import React, { ReactNode, useCallback, useContext, useEffect, useMemo, useState } from 'react'; import { AnotherVerySlowComponent, VerySlowComponent } from './components/very-slow-component'; import './styles.scss'; // store the state here const ContextData = React.createContext({ isNavExpanded: false }); // store the open/close functions here const ContextApi = React.createContext({ open: () => {}, close: () => {} }); const NavigationController = ({ children }: { children: ReactNode }) => { const [isNavExpanded, setIsNavExpanded] = useState(false); const open = useCallback(() => setIsNavExpanded(true), []); // no dependencies, close won't change const close = useCallback(() => setIsNavExpanded(false), []); // that one has a dependency on state const data = useMemo(() => ({ isNavExpanded }), [isNavExpanded]); // that one never changes - no dependencies const api = useMemo(() => ({ open, close }), [close, open]); return ( {children} ); }; const useNavigationData = () => useContext(ContextData); const useNavigationApi = () => useContext(ContextApi); const AdjustableColumnsBlock = () => { const { isNavExpanded } = useNavigationData(); return isNavExpanded ?

two block items here

three block items here

; }; const SomeComponent = () => { const { open } = useNavigationApi(); useEffect(() => { console.info("SomeComponent won't re-render on navigation expand/collapse, even though it uses Context"); }); return (

); }; const MainPart = () => { return ( <> ); }; const ExpandButton = () => { const { isNavExpanded } = useNavigationData(); const { open, close } = useNavigationApi(); useEffect(() => { console.info("Button that uses Context data re-renders. But SomeComponent won't"); }); return ; }; const SidebarLayout = ({ children }: { children: ReactNode }) => { const { isNavExpanded } = useNavigationData(); return (

{children}

); }; const Sidebar = () => { return ( {/* this one will control the expand/collapse */}

some links

); }; const Layout = ({ children }: { children: ReactNode }) => { const [scroll, setScroll] = useState(0); useEffect(() => { window.addEventListener('scroll', () => { setScroll(window.scrollY); }); }, []); return (

{children}

); }; const Page = () => { return ( ); }; export default function App() { return ( <>

Very slow "Page" component - click on expand/collapse to toggle nav

Those components that use only API won't re-render on nav expand/collapse anymore

); }

Chapter 8. Example 5