✨ Features

• Use your React Context without the additional re-renders in the consumers
• Ability to read values out of context "on-the-fly" - useful in callbacks so you don't have to bind the UI to a context value change just to use the value in a callback
• Redux-like pattern (reducer, actions, and selectors)
• Built with TypeScript

About

Here at Air, we needed a way to store multiple instances of complex global state (what React Context API does) but with the performance of Redux. react-memoized-context solves this problem.

Why not React Context?

A React Context provider renders all consumers every time it's value changes - even if the component isn't using a property on the value (if it's an object). This can cause lots of performance issues and the community is trying to solve it. We've looked at these other solutions but they're either not ready, had too many bugs or lacked features (like reading values on the fly) so we decided to roll our own.

Why not Redux?

Redux is great as a global store when multiple components want to read and write to a single centralized value. But when you want to have multiple global values with the same structure, Redux isn't as flexible because you need to duplicate your reducers, actions, and selectors. That's where React Context is nice because you can just wrap around another Provider.

Install

npm install --save @air/react-memoized-context

yarn add @air/react-memoized-context

Usage

1. Create types for your context:

   • create type for value, which you want to store:

     export interface User {
       id: string;
       name: string;
       score: number;
     }
       
     export interface UsersTeamContextValue {
       users: User[];
     }

   • create type for actions you want to provide to update value:

     export interface UsersTeamContextActions {
       addUser: (user: User) => void;
       assignScore: (userId: User['id'], score: number) => void;
     }

   • create type for your context - remember to extend MemoizedContextType:

     export interface UsersTeamContextType extends MemoizedContextType<UsersTeamContextValue>, UsersTeamContextActionsType {}

   • create default value for your context:

     export const defaultUsersTeamContextValue: UsersTeamContextType = {
       ...defaultMemoizedContextValue,
       getValue: () => ({
         users: [],
       }),
       addUser: () => {},
       assignScore: () => {},
      };

   • create types for your actions - you will use them to modify context value:

     export interface AddUserAction extends MemoizedContextAction {
       type: 'addUser';
       data?: { user: User };
     }
       
     export interface AssignScoreAction extends MemoizedContextAction {
       type: 'assignScore';
       data?: { userId: User['id']; score: number };
     }
       
     export type UserTeamContextActions = AddUserAction | AssignScoreAction;

2. Create your context:

   const UsersTeamContext = createContext<UsersTeamContextType>(defaultUsersTeamContextValue);
      
   const useUsersTeamContext = () => useContext(UsersTeamContext);

3. Create your dispatch method. It should work as redux dispatch - takes an action, modifies state value and returns a new state:

   export const usersTeamContextDispatch = (state: UsersTeamContextValue, action: UserTeamContextActions) => {
     switch (action.type) {
       case 'assignScore':
         return {
           ...state,
           users: state.users.map((user) => {
             if (user.id === action.data?.userId) {
               return {
                 ...user,
                 score: action.data?.score ?? 0,
               };
             }
             return user;
           }),
         };
       case 'addUser':
         return {
           ...state,
           users: action.data ? [...state.users, action.data.user] : state.users,
         };
     }
   };

4. Create your provider:

   export const UsersTeamProvider = ({ children }: PropsWithChildren<{}>) => {
        
     const { contextValue } = useMemoizedContextProvider<UsersTeamContextValue>(
       // provide default value for your context
       {
         users: [],
       },
       usersTeamContextDispatch,
     );
      
     // create methods you want to expose to clients
     const addUser = useCallback((user: User) => contextValue.dispatch({ type: 'addUser', data: { user } }), [contextValue]);
      
     const assignScore = useCallback(
       (userId: User['id'], score: number) => contextValue.dispatch({ type: 'assignScore', data: { userId, score } }),
       [contextValue],
     );
      
     // memoize your final value that will be available for clients
     // just return what's in contextValue and add your methods
     const value = useMemo<UsersTeamContextType>(
       () => ({
         ...contextValue,
         addUser,
         assignScore,
       }),
       [addUser, assignScore, contextValue],
     );
      
     return <UsersTeamContext.Provider value={value}>{children}</UsersTeamContext.Provider>;
   };

5. To retrieve data from context, you need selectors:

   export const usersTeamUsersSelector = (state: UsersTeamContextValue) => state.users;

   usage in component:

   const context = useUsersTeamContext();
   // pass context to useMemoizedContextSelector
   const users = useMemoizedContextSelector(context, usersTeamUsersSelector);

   to simplify it, you can create a helper:

   export function useUsersTeamContextSelector<T>(selector: (st: UsersTeamContextValue) => T) {
     const context = useUsersTeamContext();
     return useMemoizedContextSelector(context, selector);
   }
      

   then, to retrieve users from context you can do:

   const users = useUsersTeamContextSelector(usersTeamUsersSelector);

6. Start using your context!

   Wrap your components with your Provider component, as you do with React Context:

   <UsersTeamProvider>
       <UsersTeam name="Team 1" />
   </UsersTeamProvider>

   To modify context value, use any of your actions:

   import { useUsersTeamContextSelector } from "./usersTeamContext";
      
   const { addUser } = useUsersTeamContext()
      
   const onClick = () => {
     addUser({ name: 'John' })
   }
      

   You can read context values on the fly if you need. For example, we will create a user with users.length as id. We can use usersTeamUsersSelector, but the component would be rerendered every time when any user changes. We don't want that - we need just users length. We could create a selector that gets users length, but again - everytime we add a user, the component will rerender. For us, it's enough to know users length by the time we create a user:

    // get whole context value - it will not cause any rerender!
    const contextValue = useUsersTeamContext();
      
     const addNewUser = () => {
       // read users array when we need it
       const users = contextValue.getValue().users;
       // call addUser action to add a new user
       contextValue.addUser({ id: users.length + 1, name: userName, score: 0 });
     };