Making your App really Sexy with a React Singleton Hook
Tags: reactnative, react, frontend, javascript
Published: 2021-03-04 08:10:13



*Create singleton hook from regular react hook*

## What is a singleton hook

* Singleton hooks very similar to React Context in terms of functionality. Each singleton hook has a body, you might think of it as of Context Provider body. Hook has a return value, it’s similar to the value provided by context. Using a singleton hook from a component is like consuming a context.

* Singleton hooks are lazy. The body is not executed until the hook is called by some component or other hook. Once loaded, the hook body remains loaded forever. If you want to eager-load some Singleton hooks, use them at the top-level component of your App.

* Singleton hooks do not require a provider or a special App structure. Under the hood, it uses useState/useRef/useEffect and some less-known react features for performance and portability.

* It’s possible to mix into single app Singleton hooks, React-Redux hooks api, React Context hooks and any custom

## Installation

To use React Singleton Hook with your React app, install it as a dependency:

```
    # If you use npm:
    npm install react-singleton-hook

    # Or if you use Yarn:
    yarn add react-singleton-hook
```

This assumes that you’re using [npm](http://npmjs.com/) package manager with a module bundler like [Webpack](https://webpack.js.org/) or [Browserify](http://browserify.org/) to consume [CommonJS modules](https://webpack.js.org/api/module-methods/#commonjs).

![](https://cdn-images-1.medium.com/max/2000/1*C4I_k62Y8poGBhRpckFOOg.png)

**Get the book**: [React js Hooks PDF](https://leanpub.com/react-hooks-tutorial)

## Examples

### convert any custom hook into singleton hook

In the code below, the user profile is not fetched until useUserProfile used by some component, and once fetched it is never reloaded again, the hook remains mounted forever into hidden component.

```
    import  { useEffect, useState } from 'react';
    import { singletonHook } from 'react-singleton-hook';
    const api = { async getMe() { return { name: 'test' }; } };

    const init = { loading: true };

    const useUserProfileImpl = () => {
      const [profile, setProfile] = useState(init);
      useEffect(() => {
        api.getMe()
          .then(profile => setProfile({ profile }))
          .catch(error => setProfile({ error }));
      }, []);

      return profile;
    };


    export const useUserProfile = singletonHook(init, useUserProfileImpl);
```

### dark/light mode switch

Whenever Configurator changes darkMode, all subscribed components are updated.

```
    /***************    file:src/services/darkMode.js    ***************/  
    import { useState } from 'react';
    import { singletonHook } from 'react-singleton-hook';

    const initDarkMode = false;
    let globalSetMode = () => { throw new Error('you must useDarkMode before setting its state'); };

    export const useDarkMode = singletonHook(initDarkMode, () => {
      const [mode, setMode] = useState(initDarkMode);
      globalSetMode = setMode;
      return mode;
    });

    export const setDarkMode = mode => globalSetMode(mode);
    

    /***************    file:src/compoents/App.js    ***************/

    import  React from 'react';
    import { useDarkMode, setDarkMode } from 'src/services/darkMode';

    const Consumer1 = () => {
      const mode = useDarkMode();
      return <div className={`is-dark-${mode}`}>Consumer1 - {`${mode}`}</div>;
    };

    const Consumer2 = () => {
      const mode = useDarkMode();
      return <div className={`is-dark-${mode}`}>Consumer2 - {`${mode}`}</div>;
    };

    const Configurator = () => {
      const mode = useDarkMode();
      return <button onClick={() => setDarkMode(!mode)}>Toggle dark/light</button>;
    };
```

### imperatively read hook state for non-react code

```
    import { useState } from 'react';
    import { singletonHook } from 'react-singleton-hook';

    const initDarkMode = false;
    let currentMode = initDarkMode;
    let globalSetMode = () => { throw new Error(`you must useDarkMode before setting its state`); };

    export const useDarkMode = singletonHook(initDarkMode, () => {
      const [mode, setMode] = useState(initDarkMode);
      globalSetMode = setMode;
      currentMode = mode;
      return mode;
    });

    export const setDarkMode = mode => globalSetMode(mode);
    export const getDarkMode = () => currentMode;
```

### use react-redux (or any other context) inside singletonHook

To use react-redux or any other context-based functionality, singleton hooks should be mounted under provider in your app. To do that, import SingletonHooksContainer from react-singleton-hook and mount anywhere in you app. SingletonHooksContainer must be rendered ealier then any component using singleton hook! By default you are not required to dial with a SingletonHooksContainer, we run this component internally in separate react app.

```
    /***************    file:src/services/currentUser.js    ***************/
    import { singletonHook } from 'react-singleton-hook';
    import { useSelector } from 'react-redux';

    const init = { loading: true };
    const useCurrentUserImpl = () => {
      const session = useSelector(state => state.session);
      if (session.loading) return init;
      return session.user;
    };

    export const useCurrentUser = singletonHook(init, useCurrentUserImpl);

    /***************    file:src/App.js    ***************/

    import React from 'react';
    import ReactDOM from 'react-dom';
    import { SingletonHooksContainer } from 'react-singleton-hook';
    import { Provider } from 'react-redux';
    import store from 'src/store';
    import App from 'src/views';

    const app = (
      <Provider store={store}>
        <>
          <SingletonHooksContainer/>
          <App/>
        </>
      </Provider>
    );

    ReactDOM.render(app, document.getElementById('root'));

### top-level components updated before low-level components

    /***************    file:src/services/session.js    ***************/

    import { useEffect, useState } from 'react';
    import { singletonHook } from 'react-singleton-hook';

    const initState = { loading: true };
    let setSessionGlobal = () => { throw new Error('you must useSession before login'); };

    const useSessionImpl = () => {
      const [session, setSession] = useState(initState);
      setSessionGlobal = setSession;
      useEffect(() => { setSession({ loggedIn: false }); }, []);
      return session;
    };

    export const useSession = singletonHook(initState, useSessionImpl);

    export const login = (name, pass) => {
      setSessionGlobal({ loggedIn: true, user: { name: 'test' } });
    };

    /***************    file:src/index.js    ***************/
    import React, { useEffect } from 'react';
    import ReactDOM from 'react-dom';
    import { login, useSession } from 'src/services/session';

    const LoggedInView = () => {
      const session = useSession();
      console.log(`LoggerInView rendered with ${JSON.stringify(session)}`);
      return null;
    };

    const LoggedOutView = () => {
      const session = useSession();
      console.log(`LoggedOutView rendered with ${JSON.stringify(session)}`);
      return null;
    };

    const WaitingForSessionView = () => {
      const session = useSession();
      console.log(`WaitingForSessionView rendered with ${JSON.stringify(session)}`);
      return null;
    };

    const MainComponent = () => {
      const session = useSession();

      useEffect(() => {
        setTimeout(() => { login('testuser'); }, 2000);
      }, []);

      console.log(`MainComponent rendered with ${JSON.stringify(session)}`);

      if (session.loading) return <WaitingForSessionView/>;
      if (session.loggedIn) return <LoggedInView/>;
      return <LoggedOutView/>;
    };
    

    ReactDOM.render(<MainComponent/>, document.getElementById('root'));

    /***************    console.log    ***************/
    /*

    MainComponent rendered with {"loading":true}
    WaitingForSessionView rendered with {"loading":true}
    MainComponent rendered with {"loggedIn":false}
    LoggedOutView rendered with {"loggedIn":false}
    MainComponent rendered with {"loggedIn":true,"user":{"name":"test"}}
    LoggerInView rendered with {"loggedIn":true,"user":{"name":"test"}}

    */
```

## Initial state callback

As of version 3.0, singletonHook accepts a callback that calculates initial state instead of predefined initial state. This callback is called once and only when the value is required. You can use it to calculate expensive initial values or avoid an extra render (and a state flickering) when initial state changes before any component consumes the hook:

### example: subscribe components to pre-existing get/set data module

```
    /***************    file:src/services/darkMode.js    ***************/

    import { useState } from 'react';
    import { singletonHook } from 'react-singleton-hook';

    let isDarkMode = false; // the state of the module
    let updateSubscribers = (mode) => {}; //update subscribers callback - do nothing by default

    // pre-existing functions to manipulate the state
    export const getDarkMode = () => isDarkMode;

    export const setDarkMode = (newMode) => {
     isDarkMode = newMode;
     updateSubscribers(isDarkMode); // call updateSubscribers when setting new state
    };

    // new function - custom hook for components to subscribe.
    // using getDarkMode as an init callback to get most relevant state
    export const useDarkMode = singletonHook(getDarkMode, () => {
     const [mode, setMode] = useState(getDarkMode);
     updateSubscribers = setMode; // subscribing for further updates
     return mode;
    });

    /***************    file:src/index.js    ***************/

    // you can call setter and getter any time
    setDarkMode(true);
    setInterval(() => setDarkMode(!getDarkMode()), 2000);

    const App = () => {
     // component will be updated on darkMode change 
     // on first render "mode" is set to the current value getDarkMode returns
     const mode = useDarkMode();
     return <div className={`is-dark-${mode}`}>App - {`${mode}`}</div>;
    };
```

Github: [https://github.com/Light-Keeper/react-singleton-hook](https://github.com/Light-Keeper/react-singleton-hook)

**Additional Resources to learn React:**

### [Django with React | An Ecommerce Website](https://click.linksynergy.com/deeplink?id=NcMZU1aTATA&mid=39197&murl=https%3A%2F%2Fwww.udemy.com%2Fcourse%2Fdjango-with-react-an-ecommerce-website%2F)

### [Redux Saga (with React and Redux): Fast-track intro course](https://coursesim.com/course/redux-saga-with-react-and-redux-fast-track-intro-course/)