IT技术 - 使用 react-redux、reselect 和 React.memo() 记忆功能组件 - 吾爱随笔录

使用 react-redux、reselect 和 React.memo() 记忆功能组件

IT技术 reactjs react-redux memoization reselect

2021-05-12 05:53:28

我已经在 ReactJS 16.8.5 和 React-Redux 3.7.2 上构建了一个应用程序。当应用加载应用挂载时，会设置初始存储并针对 Firebase 实时数据库设置数据库订阅。该应用程序包含标题Sidebar和内容部分。
我已经与React.memo一起实现了reselect以避免在 props 更改时重新渲染，但组件仍在重新渲染。使用React profiler API和React.memo 中的比较函数，我可以看到尽管 props 相等，但它被渲染了几次。SidebarareEqualSidebar

app.js

//Imports etc...
const jsx = (
  <React.StrictMode>
    <Provider store={store}>
      <AppRouter />
    </Provider>
  </React.StrictMode>
)

let hasRendered = false
const renderApp = () => {
  if (!hasRendered) { //make sure app only renders one time
    ReactDOM.render(jsx, document.getElementById('app'))
    hasRendered = true
  }
}

firebase.auth().onAuthStateChanged((user) => {
  if (user) {
    // Set initial store and db subscriptions
    renderApp()
  }
})

AppRouter.js

//Imports etc...
const AppRouter = ({}) => {
  //...
  return (
    <React.Fragment>
      //uses Router instead of BrowserRouter to use our own history and not the built in one
      <Router history={history}>    
        <div className="myApp">
          <Route path="">
            <Sidebar ...props />
          </Route>
          //More routes here...
        </div>
      </Router>
    </React.Fragment>
  )
}
//...
export default connect(mapStateToProps, mapDispatchToProps)(AppRouter)

Sidebar.js

//Imports etc...
export const Sidebar = (props) => {
  const onRender = (id, phase, actualDuration, baseDuration, startTime, commitTime) => {
    if (id !== 'Sidebar') { return }
    console.log('onRender', phase, actualDuration)
  }
  return (
    <Profiler id="Sidebar" onRender={onRender}>
      <React.Fragment>
        {/* Contents of Sidebar */}
      </React.Fragment>
    </Profiler>
}

const getLang = state => (state.usersettings) ? state.usersettings.language : 'en'
const getMediaSize = state => (state.route) ? state.route.mediaSize : 'large'
const getNavigation = state => state.navigation
const getMyLang = createSelector(
  [getLang], (lang) => console.log('Sidebar lang val changed') || lang
)
const getMyMediaSize = createSelector(
  [getMediaSize], (mediaSize) => console.log('Sidebar mediaSize val changed') || mediaSize
)
const getMyNavigation = createSelector(
  [getNavigation], (navigation) => console.log('Sidebar navigation val changed') || navigation
)
const mapStateToPropsMemoized = (state) => {
  return {
    lang: getMyLang(state),
    mediaSize: getMyMediaSize(state),
    navigation: getMyNavigation(state)
  }
}

const areEqual = (prevProps, nextProps) => {
  const areStatesEqual = _.isEqual(prevProps, nextProps)
  console.log('Sidebar areStatesEqual', areStatesEqual)
  return areStatesEqual
}
export default React.memo(connect(mapStateToPropsMemoized, mapDispatchToProps)(Sidebar),areEqual)

初始渲染看起来没问题，直到Sidebar navigation val changed- 之后组件重新渲染了一大堆 - 为什么！？

Console output - initial render

onRender Sidebar mount 572 
Sidebar mediaSize val changed 
Profile Sidebar areEqual true 
Sidebar navigation val changed 
onRender Sidebar update 153 
Sidebar navigation val changed 
onRender Sidebar update 142 
onRender Sidebar update 103 
onRender Sidebar update 49 
onRender Sidebar update 5 
onRender Sidebar update 2 
onRender Sidebar update 12 
onRender Sidebar update 3 
onRender Sidebar update 2 
onRender Sidebar update 58 
onRender Sidebar update 2 
onRender Sidebar update 4 
onRender Sidebar update 5 
onRender Sidebar update 4

后续渲染不会影响 store 中映射到 props（位置）的任何部分，但组件仍在重新渲染。

Console output - subsequent render

Profile Sidebar areEqual true
onRender Sidebar update 76
onRender Sidebar update 4

我希望Sidebar在初始加载期间在商店的装载/更新期间被记住并且只渲染/重新渲染几次。

为什么Sidebar组件被渲染了这么多次？

亲切的问候/K

1个回答

不需要 React.memo，因为 react-redux connect 将返回一个纯组件，只有在您更改传递的 props 或在调度的操作导致状态发生任何更改后才会重新渲染。

你mapStateToPropsMemoized应该工作（见更新），但可能更好地写成这样：

const mapStateToPropsMemoized = createSelector(
  getMyLang,
  getMyMediaSize,
  getMyNavigation,
  (lang, mediaSize, navigation) => ({
    lang,
    mediaSize,
    navigation,
  })
);
//using react.redux connect will return a pure component and passing that
//  to React.memo should cause an error because connect does not return a
//  functional component.
export default connect(
  mapStateToPropsMemoized,
  mapDispatchToProps
)(Sidebar);

更新

你的 getState 应该可以工作。

我无法使用您的代码重现组件重新渲染。从 mapState 返回的对象每次都是一个新对象，但它的直接属性永远不会改变，因为选择器总是返回记忆化的结果。请参阅下面的示例

显示代码片段

const { useRef, useEffect } = React;
const {
  Provider,
  useDispatch,
  connect,
  useSelector,
} = ReactRedux;
const { createStore } = Redux;
const { createSelector } = Reselect;
const state = { someValue: 2, unrelatedCounter: 0 };
//returning a new state every action someValue
//  never changes, only counter
const reducer = (state) => ({
  ...state,
  unrelatedCounter: state.unrelatedCounter + 1,
});
const store = createStore(
  reducer,
  { ...state },
  window.__REDUX_DEVTOOLS_EXTENSION__ &&
    window.__REDUX_DEVTOOLS_EXTENSION__()
);
//selectors
const selectSomeValue = (state) => state.someValue;
//selectors only return a new object if someValue changes
const selectA = createSelector(
  [selectSomeValue],
  () => ({ value: 'A' }) //returns new object if some value changes
);
const selectB = createSelector(
  [selectSomeValue],
  () => ({ vale: 'B' }) //returns new object if some value changes
);
const selectC = createSelector(
  [selectSomeValue],
  () => ({ vale: 'C' }) //returns new object if some value changes
);
const Counter = () => {
  const counter = useSelector(
    (state) => state.unrelatedCounter
  );
  return <h4>Counter: {counter}</h4>;
};
const AppComponent = (props) => {
  const dispatch = useDispatch();
  const r = useRef(0);
  //because state.someValue never changes this component
  //  never gets re rendered
  r.current++;
  useEffect(
    //dispatch an action every second, this will create a new
    //  state but state.someValue never changes
    () => {
      setInterval(() => dispatch({ type: 88 }), 1000);
    },
    [dispatch] //dispatch never changes but linting tools don't know that
  );
  return (
    <div>
      <h1>Rendered {r.current} times</h1>
      <Counter />
      <pre>{JSON.stringify(props, undefined, 2)}</pre>
    </div>
  );
};
const mapStateToProps = (state) => {
  return {
    A: selectA(state),
    B: selectB(state),
    C: selectC(state),
  };
};

const App = connect(mapStateToProps)(AppComponent);
ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/redux/4.0.5/redux.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-redux/7.2.0/react-redux.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/reselect/4.0.0/reselect.min.js"></script>


<div id="root"></div>

其它你可能感兴趣的问题

上一篇使用实时 Firebase 数据库的 React 应用程序内存泄漏下一篇如何导航到另一个页面而无需在 NextJs 中重新加载整个页面？