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 (
 <div>
 <Buttons store={store} />
 <Result store={store} />
 </div>
 )
 }
}

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

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

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 (
 <div>{this.state.count}</div>
 );
 };
}