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tldr; How do I simulate componentDidUpdate or otherwise use key with an array to force my component to be reset?

I'm implementing a component which displays a timer and executes a callback when it reaches zero. The intent is for the callback to update a list of objects. The latter component is made of the new React hooks useState and useEffect.

The state contains a reference to the time at which the timer was started, and the time remaining. The effect sets an interval called every second to update the time remaining, and to check whether the callback should be called.

The component is not meant to reschedule a timer, or keep the interval going when it reaches zero, it's supposed to execute the callback and idle. In order for the timer to refresh, I was hoping to pass an array to key which would cause the component's state to be reset, and thus the timer would restart. Unfortunately key must be used with a string, and therefore whether or not my array's reference has changed produces no effect.

I also tried to push changes to the props by passing the array that I was concerned about, but the state was maintained and thus the interval was not reset.

What would be the preferred method to observe shallow changes in an array in order to force a state to be updated solely using the new hooks API?

import React, { useState, useEffect } from 'react';

import PropTypes from 'prop-types';



function getTimeRemaining(startedAt, delay) {

    const now = new Date();

    const end = new Date(startedAt.getTime() + delay);

    return Math.max(0, end.getTime() - now.getTime());

}



function RefresherTimer(props) {

    const [startedAt, setStartedAt] = useState(new Date());

    const [timeRemaining, setTimeRemaining] = useState(getTimeRemaining(startedAt, props.delay));



    useEffect(() => {



        if (timeRemaining <= 0) {

            // The component is set to idle, we do not set the interval.

            return;

        }



        // Set the interval to refresh the component every second.

        const i = setInterval(() => {

            const nowRemaining = getTimeRemaining(startedAt, props.delay);

            setTimeRemaining(nowRemaining);



            if (nowRemaining <= 0) {

                props.callback();

                clearInterval(i);

            }

        }, 1000);



        return () => {

            clearInterval(i);

        };

    });



    let message = `Refreshing in ${Math.ceil(timeRemaining / 1000)}s.`;

    if (timeRemaining <= 0) {

        message = 'Refreshing now...';

    }



    return <div>{message}</div>;

}



RefresherTimer.propTypes = {

    callback: PropTypes.func.isRequired,

    delay: PropTypes.number

};



RefresherTimer.defaultProps = {

    delay: 2000

};



export default RefresherTimer;

Attempted to use with key:

<RefresherTimer delay={20000} callback={props.updateListOfObjects} key={listOfObjects} />

Attempted to use with a props change:

<RefresherTimer delay={20000} callback={props.updateListOfObjects} somethingThatChanges={listOfObjects} />

listOfObjects refers to an array of objects, where the objects themselves won't necessarily change, so the array should be compared with !==. Typically, the value will be coming from Redux, where the action updateListOfObjects causes the array to be reinitialised like so: newListOfObjects = [...listOfObjects].

A way to remount a component is to provide new key property. It's not necessarily a string but it will be coerced to a string internally, so if listOfObjects is a string, it's expected that key is compared internally with listOfObjects.toString().

Any random key can be used, e.g. uuid or Math.random(). Shallow comparison of listOfObjects can be performed in parent component to provide new key. useMemo hook can be used in parent state to conditionally update remount key, and listOfObjects can be used as a list of parameters that need to be memoized. Here's an example:

  const remountKey = useMemo(() => Math.random(), listOfObjects);



  return (

    <div>

      <RefresherTimer delay={3000} callback={() => console.log('refreshed')} key={remountKey} />

    </div>

  );

As an alternative to remount key, child component could be able to reset own state and expose a callback to trigger a reset.

Doing shallow comparison of listOfObjects inside child component would be an antipattern because this requires it to be aware of parent component implementation.

In short, you want to reset your timer when the reference of the array changes, right ?
If so, you will need to use some diffing mechanism, a pure hooks based solution would take advantage of the second parameter of useEffect, like so:

function RefresherTimer(props) {

    const [startedAt, setStartedAt] = useState(new Date());

    const [timeRemaining, setTimeRemaining] = useState(getTimeRemaining(startedAt, props.delay));



    //reset part, lets just set startedAt to now

    useEffect(() => setStartedAt(new Date()),

      //important part

      [props.listOfObjects] // <= means: run this effect only if any variable

      // in that array is different from the last run

    )



    useEffect(() => {

    // everything with intervals, and the render

}

More information about this behaviour here https://reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects

The withRef creates an "instance variable" in functional component. It acts a flag to indicate whether it is in mount or update phase without updating state.

const mounted = useRef();

useEffect(() => {

  if (!mounted.current) {

    mounted.current = true;

  } else {

    // do componentDidUpate logic

  }

});