一、简介

        普通的action处理会自动分派给对应的reducer处理。异步的action会经过Middlewares进行处理，异步完成后再交由对应的reducer处理。

1.Middleware

        (1) 截获action

                判断action是否是一个promise操作。

        (2) 发出action

二、代码实现

        举个例子，获取文章列表。action返回的是一个promise，有开始请求列表、获取列表成功、获取列表失败三个action。当ajax异步请求执行后，会发送对应的action，reducer会根据action处理state，ui会渲染对应state的变更。

        

//redux state
const initialState = {data:null, fetchRebbitListPending:false, fetchRebbitListError:null}

// 获取文章列表 action
export function fetchRedditList(args = {}){
    return dispatch => {
        // 发送一个开始获取列表事件
        dispatch({type: FETCH_REDDIT_LIST_BEGIN});
        // 发送ajax请求，获取数据
        const promise = new Promise({resolve,reject} =>{

            const doRequest = axios.get("http://www.reddit.com/r/reactjs.json");
            doRequest.then(
                res => {
                    dispatch({
                        type: FETCH_REDDIT_LIST_SUCCESS,
                        data: res.data
                    });
                },
                err => {
                    dispatch({
                        type: FETCH_REDDIT_LIST_FAILURE,
                        data: {error:err}
                        }
                    );
          
                }
            );
        };
        return promise;
}


// redux reducer
export function reducer(state, action){
    switch (action.type){
        case FETCH_REDDIT_LIST_BEGIN:
            return (data:[action.res.data], fetchRebbitListPending: true, fetchRebbitListError:null);
        case FETCH_REDDIT_LIST_SUCCESS:
            return (...state, fetchRebbditListPending: false, fetchRebbitListError:null);
        case FETCH_REDDIT_LIST_ERROR:
            return (...state, fetchRebbitListPending: false, 
                    fetchRebbitListError:action.data.error);
    
    }
    
}

// REDUX STORE，指定middle
const store = createStore(reducer, applyMiddleware(thunk));


<div>
    <button onClick = {fetchRedditList}>获取文章列表</button>
    {initialState.fetchRebbitListError && <span>获取文章列表失败</span>}
    {initialState.fetchRebbditListPending && <span>加载中</span>}
    {initialState.data 
        && 
        <ul>
            initialState.data.map(item -> <li>item.title</li)
        </ul>
    }
</div>