背景：由于react官方并没有提供缓存组件相关的api（类似vue中的keepalive），在某些场景，会使得页面交互性变的很差，比如在有搜索条件的表格页面，点击某一条数据跳转到详情页面，再返回表格页面，会重新请求数据，搜索条件也将清空，用户得重新输入搜索条件，再次请求数据，大大降低办公效率，如图：

目标：封装keepalive缓存组件，实现组件的缓存，并暴露相关方法，可以手动清除缓存。

版本：React 17，react-router-dom 5

结构：

代码：

cache-types.js

// 缓存状态
export const CREATE = 'CREATE';        // 创建
export const CREATED = 'CREATED';      // 创建成功
export const ACTIVE = 'ACTIVE';        // 激活
export const DESTROY = 'DESTROY';      // 销毁

 CacheContext.js

import React from 'react';
const CacheContext = React.createContext();
export default CacheContext;

cacheReducer.js

import * as cacheTypes from "./cache-types";
function cacheReducer(cacheStates = {}, { type, payload }) {
  switch (type) {
    case cacheTypes.CREATE:
      return {
        ...cacheStates,
        [payload.cacheId]: {
          scrolls: {},              // 缓存滚动条，key: dom, value: scrollTop
          cacheId: payload.cacheId, // 缓存Id
          element: payload.element, // 需要渲染的虚拟DOM
          doms: undefined,          // 当前的虚拟dom所对应的真实dom
          status: cacheTypes.CREATE,// 缓存状态
        },
      };
    case cacheTypes.CREATED:
      return {
        ...cacheStates,
        [payload.cacheId]: {
          ...cacheStates[payload.cacheId],
          doms: payload.doms,
          status: cacheTypes.CREATED,
        },
      };
    case cacheTypes.ACTIVE:
      return {
        ...cacheStates,
        [payload.cacheId]: {
          ...cacheStates[payload.cacheId],
          status: cacheTypes.ACTIVE,
        },
      };
    case cacheTypes.DESTROY:
      return {
        ...cacheStates,
        [payload.cacheId]: {
          ...cacheStates[payload.cacheId],
          status: cacheTypes.DESTROY,
        },
      };
    default:
      return cacheStates;
  }
}
export default cacheReducer;

KeepAliveProvider.js

import React, { useReducer, useCallback } from "react";
import CacheContext from "./CacheContext";
import cacheReducer from "./cacheReducer";
import * as cacheTypes from "./cache-types";
function KeepAliveProvider(props) {
  let [cacheStates, dispatch] = useReducer(cacheReducer, {});
  const mount = useCallback(
    ({ cacheId, element }) => {
      // 挂载元素方法，提供子组件调用挂载元素
      if (cacheStates[cacheId]) {
        let cacheState = cacheStates[cacheId];
        if (cacheState.status === cacheTypes.DESTROY) {
          let doms = cacheState.doms;
          doms.forEach((dom) => dom.parentNode.removeChild(dom));
          dispatch({ type: cacheTypes.CREATE, payload: { cacheId, element } }); // 创建缓存
        }
      } else {
        dispatch({ type: cacheTypes.CREATE, payload: { cacheId, element } }); // 创建缓存
      }
    },
    [cacheStates]
  );
  let handleScroll = useCallback(
    // 缓存滚动条
    (cacheId, { target }) => {
      if (cacheStates[cacheId]) {
        let scrolls = cacheStates[cacheId].scrolls;
        scrolls[target] = target.scrollTop;
      }
    },
    [cacheStates]
  );
  return (
    <CacheContext.Provider
      value={{ mount, cacheStates, dispatch, handleScroll }}
    >
      {props.children}
      {/* cacheStates维护所有缓存信息， dispatch派发修改缓存状态*/}
      {Object.values(cacheStates)
        .filter((cacheState) => cacheState.status !== cacheTypes.DESTROY)
        .map(({ cacheId, element }) => (
          <div
            id={`cache_${cacheId}`}
            key={cacheId}
            // 原生div中声明ref，当div渲染到页面，会执行ref中的回调函数，这里在id为cache_${cacheId}的div渲染完成后，会继续渲染子元素
            ref={(dom) => {
              let cacheState = cacheStates[cacheId];
              if (
                dom &&
                (!cacheState.doms || cacheState.status === cacheTypes.DESTROY)
              ) {
                let doms = Array.from(dom.childNodes);
                dispatch({
                  type: cacheTypes.CREATED,
                  payload: { cacheId, doms },
                });
              }
            }}
          >
            {element}
          </div>
        ))}
    </CacheContext.Provider>
  );
}
const useCacheContext = () => {
  const context = React.useContext(CacheContext);
  if (!context) {
    throw new Error("useCacheContext必须在Provider中使用");
  }
  return context;
};
export { KeepAliveProvider, useCacheContext };

withKeepAlive.js

import React, { useContext, useRef, useEffect } from "react";
import CacheContext from "./CacheContext";
import * as cacheTypes from "./cache-types";
function withKeepAlive(
  OldComponent,
  { cacheId = window.location.pathname, scroll = false }
) {
  return function (props) {
    const { mount, cacheStates, dispatch, handleScroll } =
      useContext(CacheContext);
    const ref = useRef(null);
    useEffect(() => {
      if (scroll) {
        // scroll = true, 监听缓存组件的滚动事件，调用handleScroll()缓存滚动条
        ref.current.addEventListener(
          "scroll",
          handleScroll.bind(null, cacheId),
          true
        );
      }
    }, [handleScroll]);
    useEffect(() => {
      let cacheState = cacheStates[cacheId];
      if (
        cacheState &&
        cacheState.doms &&
        cacheState.status !== cacheTypes.DESTROY
      ) {
        // 如果真实dom已经存在，且状态不是DESTROY，则用当前的真实dom
        let doms = cacheState.doms;
        doms.forEach((dom) => ref.current.appendChild(dom));
        if (scroll) {
          // 如果scroll = true, 则将缓存中的scrollTop拿出来赋值给当前dom
          doms.forEach((dom) => {
            if (cacheState.scrolls[dom])
              dom.scrollTop = cacheState.scrolls[dom];
          });
        }
      } else {
        // 如果还没产生真实dom，派发生成
        mount({
          cacheId,
          element: <OldComponent {...props} dispatch={dispatch} />,
        });
      }
    }, [cacheStates, dispatch, mount, props]);
    return <div id={`keepalive_${cacheId}`} ref={ref} />;
  };
}
export default withKeepAlive;

index.js

export { KeepAliveProvider } from "./KeepAliveProvider";
export {default as withKeepAlive} from './withKeepAlive';

使用：

　　1.用<KeepAliveProvider></KeepAliveProvider>将目标缓存组件或者父级包裹；

　　2.将需要缓存的组件，传入withKeepAlive方法中，该方法返回一个缓存组件；

　　3.使用该组件；

App.js

import React from "react";
import {
  BrowserRouter,
  Link,
  Route,
  Switch,
} from "react-router-dom";
import Home from "./Home.js";
import List from "./List.js";
import Detail from "./Detail.js";
import { KeepAliveProvider, withKeepAlive } from "./keepalive-cpn";

const KeepAliveList = withKeepAlive(List, { cacheId: "list", scroll: true });

function App() {
  return (
    <KeepAliveProvider>
      <BrowserRouter>
        <ul>
          <li>
            <Link to="/">首页</Link>
          </li>
          <li>
            <Link to="/list">列表页</Link>
          </li>
          <li>
            <Link to="/detail">详情页A</Link>
          </li>
        </ul>
        <Switch>
          <Route path="/" component={Home} exact></Route>
          <Route path="/list" component={KeepAliveList}></Route>
          <Route path="/detail" component={Detail}></Route>
        </Switch>
      </BrowserRouter>
    </KeepAliveProvider>
  );
}

export default App;

效果：

假设有个需求，从首页到列表页，需要清空搜索条件，重新请求数据，即回到首页，需要清除列表页的缓存。

上面的KeepAliveProvider.js中，暴露了一个useCacheContext()的hook，该hook返回了缓存组件相关数据和方法，这里可以用于清除缓存：

Home.js

import React, { useEffect } from "react";
import { DESTROY } from "./keepalive-cpn/cache-types";
import { useCacheContext } from "./keepalive-cpn/KeepAliveProvider";

const Home = () => {
  const { cacheStates, dispatch } = useCacheContext();

  const clearCache = () => {
    if (cacheStates && dispatch) {
      for (let key in cacheStates) {
        if (key === "list") {
          dispatch({ type: DESTROY, payload: { cacheId: key } });
        }
      }
    }
  };
  useEffect(() => {
    clearCache();
    // eslint-disable-next-line
  }, []);
  return (
    <div>
      <div>首页</div>
    </div>
  );
};

export default Home;

效果：

至此，react简易版的keepalive组件已经完成啦~

脚踏实地行，海阔天空飞