react系列笔记：第二记-中间件

中间件所做的事情就是在action发起后，到reducer之前做扩展，实现的方式是对store的dispatch进行包装

     store.dispatch => 【middlewales】 => return new store.dispatch

所以从上面所知，middlewales是需要接受store的dispatch为参数的，为了进行state的一些操作比如跟踪state变化，则把getState也一同传入

　　middlewales(store.dispatch,store.getState) = > return new dispatch

然后applyMiddleWales基本上做了一件事就是遍历了middlewales

　　applyMiddleWales(store,[middlewals1,.....])  => return new store.dispatch

官网上也在实现上分了两步进行解释大致的实现

第一部分：这个比较简单，就是把store传下去，然后把dispatch重新返回来，当然在这之间可以做些什么再返回来，比如添加log,捕获错误等

const logger1 =(store) => {
  const next = store.dispatch
  return (action)=>{
    console.log('log1---start')
    let o = next(action);
    console.log('log1---end')
    return o;
  }
}

const logger2 = (store) => {
  const next = store.dispatch;
  return (action) =>{
    console.log('log2----start');
    let o = next(action);
    console.log('log2-----end')
    return o;
  }
}

const store = {
    dispatch:()=>{console.log('dispatch----')}
}

function middle(store,wales){
  wales.forEach(item => {
    store.dispatch = item(store)
  })
}

middle(store,[logger1,logger2])

//打印：
//log2 --start
//log1--start
//dispacht
//log1--end
//log2--end

第二步：这里把middlewales的调用方式改变了一下，前面是middle(store) => return dispatch，这里是middls(store)(dispatch) => return dispatch

这么做的好处是不是直接在store身上直接去扩展dispatch，而是把dispatch从源头传出来，然后返回新的dispatch，最后生成的也是store的一个副本。

const newLog1 = (store)=>{
    return (next) => {
        return (action) =>{
            console.log('newLog1---start')
            let o = next(action);
            console.log('newLog1---end');
            return o;
        }
    }
}

const newLog2 = (store)=>{
    return (next) => {
        return (action) =>{
            console.log('newLog2---start')
            let o = next(action);
            console.log('newLog2---end');
            return o;
        }
    }
}

newLog2(store);//此时这个返回的是一个函数，（next） => return (action)=>{}
newLog2(store)(store.dispatch)//此时返回的是一个函数  (action)=>{}

const newStore = {
    dispatch:() => console.log('newStore dispatch-----')
}
const newMiddle = (store,wales) =>{
    let dispatch = store.dispatch;
    wales.forEach(item => {
        dispatch = item(store)(dispatch);
    })
    return {...store,dispatch};
}
const ns = newMiddle(newStore,[newLog1,newLog2]);