====== Redux ======
  - Install redux, react-redux, and update the react-dom
  - Create a reducer for storing the state, and handle the dispatched task
  - Create the store in index.js, and provide it to the app by <Provider...></Provider>
  - Using redux in your component:
    - Class-based component: In your component, create mapStateToProps, and mapDispatchToProps. Then use import {connect} from 'react-redux'; to connect your component to the store.
    - Functional-based component: Use {useSelector, useDispatch} from 'react-redux'. ''useSelector'' obtains the values, and ''useDispatch'' dispatches the actions.
===== Install =====
<code>
npm install react react-redux
npm update react react-dom
</code>

===== Create Reducer =====
create a file call ''reducer.js'', put in under /src/store/. We also put the initial state there.
<code>
const initialState = {
    counter: 0
}

const reducer = (state = initialState, action) => {
    switch (action.type) {
        case 'INCREMENT':
            return {
                ...state,
                counter: state.counter + 1
            }
        case 'DECREMENT':
            return {
                ...state,
                counter: state.counter - 1
            }
        case 'INCREMENT5':
            return {
                ...state,
                counter: state.counter + action.value
            }
        case 'DECREMENT5':
            return {
                ...state,
                counter: state.counter - action.value
            }
        default:
            return state;
    }
}

export default reducer;
</code>

===== Create Store in index.js, and Provider it to the app =====
Here we create the store by using createStore, passing out reducer to it. Finally provide it to the app by <Provider store={...}...></Provider>
<code>
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import registerServiceWorker from './registerServiceWorker';

import {createStore} from "redux";
import reducer from "./store/reducer";
import {Provider} from 'react-redux';

const store = createStore(reducer);
ReactDOM.render(
    <Provider store={store}>
        <App/>
    </Provider>, document.getElementById('root'));
registerServiceWorker();
</code>

===== Connect Out Component to it =====
==== Old Way: For Class Base Component ====
The key here is to create a ''mapStateToProps'' to get the state from redux, and a ''mapDispatchToProps'' to dispatch actions to the redux. We connect redux to our component by using export default connect(mapStateToProps, mapDispatchToProps)(Counter); at the end.

Since the state and action are now the props, we can use them and call them in our component like ''this.props...''.

The 'action' we dispatch usual has a ''type'' for the type of action, and any addition fields if needed.
<code>
import React, {Component} from 'react';

import CounterControl from '../../components/CounterControl/CounterControl';
import CounterOutput from '../../components/CounterOutput/CounterOutput';
import {connect} from 'react-redux';

class Counter extends Component {

    render() {
        return (
            <div>
                <CounterOutput value={this.props.ctr}/>
                <CounterControl label="Increment" clicked={this.props.onIncrementCount}/>
                <CounterControl label="Decrement" clicked={() => this.props.onDecrementCount()}/>
                <CounterControl label="Add 5" clicked={() => this.props.onInc5Count()}/>
                <CounterControl label="Subtract 5" clicked={() => this.props.onDec5Count()}/>
            </div>
        );
    }
}

const mapStateToProps = state => {
    return {
        ctr: state.counter
    }
};

const mapDispatchToProps = dispatch => {
    return {
        onIncrementCount: () => dispatch({type: 'INCREMENT'}),
        onDecrementCount: () => dispatch({type: 'DECREMENT'}),
        onInc5Count: () => dispatch({type: 'INCREMENT5', value:5}),
        onDec5Count: () => dispatch({type: 'DECREMENT5', value:10}),

    };
}
export default connect(mapStateToProps, mapDispatchToProps)(Counter);
</code>

The action.type needs to match the one in the reducer! Any spelling mistake/mismatch would cause the reducer to do nothing. You might want to use a separated file, and include this file on both the reducer and the component.

==== New Way: Use Hooks For Functional Base Component ====
People in these days use functional base component for react, and here is how to use redux in it by using ''useSelector'', ''useDispatch'' and hooks.
<code>
import './App.css';
import {useSelector, useDispatch} from 'react-redux'

function App() {

    const count = useSelector(state => {
            return state.counter;
        }
    )
    const dispatch = useDispatch();

    const click = (action, value) => {
        if (value) {
            dispatch({type: action, value: value});
        } else {
            dispatch({type: action});
        }
    }

    return (
        <>
            <div>The counter is: {count}</div>
            <button onClick={() => click('INCREMENT')}>INCREMENT</button>
            <button onClick={() => click('DECREMENT')}>DECREMENT</button>
            <button onClick={() => click('INCREMENT5', 5)}>INCREMENT 5</button>
            <button onClick={() => click('DECREMENT5', 5)}>DECREMENT 5</button>
        </>
    )
}

export default App;
</code>