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README.md
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README.md

Memoization dans React (1)

Nous allons parler de :

  • React hook useCallback
  • React hook useMemo
  • Higher Order Component memo

useCallback

Situation type 1 :
Le component reçoit une fonction via props et elle est utilisée dans useEffect.

Situation type 2 :
Je crée un hook qui retourne une fonction qui peut être utilisée dans useEffect.

Situation type 3 :
Le component emballé par React.memo reçoit une fonction via props.

const memoizedDoSomething = useCallback(() => {
  doSomething(a, b)
}, [a, b])
  • prend toujours 2 paramètres
  • le premier est une fonction à memoize
  • le deuxième est un array de deps
    • [] : function est memoized une seule fois quand le component mounts
    • [a, b] : nouvelle version est memoized à chaque fois que la changement de a ou de b est détecté.

Le problème (Exemple Star Wars Planet)

Regardons ensemble le code dans ce repo.

// Parent Component
import { useReducer } from "react"
import Planets from "./Planets"
import { reducer } from "../reducers"

const PlanetsApp = () => {
  const initialState = {
    planets: [],
    loading: false,
    error: "",
  }
  const [state, dispatch] = useReducer(reducer, initialState)
  const { planets, loading, error } = state

  const initFetch = () => dispatch({ type: "FETCH_INIT" })
  const fetchPlanets = () => {
    fetch(`https://swapi.dev/api/planets/`)
      .then(response => {
        if (!response.ok) {
          throw new Error(
            `Nous n'avons pas pu lire le registre des planètes, status : ${response.status}`
          )
        }
        return response.json()
      })
      .then(data => {
        dispatch({ type: "FETCH_SUCCESS", payload: data.results })
      })
      .catch(error => {
        dispatch({ type: "FETCH_FAILURE", payload: error.message })
      })
  }
  return (
    <>
      <Planets
        planets={planets}
        initFetch={initFetch}
        fetchData={fetchPlanets}
      />
      {loading && <p className="text-center">loading...</p>}
      {!!error && <p className="alert alert-danger">{error}</p>}
    </>
  )
}

export default PlanetsApp
// Child Component
import { useEffect } from "react"
import Planet from "./Planet"

const Planets = ({ planets, initFetch, fetchData }) => {
  useEffect(() => {
    initFetch()
    fetchData()
  }, []) // 👈 ⚠️ warning, nous devons ajouter des deps
  return (
    <>
      <div className="row">
        {planets.map(planet => {
          return <Planet key={planet.name} planet={planet} />
        })}
      </div>
    </>
  )
}

export default Planets

alors :

// Child Component
import { useEffect } from "react"
import Planet from "./Planet"

const Planets = ({ planets, initFetch, fetchData }) => {
  useEffect(() => {
    initFetch()
    fetchData()
  }, [initFetch, fetchData]) // no more warnings !!!
  return (
    <>
      <div className="row">
        {planets.map(planet => {
          return <Planet key={planet.name} planet={planet} />
        })}
      </div>
    </>
  )
}

export default Planets

No more warnings 🎉 mais ....

Pourquoi ?

https://codepen.io/alyra/pen/WNpXOyP

Voici un schema d'une situation type:

const ParentComponent = () => {
  // ici pour la simplicité, state est un nombre
  const [state, setState] = useState(0)
  const someFunction = () => {
    /* qui à l'occasion met à jour state */
    setState(s => s + 1)
  }
  return <ChildComponent someFunction={someFunction} />
}
const ChildComponent = ({ someFunction }) => {
  React.useEffect(() => {
    someFunction()
  }, [someFunction])
  return <div>Hello !</div>
}
ParentComponent mounts (state => 0)
  someFunction is created 🆕
  ChildCompoment mounts
    useEffect runs
      state => 1

---

ParentComponent re-renders (state changed)
  someFunction is created 🆕
  ChildCompoment rerenders
    useEffect runs because someFunction 🆕 changed
      state => 2

---

ParentComponent re-renders (state changed)
  someFunction is created 🆕
  ChildCompoment rerenders
    useEffect runs because someFunction 🆕 changed
      state => 3

---

.... infinite loop 🤯

https://codepen.io/alyra/pen/Rwpjgmy

Solution

const ParentComponent = () => {
  // ici pour la simplicité, state est un nombre
  const [state, setState] = useState(0)
  const someFunction = useCallback(() => {
    /* qui à l'occasion met à jour state */
    setState(s => s + 1)
  }, [])
  return <ChildComponent someFunction={someFunction} />
}
const ChildComponent = ({ someFunction }) => {
  React.useEffect(() => {
    someFunction()
  }, [someFunction])
  return <div>Hello !</div>
}
ParentComponent mounts (state => 0)
  someFunction is created 🆕 and stored ♻ with useCallback
  ChildCompoment mounts
    useEffect runs
      state => 1

---

ParentComponent re-renders (state changed)
  someFunction is restored ♻ from useCallback
  ChildCompoment rerenders
    useEffect does not run because someFunction ♻ didn't change

Star Wars Planets

PlanetsApp mounts (state => {...})
  fetchInit is created 🆕
  fetchData is created 🆕
  Planets mounts
    useEffect runs
      fetch data calls dispatch
        state changes {...}

---

ParentComponent re-renders (state changed)
  fetchInit is created 🆕
  fetchData is created 🆕
  Planets mounts
    useEffect runs because fetchInit changed and because fetchData changed
      fetch data calls dispatch
        state changes {...}

---

ParentComponent re-renders (state changed)
  fetchInit is created 🆕
  fetchData is created 🆕
  Planets mounts
    useEffect runs because fetchInit changed and because fetchData changed
      fetch data calls dispatch
        state changes {...}
---

.... infinite loop 🤯

Solution

// Parent Component
import { useReducer, useCallback } from "react"
import Planets from "./Planets"
import { reducer } from "../reducers"

const PlanetsApp = () => {
  const initialState = {
    planets: [],
    loading: false,
    error: "",
  }
  const [state, dispatch] = useReducer(reducer, initialState)
  const { planets, loading, error } = state

  const initFetch = useCallback(() => dispatch({ type: "FETCH_INIT" }), [])
  const fetchPlanets = useCallback(() => {
    fetch(`https://swapi.dev/api/planets/`)
      /* comme avant */
  }, [])

  return ( /* comme avant */ )
}

export default PlanetsApp

Changement des pages

git checkout page

Nous avons ajouté un component <SwitchPage /> et une variable de state page afin de pouvoir visualiser les données de toutes les planètes. Mais ceci ne fonctionne pas correctement.

Comment corriger le code suivant?

import { useReducer, useState, useCallback } from "react"
import Planets from "./Planets"
import { reducer } from "../reducers"
import SwitchPage from "./SwitchPage"

const PlanetsApp = () => {
  const initialState = {
    planets: [],
    loading: false,
    error: "",
  }
  const [page, setPage] = useState(1)
  const [state, dispatch] = useReducer(reducer, initialState)
  const [active, setActive] = useState(false)
  const { planets, loading, error } = state

  const initFetch = useCallback(() => dispatch({ type: "FETCH_INIT" }), [])

  const fetchPlanets = useCallback(() => {
    fetch(`https://swapi.dev/api/planets/?page=${page}`)
      .then(response => {
        if (!response.ok) {
          throw new Error(
            `Nous n'avons pas pu lire le registre des planètes, status : ${response.status}`
          )
        }
        return response.json()
      })
      .then(data => {
        dispatch({ type: "FETCH_SUCCESS", payload: data.results })
      })
      .catch(error => {
        dispatch({ type: "FETCH_FAILURE", payload: error.message })
      })
  }, [])
  return (
    <>
      {!active ? (
        <button className="btn btn-dark" onClick={() => setActive(true)}>
          Afficher des planètes
        </button>
      ) : (
        <>
          <SwitchPage page={page} setPage={setPage} />
          <Planets
            planets={planets}
            initFetch={initFetch}
            fetchData={fetchPlanets}
          />
        </>
      )}
      {loading && <p className="text-center">loading...</p>}
      {!!error && <p className="alert alert-danger">{error}</p>}
    </>
  )
}

export default PlanetsApp

Solution

import { useReducer, useState, useCallback } from "react"
import Planets from "./Planets"
import { reducer } from "../reducers"
import SwitchPage from "./SwitchPage"

const PlanetsApp = () => {
  const initialState = {
    planets: [],
    loading: false,
    error: "",
  }
  const [page, setPage] = useState(1)
  const [state, dispatch] = useReducer(reducer, initialState)
  const [active, setActive] = useState(false)
  const { planets, loading, error } = state

  const initFetch = useCallback(() => dispatch({ type: "FETCH_INIT" }), [])

  const fetchPlanets = useCallback(() => {
    fetch(`https://swapi.dev/api/planets/?page=${page}`)
      .then(response => {
        if (!response.ok) {
          throw new Error(
            `Nous n'avons pas pu lire le registre des planètes, status : ${response.status}`
          )
        }
        return response.json()
      })
      .then(data => {
        dispatch({ type: "FETCH_SUCCESS", payload: data.results })
      })
      .catch(error => {
        dispatch({ type: "FETCH_FAILURE", payload: error.message })
      })
  }, [page]) // 👈
  return /* comme avant */
}

export default PlanetsApp

useCallback dans custom hooks

Revenons sur notre exemple Todo with API.

Nous allons mettre en place un hook qui permet de controller la fonction dispatch, elle devrait être exécutée uniquement si component est dans la page.

// src/hooks/useSafeDispatch.js
import { useCallback } from "react"
import { useIsMounted } from "./useIsMounted"

export const useSafeDispatch = dispatch => {
  const isMounted = useIsMounted()
  return useCallback(
    action => (isMounted.current ? dispatch(action) : undefined),
    [dispatch, isMounted]
  )
}