function createStore(reducer) {
    let currentState;
    let currentListeners = []
    // get
    function getState() {
        return currentState
    }
    // set
    function dispatch(action) {
        currentState = reducer(currentState, action);
        currentListeners.forEach(listener => listener());
        return action;
    }

    // 订阅和取消订阅必须成对出现
    function subscribe(listener) {
        currentListeners.push(listener);
        return () => {
            currentListeners = currentListeners.filter(_fn => _fn !==listener)
        }
    }

    return {
        getState,
        dispatch,
        subscribe
    }
}