Redux（七）：源码分析之applyMiddleware
一、源码分析
Redux真正强大之处便是中间件系统，利用各类自定义或第三方的中间件，可以极大丰富其功能，派发的action也将不再局限于普通纯对象。
还记得createStore里enhancer参数吗？在createStore.js的53行，如果传递了enhancer，则：
return enhancer(createStore)(reducer, preloadedState)接着看下边的实例：
const initialState = {
todos:[]
};
const reducer = function(state,action){
switch (action.type){
case "TODO_ADD":
return Object.assign({},state,{
todos:state.todos.concat({text:action.text})
})
}
return state;
};
const enhancer = function(createStore){
return function(reducer, preloadedState){
return createStore(reducer,preloadedState);
}
};
const store = createStore(reducer,initialState,applyMiddleware(enhancer));可以看出来，这里store是执行enhancer的返回函数所创建的。其实enhancer就是applyMiddleware的返回结果，我们来看看applyMiddleware的源码：
export default function applyMiddleware(...middlewares) {
return createStore => (...args) => {
const store = createStore(...args)
let dispatch = () => {
throw new Error(
`Dispatching while constructing your middleware is not allowed. ` +
`Other middleware would not be applied to this dispatch.`
)
}
const middlewareAPI = {
getState: store.getState,
dispatch: (...args) => dispatch(...args)
}
const chain = middlewares.map(middleware => middleware(middlewareAPI))
dispatch = compose(...chain)(store.dispatch)
return {
...store,
dispatch
}
}
}这个看起来不够直观，转成ES5，那么就是这样的：
function applyMiddleware(...middlewares){
return function(createStore){
return function(reducer,preloadedState){
const store = createStore(reducer,preloadedState);
let dispatch = function(){
throw new Error(
`Dispatching while constructing your middleware is not allowed. ` +
`Other middleware would not be applied to this dispatch.`
)
};
const middlewareAPI = {
getState: store.getState,
dispatch:function(...args){
return dispatch(...args);
}
};
const chain = middlewares.map(function(middleware){
return middleware(middlewareAPI);
});
dispatch = compose(...chain)(store.dispatch);
return {
...store,
dispatch
}
}
}
}对比我们的enhancer函数，是不是很像呢，接着逐行分析最里面的代码。
first step：
const store = createStore(reducer,preloadedState);
let dispatch = function(){
throw new Error(
`Dispatching while constructing your middleware is not allowed. ` +
`Other middleware would not be applied to this dispatch.`
)
};创建store，自定义dispatch错误函数，这个函数的意思：不能在中间件里面直接调用根dispatch，应该调用形参dispatch。
second step：
const middlewareAPI = {
getState: store.getState,
dispatch:function(...args){
return dispatch(...args);
}
};
const chain = middlewares.map(function(middleware){
return middleware(middlewareAPI);
});定义middlewareAPI对象，包含getState和dispatch俩个属性。
定义chain数组，每个元素是middleware的执行结果（依然是函数），参数就是middlewareAPI。在中间件里面不能直接调用middlewareAPI的dispatch。
third step：
dispatch = compose(...chain)(store.dispatch);
return {
...store,
dispatch
}结合上一章compose的解读，那么中间件的执行顺序就是由后向前的，执行队列的每个dispatch形参都由上个中间件的执行结果返回。
二、自定义中间件格式
结合上边的分析，那么一个最最简单的中间件应该是这种格式的。
function middleware(middlewareAPI){
return function(next){
return function(action){
next(action);
}
}
}我们定义俩个中间件：
function middleware1(middlewareAPI){
return function ware1(next){
return function(action){
console.log("middleware1开始");
next(action);
console.log("middleware1结束");
}
}
}
function middleware2(middlewareAPI){
return function ware2(next){
return function(action){
console.log("middleware2开始");
next(action);
console.log("middleware2结束");
}
}
}结合上边的源码，那么在applyMiddlewares中数组常量chain的格式就是这样的：
const chain = [
function ware1(next){
return function(action){
console.log("middleware1开始");
next(action);
console.log("middleware1结束")
}
},
function ware2(dispatch){
return function(next){
console.log("middleware2开始");
next(action);
console.log("middleware2结束")
}
},
];接着经过compose函数的运算，所以原始的dispatch会作为ware2中间件的参数，然后ware2的返回值会作为ware1的参数，那么最终dispatch函数就是这样子的：
const dispatch = function(action){
console.log("middleware1开始");
void function(next){
console.log("middleware2结束");
next(action);
console.log("middleware2结束")
}(next)
console.log("middleware1结束")
};那么当派发一个action之后，正确的执行顺序就是：
- 打印middleware1开始
- 打印middleware2开始
- 执行原始的dispatch函数
- 执行subscribe注册的回调函数
- 打印middleware2结束
- 打印middleware1结束
总结
综上，调用next之前可以获取到当前state树，调用结束后可以获取到更新后的state树，基本中间件格式：
function middleware(middlewareAPI){
return function(next){
return function(action){
console.log("middleware开始",middlewareAPI.getState());
next(action);
console.log("middleware结束",middlewareAPI.getState());
}
}
}同时中间件的引用顺序也是有讲究的，第一个中间的action回传给redux必须是纯对象。
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