---

Mcrel is a state management library for React and TypeScript. The main idea is using type checking for doing deep updates so you can safely use good old setState-like API but with nesting. Mcrel checks statically that value of the update is a strict subset of the state and does all the tedious work of recreating underlying objects.

interface User {
  name: {
    firstName: string;
    lastName: string;
  };
  age: number;
}

interface StoreState {
  user?: User;
  loaded: boolean;
}

const store = new Store<StoreState>({
  loaded: false,
});

store.setState({
  user: {
    name: {
      firstName: 'Solid',
      lastName: 'Snake',
    },
    age: 42,
  },
  loaded: true,
}); // Ok: user is User, loaded is boolean

store.setState({ user: { name: 'Liquid Snake', age: 33 } }); // Error: user.name is object

store.setState({ user: { name: { firstName: 'Liquid' }, age: 33 } }); // Ok: user.name.firstName is string, user.age is number

store.setState({ user: { realName: 'Big Boss' } }); // Error: user has no field realName

store.setState({ user: undefined, loaded: false }); // Ok: user is undefined, loaded is boolean

Example

There is classic todo-list example is available.

Usage

Mcrel can be used as a global state (like Redux) and as a local state (like useState). TypeScript strictNullChecks is required for Mcrel to work properly.

Global state

1. Define State and create a Store instances.

   import { Store } from 'mcrel';
   
   interface StoreState {
     value: number;
     timestamp: Date;
   }
   
   export const store = new Store<StoreState>({ value: 0, timestamp: new Date() });

2. Put Provider at top level of your app and pass store from step one to its props.

   import React from 'react';
   
   import { Provider } from 'mcrel';
   import ReactDOM from 'react-dom';
   
   import App from './App';
   import { store } from './store';
   
   const rootElement = document.getElementById('root');
   
   ReactDOM.render(
     <Provider store={store}>
       <App />
     </Provider>,
     rootElement,
   );

3. Use useMcrelSelector hook or connect function to read values from Store.

   import React from 'react';
   
   import { useMcrelSelector } from 'mcrel';
   
   export default function Year() {
     const timestamp = useMcrelSelector(({ timestamp }) => timestamp);
   
     return <div>{timestamp.getYear()}</div>;
   }

   or

   import React from 'react';
   
   import { connect } from 'mcrel';
   
   interface Props {
     value: number;
   }
   
   function Value({ value }: Props) {
     return <div>{value}</div>;
   }
   
   export default connect(
     Value,
     ({ value }) => ({ value }),
   );

4. Use Store.setState to update the store.

   import React from 'react';
   
   import { store } from './store';
   
   function UpButton() {
     return (
       <button
         onClick={() => {
           store.setState(({ value }) => ({ value: value + 1, timestamp: new Date() }));
         }}
       >
         Up
       </button>
     );
   }

Local state

Just use useMcrelState hook as a useState but with all Mcrel update functionality.

import React from 'react';

import { useMcrelState } from 'mcrel';

function MyComponent() {
  const [data, setData] = useMcrelState({
    value: {
      count: 0,
      color: 'red',
    },
    timestamp: new Date(),
  });

  return (
    <div>
      <div style={{ backgroundColor: data.value.color }}>{data.value.count}</div>
      <div>{data.timestamp}</div>

      <button
        onClick={() => {
          const { count } = data.value;

          setData({
            value: { count: count + 1, color: count % 2 ? 'aqua' : 'maroon' },
            timestamp: new Date(),
          });
        }}
      >
        Up
      </button>
    </div>
  );
}

API

Store

Store contains the global state of the app. Its instance is intended to be used as singleton.

new Store<T, G>(state: T, atomicGuards?: G): Store<T, G>

Straight forward way for creating the Store. Set state as initial store state. With optional atomicGuards parameters you can specify the list of atomic objects for store (Array of type guard functions).

import { Store } from 'mcrel';

interface State {
  value: number;
}

const store = new Store<State>({ value: 0 });

createStore<T>(state: T): (<G>(atomicGuards: G) => Store<T, G>)

Factory function for creating Store. Helpful when setting atomic guards so you can explicitly set type of the Store state and omit type of the atomic guards list (Array of type guard functions).

import { DateTime } from 'luxon';
import { createStore } from 'mcrel';

interface State {
  date: DateTime;
}

function isDateTime(value: any): value is DateTime {
  return value instanceof DateTime;
}

const store = createStore<State>({
  date: DateTime.local(),
})([isDateTime]);

Store.setState<P>(arg: P | ((state: DeepReadonlyObject<T>) => P): DeepReadonlyObject<T>

Set store state and notify all subscriptions. arg is either a object that should be strict subset of the State or a function that receives store state as argument and should return object that should be strict subset of the State. Returns new store state.

import { DeepReadonlyObject, Store } from 'mcrel';

interface State {
  num: number;
  obj: {
    bool: boolean;
    str: string;
  };
}

const store = new Store<State>({
  num: 0,
  obj: {
    bool: true,
    str: '',
  },
});

store.setState({
  num: 1,
  obj: {
    str: 'hello',
  },
});

store.setState((state: DeepReadonlyObject<State>) => ({
  num: state.num + 1,
  obj: {
    bool: !state.obj.bool,
  },
}));

Store.subscribe(callback: (state: DeepReadonlyObject<T>) => void): Unsubscribe

Subscribe for store updates. callback will be called on each store state change. Returns function that should be called when you want to unsubscribe.

import { DeepReadonlyObject, Store } from 'mcrel';

interface State {
  user: string;
}

const store = new Store<State>({ user: 'Darth Sidious' });

const unsubscribe = store.subscribe((state: DeepReadonlyObject<State>) => {
  console.log(state);
});

Components

<Provider store={store} />

All components that use useMcrelSelector or connect must be descendants of the Provider.

import React from 'react';

import { Provider } from 'mcrel';

import { store } from './store';
import Root from './Root';

export default function App() {
  return (
    <Provider store={store}>
      <Root />
    </Provider>
  );
}

HOC

connect<P, R, S>(Component: ComponentType<P & R>, selector: Selector<S, R>, compareFunction: CompareFunction<R>): FunctionComponent<P>

Create connected to store component. Mostly exist for Class components. Component is React component that will receive values selected in selector function in props. compareFunction is used to to determine whether the selected data has changed or not. By default connect use shallowEqual for compareFunction but you can specify your own function to optimize updates.

import React from 'react';

import { connect } from 'mcrel';

interface Props {
  starsCount: number;
}

function StarsCounter({ starsCount }: Props) {
  return <div>{starsCount}</div>;
}

export default connect(
  StarsCounter,
  ({ starsCount }) => ({ starsCount }),
);

Hooks

useMcrelSelector<S, R>(selector: Selector<S, R>, compareFunction: CompareFunction<R>): R

Select values from store and subscribe for its updates. selector is a function that receives store state as argument and should return any value. The hook use compareFunction to determine whether the selected data has changed or not. The default compareFunction is by value comparison (i.e. ===) but you can specify your own function to optimize updates.

import React from 'react';

import { useMcrelSelector } from 'mcrel';

export default function StarsCounter {
  const starsCount = useMcrelSelector(({ starsCount }) => starsCount);

  return <div>{starsCount}</div>;
}

useMcrelState<T>(state: T): [DeepReadonlyObject<T>, <P>(arg: P | ((state: DeepReadonlyObject<T>) => P)) => void]

Use Mcrel for local state management.

import React from 'react';

import { useMcrelState } from 'mcrel';

interface State {
  numbers: {
    value: number;
  }[];
  timestamp: Date;
}

function Component() {
  const [data, setData] = useMcrelState<State>({
    numbers: [],
    timestamp: new Date(),
  });

  return (
    <div>
      <div>
        {data.number.map(({ value }) => (
          <span>value</span>
        ))}
      </div>
      <div>{data.timestamp}</div>

      <button
        onClick={() => {
          const numbers = Array.from(data.numbers);
          numbers.push({ value: Math.random() });

          setData({
            numbers,
            timestamp: new Date(),
          });
        }}
      >
        One more
      </button>
    </div>
  );
}

Utility

pierce<K, V>(key: K, value: V): Pierce<K, V>

Piercing is a powerful feature that allows using Mcrel for updating through key-value collections (Array and Map). Pass key of updating object as first parameter and Mcrel update value as second.

import { Store, pierce } from 'mcrel';

interface State {
  jedi: Map<number, { name: string; message: string }>;
}

const store = new Store<State>({
  jedi: new Map([
    [1, { name: 'Obi-Wan Kenobi', message: 'Hello there!' }],
    [2, { name: 'Anakin Skywalker', message: 'I HATE YOU!' }],
  ]),
});

store.setState({ jedi: pierce(2, { name: 'Darth Vader' }) });

shallowEqual(objA: any, objB: any): boolean

Performs equality by iterating through keys on an object and returning false when any key has values which are not strictly equal between the arguments. Returns true when the values of all keys are strictly equal.

Atomic Guards

Atomic guard is a TypeScript guard function that define custom atomic object for Mcrel. Atomics can't be updated partially and always set using direct assignment. Mcrel already supports built in JavaScript atomics like Date, Regex and other.

import { createStore } from 'mcrel';

class MyAtomicType {
  private counter = 0;

  up() {
    this.counter++;
  }
}

interface State {
  value: MyAtomicType;
}

function isMyAtomicType(value: any): value is MyAtomicType {
  return value instanceof MyAtomicType;
}

const store = createStore<State>({
  value: new MyAtomicType(),
})([isMyAtomicType]);

Acknowledgements

Special thanks to Nastya Loginova for creating a project logo.

License

Mcrel is MIT License.