applyMiddleware 沉思录

let newStore = applyMiddleware(mid1, mid2, mid3, ...)(createStore)(reducer, null);

给({ getState, dispatch }) => next => action签名的函数组分步赋值并进行串联的过程；

整体上是dispatch的method swizzling；

export default function applyMiddleware(...middlewares) {

return (next)  =>

(reducer, initialState) => {

var store = next(reducer, initialState);

var dispatch = store.dispatch;

var chain = [];

//({ getState, dispatch })

var middlewareAPI = {

getState: store.getState,

dispatch: (action) => dispatch(action)

};

//({ getState, dispatch })

chain = middlewares.map(middleware => middleware(middlewareAPI));

//next

dispatch = compose(...chain, store.dispatch);

return {

...store,

dispatch

};

};

}

export default store => next => action => {

console.log('dispatch:', action);

next(action);

console.log('finish:', action);

}

export default ({ getState, dispatch }) => next => action => {

console.log('dispatch:', action);

next(action);

console.log('finish:', action);

}

function compose(...funs) {

return arg => funcs.reduceRight( (compose, f) => f(composed), arg)

}

https://www.cnblogs.com/fe-linjin/p/10494763.html

https://stackoverflow.com/questions/41139585/what-is-the-next-in-applymiddleware

...middlewares (arguments): 遵循 Redux middleware API 的函数。每个 middleware 接受 Store 的 dispatch 和 getState 函数作为命名参数，并返回一个函数。该函数会被传入 被称为 next 的下一个 middleware 的 dispatch 方法，并返回一个接收 action 的新函数，这个函数可以直接调用 next(action)，或者在其他需要的时刻调用，甚至根本不去调用它。调用链中最后一个 middleware 会接受真实的 store 的 dispatch 方法作为 next 参数，并借此结束调用链。所以，middleware 的函数签名是 ({ getState, dispatch }) => next => action。

https://www.redux.org.cn/docs/api/applyMiddleware.html