX.

import {
  action,
  autorun,
  computed,
  observable,
} from 'mobx';
import { observer } from 'mobx-react-lite';
import {
  ReactStateObject,
  useMountStateObject,
} from 'react-state-object';

class CounterState extends ReactStateObject {
  @observable accessor count = 0;

  @computed get doubled(): number {
    return this.count * 2;
  }

  protected mount(): void {
    // Runs once when the React component mounts.
    this.withCleanup(() => {
      // `autorun` returns a disposer. `withCleanup` stores it
      // and calls it automatically on unmount.
      return autorun(() => {
        console.log('count is', this.count);
      });
    });
  }

  @action increment(): void {
    this.count += 1;
  }

  @action decrement(): void {
    this.count -= 1;
  }
}

export const CounterScreen = observer((): JSX.Element => {
  const state = useMountStateObject(
    () => new CounterState()
  );

  return (
    <div>
      <p>Count: {state.count}</p>
      <p>Doubled: {state.doubled}</p>
      <button onClick={() => state.decrement()}>-</button>
      <button onClick={() => state.increment()}>+</button>
    </div>
  );
});

Core Concepts

ReactStateObject

Base class for state objects that need lifecycle and child-state propagation.

class MyState extends ReactStateObject {
  protected mount(): void {
    // setup subscriptions, observers, timers, etc.
  }

  protected unmount(): void {
    // optional manual cleanup if needed
  }
}

What it adds on top of a plain MobX class

• mount() and unmount() hooks
• nested child state object detection and recursive mount/unmount
• withCleanup(...) helper to register disposer functions
• hookIntoLifecycle(...) for advanced lifecycle registration

Automatic child lifecycle propagation

In larger apps, state objects often contain smaller ones (for example, a layout state containing an element-size observer, a sidebar state object, and other focused child state objects).

If a property value is another ReactStateObject, it is considered a child and will mount/unmount automatically.

class FiltersState extends ReactStateObject {
  @observable accessor query = '';
}

class TableState extends ReactStateObject {
  @observable accessor filters = new FiltersState();
}

// Mounting `TableState` also mounts `filters`.

This makes it easy to create a tree of state objects without manually coordinating lifecycle for every child.

useMountStateObject(factory)

This is the React hook that creates and owns a ReactStateObject instance.

const state = useMountStateObject(() => new MyState());

What it does

• creates the instance once per component instance
• records any React hooks used by @fromHook(...) decorators during initialization
• replays those hooks on subsequent renders to preserve hook order
• calls the state object lifecycle (mount / unmount)

Important rule (always)

A ReactStateObject should never be initialized outside of useMountStateObject(...) when used from React.

Always create it with:

const state = useMountStateObject(() => new MyState());

Do not create a ReactStateObject with plain new inside a React component (including useMemo, useRef, module-level singletons used as UI state, or conditional branches).

Lifecycle Helpers

withCleanup(...) (most common)

This is the main lifecycle helper you will use.

It runs setup logic now and stores the returned cleanup function for unmount.

protected mount(): void {
  this.withCleanup(() => {
    const id = window.setInterval(() => {
      console.log('tick');
    }, 1000);

    return () => {
      window.clearInterval(id);
    };
  });
}

This pattern is commonly used for:

• autorun(...) disposers
• DOM event listeners (resize, etc.)
• subscriptions / observables

hookIntoLifecycle(...) (advanced)

This is a lower-level API that lets you register mount / unmount callbacks directly.

constructor() {
  super();

  this.hookIntoLifecycle({
    onMount: () => {
      console.log('mounted');
    },
    onUnmount: () => {
      console.log('unmounted');
    },
  });
}

In most apps, withCleanup(...) is the primary pattern; hookIntoLifecycle(...) exists for more custom composition scenarios.

MobX + React Rendering (How UI Updates)

react-state-object does not automatically make your component reactive. React components still need to be wrapped with MobX's observer(...) (or use another MobX React integration pattern).

import { observer } from 'mobx-react-lite';

const UserPanel = observer(() => {
  const state = useMountStateObject(() => new UserState());

  // Reading observables here makes this component react.
  return <div>{state.userName}</div>;
});

If you read MobX observables in a component that is not an observer, React usually will not re-render when they change.

Using React Hooks Inside a Class (@fromHook(...))

A major feature of this library is the ability to bind a class accessor to a React hook result.

This is useful when your state object needs data from a React hook such as:

• routing hooks (useParams, useLocation, usePage)
• context hooks
• feature hooks that return callbacks/services

Basic example

import { observable } from 'mobx';
import { useLocation } from 'react-router-dom';
import {
  fromHook,
  ReactStateObject,
} from 'react-state-object';

class RouteState extends ReactStateObject {
  @fromHook(() => useLocation())
  @observable
  accessor location!: ReturnType<typeof useLocation>;
}

Example with this access (common app pattern)

@fromHook(...) can receive a function that uses the state object instance as this.

This is useful when the hook needs the state object.

class ModalState extends ReactStateObject {
  @fromHook(function (this: ModalState) {
    return useModalLauncher(this);
  })
  @observable
  accessor openModal!: () => void;
}

This pattern is useful when a feature state object binds a hook-derived callback or service that needs access to the state instance.

Rules for @fromHook(...)

• Use it on accessor properties.
• Create the state object with useMountStateObject(...).
• Keep hook usage stable (same hooks in same order across renders), just like normal React rules.
• Typically combine it with @observable when you want MobX reactivity on the accessor value.

Why hook-backed accessors are excluded from child mount recursion

The library does not treat @fromHook(...) accessors as child ReactStateObjects, because those values are managed by React hooks, not by the parent state object's child lifecycle traversal.

This keeps hook-managed values under React's lifecycle ownership instead of treating them like nested child state objects.

Class Instance Injection (DI) for React Trees

This library includes a simple class-instance injection system. It lets you bind an instance in React and then retrieve it later by class.

This is a strong fit for sharing app-level and page-level state (for example layout state, route/page observers, window-size observers, and feature state) without prop drilling.

InstanceInjectionRoot

This provider stores the internal registry of class -> React context mappings.

Put it near the top of your app (once).

<InstanceInjectionRoot>
  <App />
</InstanceInjectionRoot>

BindInstanceForInjection

Binds a specific class instance for descendants.

<BindInstanceForInjection instance={appState}>
  <Children />
</BindInstanceForInjection>

You can nest these to provide multiple state objects.

<InstanceInjectionRoot>
  <BindInstanceForInjection instance={appState}>
    <BindInstanceForInjection instance={sessionState}>
      <MyScreen />
    </BindInstanceForInjection>
  </BindInstanceForInjection>
</InstanceInjectionRoot>

useInjectInstance(MyClass)

Reads the nearest instance bound for a class. Throws if missing.

import { observer } from 'mobx-react-lite';
import { useInjectInstance } from 'react-state-object';

const Header = observer(() => {
  const appState = useInjectInstance(AppState);

  return <h1>{appState.title}</h1>;
});

useInjectInstanceOrNull(MyClass)

Same as useInjectInstance(...), but returns null instead of throwing if no bound instance exists on the current branch.

const maybeAppState = useInjectInstanceOrNull(AppState);

@injectInstance(...) (Decorator)

@injectInstance(...) is a convenience decorator built on @fromHook(...). It injects an instance into a class accessor.

This lets one ReactStateObject depend on another without manually calling hooks in component code.

import { observable } from 'mobx';
import {
  injectInstance,
  ReactStateObject,
} from 'react-state-object';

class AppState extends ReactStateObject {
  @observable accessor title = 'Dashboard';
}

class PageState extends ReactStateObject {
  @injectInstance(AppState)
  @observable
  accessor appState!: AppState;
}

This is a common pattern in larger apps, where feature state objects inject app-level or page observer state objects.

End-to-End Example: App State + Page State + UI

This example combines MobX, useMountStateObject, injection, and observer(...).

import { action, computed, observable } from 'mobx';
import { observer } from 'mobx-react-lite';
import {
  BindInstanceForInjection,
  injectInstance,
  InstanceInjectionRoot,
  ReactStateObject,
  useInjectInstance,
  useMountStateObject,
} from 'react-state-object';

class AppState extends ReactStateObject {
  @observable accessor appName = 'School Portal';
}

class CounterPageState extends ReactStateObject {
  @injectInstance(AppState)
  @observable
  accessor appState!: AppState;

  @observable accessor count = 0;

  @computed get title(): string {
    return `${this.appState.appName} (${this.count})`;
  }

  @action increment(): void {
    this.count += 1;
  }
}

const CounterPageBody = observer(() => {
  const pageState = useInjectInstance(CounterPageState);

  return (
    <div>
      <h2>{pageState.title}</h2>
      <button onClick={() => pageState.increment()}>
        Increment
      </button>
    </div>
  );
});

const CounterPage = observer(() => {
  const pageState = useMountStateObject(
    () => new CounterPageState()
  );

  return (
    <BindInstanceForInjection instance={pageState}>
      <CounterPageBody />
    </BindInstanceForInjection>
  );
});

export const App = () => {
  const appState = useMountStateObject(
    () => new AppState()
  );

  return (
    <InstanceInjectionRoot>
      <BindInstanceForInjection instance={appState}>
        <CounterPage />
      </BindInstanceForInjection>
    </InstanceInjectionRoot>
  );
};

Example: DOM Observer State (ResizeObserver pattern)

A common pattern is using a state object to wrap browser APIs like ResizeObserver.

import { action, observable } from 'mobx';
import {
  ReactStateObject,
  useMountStateObject,
} from 'react-state-object';
import { observer } from 'mobx-react-lite';

class ElementSizeState extends ReactStateObject {
  @observable accessor width = 0;
  @observable accessor height = 0;

  private readonly resizeObserver = new ResizeObserver(
    ([entry]) => {
      if (!entry) return;
      this.setSize(
        entry.contentRect.width,
        entry.contentRect.height
      );
    }
  );

  readonly elementRef = (
    el: HTMLDivElement | null
  ): void => {
    this.resizeObserver.disconnect();
    if (el) this.resizeObserver.observe(el);
  };

  protected unmount(): void {
    this.resizeObserver.disconnect();
  }

  @action private setSize(
    width: number,
    height: number
  ): void {
    this.width = width;
    this.height = height;
  }
}

export const MeasuredPanel = observer(() => {
  const size = useMountStateObject(
    () => new ElementSizeState()
  );

  return (
    <div>
      <div ref={size.elementRef}>Resize me</div>
      <p>
        {size.width} x {size.height}
      </p>
    </div>
  );
});

This pattern keeps browser API setup/cleanup inside a state class instead of scattering it across components.

Example: Using a Hook-Derived Service in State

This mirrors a common production pattern where a state object gets a hook-derived function (for example a toast helper) and calls it inside an action.

import { action, observable } from 'mobx';
import {
  fromHook,
  ReactStateObject,
} from 'react-state-object';

function useNotifier(): {
  success: (msg: string) => void;
} {
  return {
    success: (msg) => console.log('SUCCESS:', msg),
  };
}

class SaveState extends ReactStateObject {
  @observable accessor isSaving = false;

  @fromHook(() => useNotifier())
  @observable
  accessor notifier!: ReturnType<typeof useNotifier>;

  @action async save(): Promise<void> {
    this.isSaving = true;

    try {
      await new Promise((resolve) =>
        setTimeout(resolve, 300)
      );
      this.notifier.success('Saved successfully');
    } finally {
      this.isSaving = false;
    }
  }
}

How This Is Commonly Used in Larger Apps (Pattern Summary)

Based on common production usage patterns, a common structure is:

1. App-level state objects

   • created near app root with useMountStateObject(...)
   • bound with BindInstanceForInjection
   • examples: theme state, page observer state, window size observer

2. Page-level state objects

   • created in page/layout components
   • may inject app-level state via @injectInstance(...)
   • may compose nested child state objects

3. Feature sub-state objects

   • nested inside page state (forms, sidebar state, tables, save state, UI state)
   • mounted/unmounted automatically through parent ReactStateObject

4. React components

   • wrapped in observer(...)
   • either read state directly via props or retrieve it via useInjectInstance(...)

This architecture keeps React components focused on UI while moving orchestration/subscriptions into state classes.

Important: every ReactStateObject in this structure should still be created through useMountStateObject(...) at the React boundary that owns it. Child state objects can be instantiated inside parent state object constructors because their lifecycle is then managed by the parent ReactStateObject created via useMountStateObject(...).

API Reference

ReactStateObject

Base class with lifecycle and child-state propagation.

Methods you typically override:

• protected mount(): void
• protected unmount(): void

Helpers:

• protected withCleanup(action: () => () => void): void
• protected hookIntoLifecycle({ onMount?, onUnmount? })

useMountStateObject(factory)

Creates a ReactStateObject once and ties it to React component lifecycle.

const state = useMountStateObject(() => new MyState());

This is the required creation path for ReactStateObject instances used by React components.

fromHook(hookFn)

Accessor decorator that assigns a React hook result to a state object accessor.

@fromHook(() => useSomeHook())
@observable
accessor value!: ReturnType<typeof useSomeHook>;

injectInstance(Class)

Accessor decorator that injects a class instance from the injection tree using useInjectInstance(...).

@injectInstance(AppState)
@observable
accessor appState!: AppState;

invokeReactStateObjectHook(hook)

Low-level hook registration helper used internally by fromHook(...). Most users should not call this manually.

InstanceInjectionRoot

Root provider for the class-instance injection registry.

BindInstanceForInjection

Binds an instance for descendants.

useInjectInstance(Class)

Gets the nearest instance for a class or throws.

useInjectInstanceOrNull(Class)

Gets the nearest instance for a class or returns null.

Common Mistakes (and Fixes)

1) Component does not re-render when state changes

Cause:

• the component is not wrapped in observer(...)

Fix:

• wrap the component with observer from mobx-react-lite

2) fromHook(...) throws an error about

useMountStateObject

Cause:

• you created a ReactStateObject with new MyState() directly in a component instead of using useMountStateObject(...)

Fix:

• create it with useMountStateObject(...)

3) useInjectInstance(...) throws that no instance was

bound

Cause:

• missing InstanceInjectionRoot
• missing BindInstanceForInjection
• bound instance is on a different branch of the tree

Fix:

• add InstanceInjectionRoot near the app root
• ensure the consuming component is a descendant of the matching BindInstanceForInjection

4) Hook order errors in fromHook(...)

Cause:

• conditional hook usage inside @fromHook(...)

Fix:

• keep hook usage unconditional and stable, same as normal React hook rules

Publishing / Development

Scripts

• npm run format
• npm run lint
• npm run typecheck
• npm run build

Prettier config

This package copies the author's existing Prettier setup style, including:

• .prettierrc
• .prettierignore
• prettier-plugin-organize-imports
• matching format and lint-staged scripts

License

UNLICENSED (update before publishing publicly if needed).