171 lines
4.9 KiB
Markdown
171 lines
4.9 KiB
Markdown
---
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id: 5a24c314108439a4d4036156
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title: 使用中间件处理异步操作
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challengeType: 6
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forumTopicId: 301451
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dashedName: use-middleware-to-handle-asynchronous-actions
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---
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# --description--
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目前为止的挑战都在避免讨论异步操作,但它们是 Web 开发中不可避免的一部分。在某些时候,你需要在 Redux 应用程序中使用异步请求,那么如何处理这些类型的请求?Redux 中间件专为此目的而设计,称为 Redux Thunk 中间件。这里简要介绍如何在 Redux 中使用它。
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如果要使用 Redux Thunk 中间件,请将其作为参数传递给`Redux.applyMiddleware()`。然后将此函数作为第二个可选参数提供给`createStore()`函数,看一下编辑器底部的代码,然后,要创建一个异步的 action,你需要在 action creator 中返回一个以`dispatch`为参数的函数。在这个函数中,你可以 dispatch action 并执行异步请求。
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在此示例中,使用`setTimeout()`调用模拟异步请求。通常在执行异步行为之前 dispatch action,以便应用程序状态知道正在请求某些数据(例如,这个状态可以显示加载图标)。然后,一旦收到数据,就会发送另一个 action,这个 action 完成的时间将作为数据的一个有效值。
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请记住,你正在通过将`dispatch`作为参数传递给这个特殊的 action creator。你用于 dispatch 你的 action 时只需将 action 直接传递给 dispatch,中间件就可以处理其余的部分。
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# --instructions--
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在`handleAsync()`的 action creator 中编写两个 dispatch 事件。在`setTimeout()`(模拟 API 调用)之前 dispatch`requestingData()`。然后在收到(模拟)数据后,dispatch`receivedData()`action,传入这些数据。现在你知道如何处理 Redux 中的异步操作,其他所有操作都继续像以前一样。
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# --hints--
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`requesData` action creator 应返回类型和值都为`REQUESTING_DATA`的对象。
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```js
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assert(requestingData().type === REQUESTING_DATA);
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```
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`receivedData` action creator 应返回类型和值都等于`RECEIVED_DATA`的对象。
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```js
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assert(receivedData('data').type === RECEIVED_DATA);
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```
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`asyncDataReducer`必须是个函数。
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```js
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assert(typeof asyncDataReducer === 'function');
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```
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分发 requestedData 后 action creator 应该将 store 中的`state`的 fetching 的值更新为 `true`。
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```js
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assert(
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(function () {
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const initialState = store.getState();
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store.dispatch(requestingData());
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const reqState = store.getState();
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return initialState.fetching === false && reqState.fetching === true;
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})()
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);
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```
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如果要 dispatch `handleAsync` 应先 dispatch 数据请求的 action,然后在收到请求结果后再 dispatch 接收的数据 action。
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```js
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assert(
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(function () {
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const noWhiteSpace = handleAsync.toString().replace(/\s/g, '');
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return (
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noWhiteSpace.includes('dispatch(requestingData())') === true &&
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noWhiteSpace.includes('dispatch(receivedData(data))') === true
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);
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})()
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);
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```
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# --seed--
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## --seed-contents--
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```js
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const REQUESTING_DATA = 'REQUESTING_DATA'
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const RECEIVED_DATA = 'RECEIVED_DATA'
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const requestingData = () => { return {type: REQUESTING_DATA} }
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const receivedData = (data) => { return {type: RECEIVED_DATA, users: data.users} }
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const handleAsync = () => {
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return function(dispatch) {
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// Dispatch request action here
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setTimeout(function() {
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let data = {
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users: ['Jeff', 'William', 'Alice']
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}
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// Dispatch received data action here
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}, 2500);
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}
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};
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const defaultState = {
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fetching: false,
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users: []
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};
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const asyncDataReducer = (state = defaultState, action) => {
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switch(action.type) {
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case REQUESTING_DATA:
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return {
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fetching: true,
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users: []
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}
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case RECEIVED_DATA:
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return {
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fetching: false,
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users: action.users
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}
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default:
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return state;
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}
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};
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const store = Redux.createStore(
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asyncDataReducer,
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Redux.applyMiddleware(ReduxThunk.default)
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);
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```
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# --solutions--
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```js
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const REQUESTING_DATA = 'REQUESTING_DATA'
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const RECEIVED_DATA = 'RECEIVED_DATA'
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const requestingData = () => { return {type: REQUESTING_DATA} }
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const receivedData = (data) => { return {type: RECEIVED_DATA, users: data.users} }
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const handleAsync = () => {
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return function(dispatch) {
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dispatch(requestingData());
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setTimeout(function() {
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let data = {
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users: ['Jeff', 'William', 'Alice']
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}
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dispatch(receivedData(data));
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}, 2500);
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}
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};
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const defaultState = {
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fetching: false,
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users: []
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};
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const asyncDataReducer = (state = defaultState, action) => {
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switch(action.type) {
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case REQUESTING_DATA:
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return {
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fetching: true,
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users: []
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}
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case RECEIVED_DATA:
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return {
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fetching: false,
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users: action.users
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}
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default:
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return state;
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}
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};
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const store = Redux.createStore(
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asyncDataReducer,
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Redux.applyMiddleware(ReduxThunk.default)
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);
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```
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