ストアを使用して React コンポーネントを最適化する

function createStore(initialState) {
 let state = initialState;
 const listeners = [];

 function setState(partial) {
  state = {
   ...state,
   ...partial,
  };
  for (let i = 0; i < listeners.length; i++) {
   listeners[i]();
  }
 }

 function getState() {
  return state;
 }

 function subscribe(listener) {
  listeners.push(listener);

  return function unsubscribe() {
   const index = listeners.indexOf(listener);
   listeners.splice(index, 1);
  };
 }

 return {
  setState,
  getState,
  subscribe,
 };
}

class Counter extends React.Component {
 constructor(props) {
  super(props);

  // 初始化 store
  this.store = createStore({
   count: 0,
  });
 }

 render() {
  return (
   <p>
    <Buttons store={store} />
    <Result store={store} />
   </p>
  )
 }
}

class Buttons extends React.Component {
 handleClick = (step) => () => {
  const { store } = this.props;
  const { count } = store.getState();
  store.setState({ count: count + step });
 }

 render() {
  return (
   <p>
    <button onClick={this.handleClick(1)}>+</button>
    <button onClick={this.handleClick(1)}>-</button>
   </p>
  );
 }
}

class Result extends React.Component {
 constructor(props) {
  super(props);

  this.state = {
   count: props.store.getState().count,
  };
 }

 componentDidMount() {
  this.props.store.subscribe(() => {
   const { count } = this.props.store.getState();
   if (count !== this.state.count) {
    this.setState({ count });
   }
  });
 }

 render() {
  return (
   <p>{this.state.count}</p>
  );
 };
}