本文讲解并实现 Redux 所有 API,包括 createStore、combineReducers、applyMiddleware、compose、bindactioncreators。所有实现为了简明去除了很多非必要的逻辑,但保持了主要功能完整。本文中的源码可在这里查看。
createStore
使用示例:
export function counterReducer(state = 0, action) {
switch (action.type) {
case 'INCREMENT':
return state + 1
case 'DECREMENT':
return state - 1
default:
return state
}
};
const { dispatch, subscribe, getState } = createStore(counterReducer);
const unsubscribe = subscribe(() => console.log(getState()));
dispatch({ type: 'INCREMENT' });
unsubscribe();
复制代码
实现:
这里本身没什么难度就不多赘述,看下面👇代码即可。createStore
返回 store
,其中包含以下四个方法。
getState()
:返回应用当前的 state。dispatch(action)
:分发 action,这是触发 state 变化的惟一途径。subscribe(listener)
:添加一个变化监听器。replaceReducer(nextReducer)
:替换 store 当前用来计算 state 的 reducer。
// 保证不会与用户定义的 actionType 重名即可
const ACTION_TYPE_INIT = `@@redux/INIT${Math.random()}`;
const ACTION_TYPE_REPLACE = `@@redux/REPLACE${Math.random()}`;
function createStore(reducer, preloadedState) {
let currentReducer = reducer;
const listeners = [];
let state = preloadedState;
const getState = () => {
return state;
};
const subscribe = listener => {
listeners.push(listener);
// 用于取消订阅
const unsubscribe = () => {
const index = listeners.indexOf(listener);
listeners.splice(index, 1);
};
return unsubscribe;
}
const dispatch = action => {
state = currentReducer(state, action);
listeners.forEach(listener => listener());
};
function replaceReducer(nextReducer) {
if (typeof nextReducer !== 'function') {
throw new Error('Expected the nextReducer to be a function.')
}
currentReducer = nextReducer;
dispatch({ type: ActionTypes.REPLACE } ); // 替换 reducer 后重新初始化 state
return store;
}
// reducer 都设置了默认的 state,调用 dispatch 完成 state 初始化
dispatch({ type: ACTION_TYPE_INIT });
const store = { dispatch, subscribe, getState, replaceReducer };
return store;
};
复制代码
combineReducers(reducers)
应用较大时需要将 reducer 拆成多个 reducer 函数,以实现每个 reducer 负责独立管理 state 的其中一部分。而 combineReducers
的作用是允许你把拆分出来的多个 reducer 函数合并成一个最终的 reducer 函数供 createStore
使用。
使用示例:
// counter.js
export default function counter(state = 0, action) {
switch (action.type) {
case 'INCREMENT':
return state + 1
case 'DECREMENT':
return state - 1
default:
return state
}
}
// todos.js
export default function todos(state = [], action) {
switch (action.type) {
case 'ADD_TODO':
return state.concat([action.text])
default:
return state
}
}
const rootReducer = combineReducers({ todoList: todos, count: counter });
const store = createStore(rootReducer);
store.getState(); // { todoList: [], count: 0 }
复制代码
实现:
实现的思路就是最终要返回一个合并后的 reducer 函数(即 combination
)。这个 combination
函数接受到 action
后会调用拆分出来的所有 reducer 函数,得到最新的状态(nextState
)。
export default function combineReducers(reducers) {
const reducerKeys = Object.keys(reducers);
return function combination(state = {}, action) {
const nextState = {};
for (let i = 0; i < reducerKeys.length; i++) {
const key = reducerKeys[i];
const reducer = reducers[key];
const previousStateForKey = state[key];
const nextStateForKey = reducer(previousStateForKey, action);
nextState[key] = nextStateForKey;
}
return nextState;
};
};
复制代码
applyMiddleware(...middleware)
middleware(中间件)
Redux 中间件的本质扩展 dispatch
的功能,并且返回被增强后的 dispatch
。我们先修改 createStore
中的 dispatch
方法,方便查看中间件的运行顺序。
const dispatch = action => {
console.log('原始的 dispatch 开始工作了')
state = reducer(state, action);
listeners.forEach(listener => listener());
console.log('原始的 dispatch 结束工作了')
};
复制代码
先自定义两个中间件并使用:
function middleware1 (store) {
return next => action => {
console.log('Middleware 1号增强的 dispatch 开始工作了');
const returnValue = next(action);
console.log('Middleware 1号增强的 dispatch 结束工作了');
return returnValue;
};
}
function middleware2 (store) {
return next => action => {
console.log('Middleware 2号增强的 dispatch 开始工作了');
const returnValue = next(action);
console.log('Middleware 2号增强的 dispatch 结束工作了');
return returnValue;
};
}
const store = createStore(todosReducer, {}, applyMiddleware(middleware1, middleware2));
复制代码
当我们发起一个 store.dispatch(action)
,可以看到这样的日志:
Middleware 1号增强的 dispatch 开始工作了
Middleware 2号增强的 dispatch 开始工作了
原始的 dispatch 开始工作了
原始的 dispatch 结束工作了
Middleware 2号增强的 dispatch 结束工作了
Middleware 1号增强的 dispatch 结束工作了
复制代码
可以看出多个中间件情况下,next
就是被下一个中间件增加后的 dispatch
,最后一个中间件调用了的即使最原始的 dispatch
。原始的 dispatch
被从右往左的中间件一个个增强。
所以如果只有一个中间件的情况下,next
就是最原始的 store.dispatch
。所以上面代码等效于:
const store = createStore(todosReducer, {});
const next = store.dispatch;
store.dispatch = action => {
console.log('Middleware 1号增强的 dispatch 开始工作了');
next(action);
console.log('Middleware 1号增强的 dispatch 结束工作了');
}
复制代码
如果存在两个中间件:
const store = createStore(todosReducer, {});
const next = store.dispatch;
const next2 = action => {
console.log('Middleware 2号增强的 dispatch 开始工作了');
next(action); // 原始的 store.dispatch
console.log('Middleware 2号增强的 dispatch 结束工作了');
}
store.dispatch = action => {
console.log('Middleware 1号增强的 dispatch 开始工作了');
next2(action);
console.log('Middleware 1号增强的 dispatch 结束工作了');
}
复制代码
上面虽然实现了自定义中间的功能,但是不够灵活,因为中间件可能不止2个,顺序需要自由的调换。所以我们就需要增加一个参数支持传入被下一个中间件增强的 dispatch
:
const next = store.dispatch;
const middleware2 = next => action => {
console.log('Middleware 2号增强的 dispatch 开始工作了');
const returnValue next(action);
console.log('Middleware 2号增强的 dispatch 结束工作了');
return returnValue;
}
const middleware1 = next => action => {
console.log('Middleware 1号增强的 dispatch 开始工作了');
const returnValue = next(action);
console.log('Middleware 1号增强的 dispatch 结束工作了');
return returnValue;
}
store.dispatch = middleware1(middleware2(next));
复制代码
又考虑到每个中间内部不仅仅是执行下一个 next
,比如一个 logger 中间件需要通过 store.getState
方法获取 state,一些支持异步场景的中间件内部还需要调用 store.dispatch
...
const logger = next => action => {
return next => action => {
console.log('即将 dispatch: ', action)
const returnValue = next(action)
console.log('dispatch 后的 state: ', store.getState())
return returnValue
}
}
复制代码
这些都来自 store
,所以每个中间件还需要增加一个参数 mAPI
以接收这些 API。
const next = store.dispatch;
const middleware2 = mAPI => next => action => {
console.log('Middleware 2号增强的 dispatch 开始工作了');
const returnValue = next(action);
console.log('Middleware 2号增强的 dispatch 结束工作了');
return returnValue;
}
const middleware1 = mAPI => next => action => {
console.log('Middleware 1号增强的 dispatch 开始工作了');
const returnValue = next(action);
console.log('Middleware 1号增强的 dispatch 结束工作了');
return returnValue;
}
const mAPI = store;
const m2 = middleware2(mAPI);
const m1 = middleware1(mAPI);
store.dispatch = m1(m2(next));
复制代码
Redux 中规定中间件中可使用的 API 只有 getState
和 dispatch
。所以改进之后:
const next = store.dispatch;
// ...
let dispatch = next;
const mAPI = {
getState: store.getState,
dispatch: (action, ...args) => dispatch(action, ...args)
}
const chain = [middleware1, middleware2].map(m => m(mAPI)); // [m1, m2]
// 实现:store.dispatch = m1(m2(next))
dispatch = chain.reverse().forEach(m => {
dispatch = m(dispatch);
});
store.dispatch = dispatch;
复制代码
applyMiddleware
上面已经实现了中间件的功能,但是还不够优雅,接下来要做的就是把上面的过程收拢到一个 applyMiddleware
函数内部,期望的使用方式是:
const store = createStore(reducer, preloadedState, applyMiddleware(...middlewares));
复制代码
applyMiddleware(...middlewares)
的目的是增强 createStore
返回的 dispatch
方法。所以可以理解为 applyMiddleware(...middlewares)
返回 createStore
的增强器 enhancer
,将 createStore
传入增强器后返回 createStore
的升级版。
const enhancer = applyMiddleware(...middlewares);
const createStore2 = enhancer(createStore);
复制代码
createStore
中增加一段逻辑,当传入 enhancer
时返回增强版的 createStore
:
// const enhancer = applyMiddleware(middleware1, middleware2);
function createStore(reducer, preloadedState, enhancer) {
if (typeof applyMiddleware === 'function') {
const createStore2 = enhancer(createStore);
return createStore2(reducer, preloadedState);
}
// ...
}
复制代码
实现 applyMiddleware
:
把处理中间件,增加 dispatch
的逻辑收拢到 applyMiddleware
返回的 createStore
增强器中。
function applyMiddleware(...middlewares) {
const enhancer = createStore => (reducer, preloadedState) => {
const store = createStore(reducer, preloadedState);
const next = store.dispatch;
let dispatch = next;
const mAPI = {
getState: store.getState,
dispatch: (action, ...args) => dispatch(action, ...args),
}
const chain = middlewares.map(middleware => middleware(mAPI));
chain.reverse().forEach(m => {
dispatch = m(dispatch);
});
return {
...store,
dispatch
}
};
return enhancer;
}
复制代码
compose(...functions)
compose
的作用是从右到左把接收到的函数合成为一个最终函数,右边函数的返回值将作为一个参数提供给它左边的函数。即 compose(funcA, funcB, funcC)
变为 compose(funcA(funcB(funcC())))
。
function compose(...funcs) {
if (funcs.length === 0) return arg => arg;
if (funcs.length === 1) return funcs[0];
return funcs.reduce((a, b) => {
retrun (...args) => a(b(...args));
});
}
复制代码
applyMiddleware
处理中间件即可复用此逻辑,store.dispatch = m1(m2(store.dispatch))
,所以 applyMiddleware
可以使用 compose
修改为:
function applyMiddleware(...middlewares) {
const enhancer = createStore => (reducer, preloadedState) => {
const store = createStore(reducer, preloadedState);
let dispatch;
const mAPI = {
getState: store.getState,
dispatch: (action, ...args) => dispatch(action, ...args),
}
const chain = middlewares.map(middleware => middleware(mAPI));
dispatch = compose(...chain)(store.dispatch);
return {
...store,
dispatch
}
};
return enhancer;
}
复制代码
bindActionCreators(actionCreators, dispatch)
actionCreator
即创建 action 的函数,比如下面的两个👇。actionCreator
的好处是复用创建 action 的逻辑,使用 dispatch
时只需 dispatch(actionCreator(value))
。
export function addTodo(text) {
return {
type: 'ADD_TODO',
text
}
}
export function removeTodo(id) {
return {
type: 'REMOVE_TODO',
id
}
}
复制代码
bindActionCreators
的唯一使用场景就是当你需要把 actionCreator 传给子组件,但却不想让子组件感觉到 Redux 的存在。
import { addTodo, removeTodo } from './TodoActionCreators'
class TodoListContainer extends Component {
constructor(props) {
super(props)
const { dispatch } = props; // 由 react-redux 注入的 dispatch
// 对 actionCreator 绑定 dispatch 方法
this.boundActionCreators = bindActionCreators({
addTodo,
removeTodo,
}, dispatch);
console.log(this.boundActionCreators); // { addTodo: Function, removeTodo: Function }
}
render() {
// 由 react-redux 注入的 todos:
let { todos } = this.props
return <TodoList todos={todos} {...this.boundActionCreators} />
// 不使用 bindActionCreators 时的做法:
// 直接把 dispatch 当作 prop 传递给子组件,但这时子组件需要引入 actionCreator
// return <TodoList todos={todos} dispatch={dispatch} />;
}
}
复制代码
实现 bindActionCreators
:
bindActionCreators
的实现很简单,如果我们不想感知到 dispatch
和 actionCreator
,只需要利用闭包实现隐藏:
function bindActionCreator(actionCreator, dispatch) {
return function (this, ...args) {
return dispatch(actionCreator.apply(this, args));
}
}
// 使用:
const addTodo = bindActionCreator(addTodo, store.dispatch);
addTodo('Use Redux');
复制代码
如果有多个 actionCreator
则返回一个 boundActionCreators
对象:
export default function bindActionCreators(actionCreators, dispatch) {
if (typeof actionCreators === 'function') {
return bindActionCreator(actionCreators, dispatch);
}
if (typeof actionCreators !== 'object' || actionCreators === null) {
throw new Error('bindActionCreators expected an object or a function.');
}
const boundActionCreators = {}
for (const key in actionCreators) {
const actionCreator = actionCreators[key];
if (typeof actionCreator === 'function') {
boundActionCreators[key] = bindActionCreator(actionCreator, dispatch);
}
}
return boundActionCreators;
}
// 使用:
const boundActionCreators = bindActionCreator({
addTodo,
removeTodo,
}, dispatch);
boundActionCreators.addTodo('Use Redux');
复制代码