There is a component "DateForm" that changes the global state of "counterInfo" when the form is submitted.
//DateForm component submittal function.
const submitDate = () =>{
props.setCounterInfo(dateInfo); //passes date info to be counterInfo state in App.js
props.setShowInputForm(false); //DateInfo component is no longer rendered
}
then, in app.js the counterInfo state is passed to the Timer component
const App = () => {
const [showInputForm, setShowInputForm] = useState(false);
const [counterInfo, setCounterInfo] = useState(undefined);
return (
<>
<Timer
counterInfo = {counterInfo}
></Timer>
{showInputForm &&
<DateForm
setShowInputForm = {setShowInputForm}
setCounterInfo = {setCounterInfo}
></DateForm>}
</>
);
}
There is a useEffect hook inside of the Timer function that, on a one second interval, used the value of counterInfo.
//Inside the Timer Component
const [currTime, setCurrTime] = useState(null);
useEffect (() => {
setInterval(() => {
let timeLeft = (new Date(`${Months(props.counterInfo.year)[props.counterInfo.month-1].name} ${props.counterInfo.day} ${props.counterInfo.year} ${props.counterInfo.hour}:${props.counterInfo.minute}:${props.counterInfo.second}`).getTime()) - new Date().getTime();
setCurrTime(timeLeft);
},1000);
return(clearInterval());
}, [props, setCurrTime]);
What I intended to happen is for the value of timeLeft in Timer.js to update when the value of counterInfo is updated in DateForm, however, when the value is changed in DateForm, the result of both the new value of counterInfo and the old one both flash when the value of timeLeft is used in Timer.js. This issue isn't caused by any code in Timer.js becuase I tried moving the useEffect hook to app.js and passing the value down to Timer but the problem persisted. The only place that the setCounterInfo state is changed is in the DateForm component.
Does anyone have any idea how to fix this?
First, you have bit mis-syntax at interval decleration
useEffect (() => {
let interval = setInterval(() => {...},1000);
return () => clearInterval(interval);
}, [props, setCurrTime]);
But unrelated, React by default re-applies effects after every render. This is intentional and helps avoid a whole class of bugs that are present in React components.
When it comes to intervals, its specifical matters cause if a render was applied every time setInterval is called, it never will get a chance to actually run
In other words, this code might have some side effects as useEffect in each run cares only of the existing values in that time and forget everything else, and interval isn't like so.
For that from my point of view the best practice is to create useInterval custom hook, that inside will store the callback for the meanwhile
function useInterval(callback) {
const savedCallback = React.useRef();
useEffect(() => {
savedCallback.current = callback;
});
useEffect(() => {
function run() {
savedCallback.current();
}
let interval = setInterval(run ,1000);
return () => clearInterval(interval);
}, [])
}
//Inside the Timer Component
const [currTime, setCurrTime] = useState(null);
useInterval(()=>
setCurrTime((new Date(`${Months(props.counterInfo.year)[props.counterInfo.month-1].name} ${props.counterInfo.day} ${props.counterInfo.year} ${props.counterInfo.hour}:${props.counterInfo.minute}:${props.counterInfo.second}`).getTime()) - new Date().getTime()))
Related
Hello i am struggling to set dynamic time for settimeout function in react js.
i have long string of key value pair of time and message. i wants to display each message for specific time and loop through whole list.
here is what i am trying, but not working.
const [timer, setTimer] = useState(0)
const [time, setTime] = useState(5000)// this is default value to start which need to update with str time value
const str=[{name:"rammy", time:1000},
{name:"james", time:4000},
{name:"crown", time:2000}]
useEffect(()=>{
const getTime= str[timer].time
setTime(getTime)
},[timer])
//when timer change it should update update time state which will be used to update time for time settime out
function increment() {
useEffect(()=>{
setTimeout(() => {
setTimer((ele)=>ele+1)
}, time);
},[timer])
} // above code is for increment time state on each iteration
function ButtonHandle(){
//setRealString(itr)
increment()
} //button handler for start timer
First of all, you can't put hooks inside functions (other than your component functions). https://reactjs.org/docs/hooks-rules.html
So take the useEffect out of increment()
useEffect(()=>{
increment()
},[timer])
function increment() {
setTimeout(() => {
setTimer((ele)=>ele+1)
}, time);
}
But you also need to clear the timeout. We can return the timeout function to reference it, and clear the time out with a return inside useEffect. Clearing timeouts and intervals in react
useEffect(()=>{
const myTimeout = increment()
return () => {
clearTimeout(myTimeout)
}
},[timer])
function increment() {
return setTimeout(() => {
setTimer((ele) => ele + 1);
}, time);
}
Then we can combine the useEffects which both have a dependancy array of [timer].
useEffect(() => {
const getTime = str[timer].time;
setTime(getTime);
const myTimeout = increment();
return () => {
clearTimeout(myTimeout);
};
}, [timer]);
You don't need to use useEffect to do it. You misunderstood the useEffect usage, it's a react hook to you implement side-effects and you can't use react-hooks inside a function, it should be in the component scope.
I can increment directly from the ButtonHandle function.
// On the index state is necessary in this implementation
const [index, setIndex] = useState(-1)
const guys=[
{name: "rammy", time:1000},
{name: "james", time:4000},
{name: "crown", time:2000}
]
// useCallback memoize the increment function so that it won't
// change the instance and you can use it in the useEffect
// dependency array
const increment = useCallback(() => {
setIndex((i) => i+1)
}, [])
useEffect(() => {
// change the state if the timer is greater than -1
if (index !== -1) {
if (index >= guys.length) {
setIndex(-1);
} else {
setTimeout(() => {
increment();
}, guys[index].time); // <-- you get the time from your array
}
}
}, [index, increment]);
function handleClick(){
//setRealString(itr)
increment()
}
Even though I helped you, I don't know what you're trying to do. This implementation sounds like a code smell. We can help you better if you explain the solution you're trying to do instead of just the peace of code.
You don't need to set the time state, as you already have the time in the array; avoiding unnecessary state changes is good.
I'm creating a simple countdown timer app with React and I am having hard time understanding how setInterval works while React re-renders the component.
For example, this following code had timer continue to run even though I had used clearInterval onPause().
let startTimer;
const onStart = () => {
startTimer = setInterval( ()=>{
if ( timeRemaining === 0 ) {
clearInterval(startTimer);
setIsCounting(false)
return
}
updateTimer()
}, 1000)
setIsCounting( (prev) => !prev )
} // end of onStart
const onPause = () => {
setIsCounting( (prev) => !prev )
clearInterval(startTimer)
}
return (
{ props.isCounting ?
<button onClick={props.onPause}> Pause </button>
: <button onClick={props.onStart}> Start </button> }
)
However, the timer successfully pauses when I simply change
let starter;
to
let startTimer = useRef(null)
const onStart = () => {
startTimer.current = setInterval( ()=>{
if ( timeRemaining === 0 ) {
clearInterval(startTimer);
setIsCounting(false)
return
}
updateTimer()
}, 1000)
setIsCounting( (prev) => !prev )
} // end of onStart
const onPause = () => {
setIsCounting( (prev) => !prev )
clearInterval(startTimer.current)
}
What's happening to setInterval when React re-renders its component? Why did my timer continue to run when I didn't use useRef()?
A ref provides what's essentially an instance variable over the lifetime of a component. Without that, all that you have inside an asynchronous React function is references to variables as they were at a certain render. They're not persistent over different renders, unless explicitly done through the call of a state setter or through the assignment to a ref, or something like that.
Doing
let startTimer;
const onStart = () => {
startTimer = setInterval( ()=>{
could only even possibly work if the code that eventually calls clearInterval is created at the same render that this setInterval is created.
If you create a variable local to a given render:
let startTimer;
and then call a state setter, causing a re-render:
setIsCounting( (prev) => !prev )
Then, due to the re-render, the whole component's function will run again, resulting in the let startTimer; line running again - so it'll have a value of undefined then (and not the value to which it was reassigned on the previous render).
So, you need a ref or state to make sure a value persists through multiple renders. No matter the problem, reassigning a variable declared at the top level of a component is almost never the right choice in React.
Take the below code as an example.
import { useEffect, useState } from "react";
export default function Stopwatch(){
const [stopwatch, setStopwatch] = useState(0);
function updateStopwatch(){
console.log("Updating stopwatch from ", stopwatch, " to ", stopwatch+1);
setStopwatch(stopwatch + 1);
}
useEffect(() => {
const interval = setInterval(updateStopwatch, 1000);
return () => clearInterval(interval);
// eslint-disable-next-line react-hooks/exhaustive-deps
}, []);
return (
<div>
<h1>{stopwatch}</h1>
</div>
);
}
In this code, the function updateStopwatch will be invoked periodically without depending on any re-rendering of the component ( Although its invocation will cause a re-render ) and each time it will be invoked, it needs to retrieve the current up-to-date value of the state variable stopwatch, so it can update it accordingly. So that the stop watch will keep counting up.
However, what happens is that it only gets the value of stopwatch at the moment the function is declared. It's not really subscribed to a state variable but more like it just reads a constant once. And as a result the count show 1 and is stuck there.
So how can I make such a function get an up-to-date value of a state variable whenever it's invoked?
In your setState method, you can retrieve a prevState argument, which is always the right value, here is the way you could do your function based on your example :
function updateStopwatch(){
setStopwatch((prevState) => prevState + 1);
}
This is an issue of a stale closure over the initial stopwatch state value. Use a functional state update to correctly update from the previous state value instead of whatever value is closed over in callback scope at the time of execution. Move updateStopwatch declaration into the useEffect hook to remove it as an external dependency.
Example:
export default function Stopwatch(){
const [stopwatch, setStopwatch] = useState(0);
useEffect(() => {
function updateStopwatch(){
setStopwatch(stopwatch => stopwatch + 1);
}
const interval = setInterval(updateStopwatch, 1000);
return () => clearInterval(interval);
}, []);
return (
<div>
<h1>{stopwatch}</h1>
</div>
);
}
Need help to understand in useEffect, if I don't put counter and timerCheck in useEffect dependency then what it will effect here.
And if I put timerCheck dependency in useEffect then counter increasing 100 time faster
Also how can i run this code without any error or warning
code in codesandbox
const [counter, setCounter] = useState(0);
const [startcounter, setStartcounter] = useState(false);
const [timerCheck, setTimerCheck] = useState(0);
useEffect(() => {
let intervalStart = null;
if (startcounter) {
intervalStart = setInterval(() => {
setCounter((prevState) => (prevState += 1));
}, 1000);
setTimerCheck(intervalStart);
} else {
clearInterval(timerCheck);
}
return () => {
if (counter !== 0) clearInterval(timerCheck);
};
}, [startcounter]);
const handleStartButton = () => {
setStartcounter(true);
};
const handleStopButton = () => {
setStartcounter(false);
};
Try not to declare unnecessary states. timerCheck state is redundant.
You want to start an interval that increases counter by one every second.
When user stops the stopwatch , you want the interval to be cleared, so it would stop increasing the timer.
So your only dependancy in useEffect would be whether your stopwatch is running or not. And the only thing that matters to you, is when startCounter is ture. You start an interval when it is true and clear the interval in its cleanup.
useEffect(() => {
let intervalStart = null;
if (!startcounter) {
return () => {};
}
intervalStart = setInterval(() => {
setCounter((prevState) => (prevState += 1));
}, 1000);
return () => {
clearInterval(intervalStart);
};
}, [startcounter]);
Here is the codesandbox
But you do not really need useEffect for this. IMO it would be a much better code without using useEffect.
So useEffect with no dependency will run on every render.
If you include a dependency, it will re-render and run whenever that dependency changes. So by including timerCheck in it, you're telling the page to re-render whenever that changes, which ends up changing it, causing it to re-render again, causing it to change again, etc., etc.
More info: https://www.w3schools.com/react/react_useeffect.asp
I am currently refactoring a react app from setState to hooks. I can't understand why the state variables aren't changed. Here is an example:
import React, { useState, useEffect } from 'react';
function Hook() {
const [num, setNum] = useState(1);
useEffect(() => {
window.addEventListener("mousemove", logNum);
}, []);
const logNum = () => {
console.log(num);
}
const handleToggle = () => {
if (num == 1) {
console.log('setting num to 2');
setNum(2);
} else {
console.log('setting num to 1');
setNum(1);
}
}
return (
<div>
<button onClick={handleToggle}>TOGGLE BOOL</button>
</div>
);
}
export default Hook;
When i click the button, I was expecting the output to be something like:
// 1
// setting num to 2
// 2
// setting num to 1
// 1
But the output look like this:
Why is the updated num variable not logged?
Shouldn't the logNum() function always point to the current value of the state?
That's why effect dependencies have to be exhaustive. Don't lie about dependencies.
logNum closures over num, so on every rerender there is a new num variable containing the new value, and a new logNum function logging that value. Your effect however gets initialized only once, thus it only knows the first logNum. Therefore, you have to add logNum as a dependency, so that the effect gets updated whenever num and thus logNum changes:
useEffect(() => {
window.addEventListener("mousemove", logNum);
}, [logNum]);
You'll notice that your effect does not correctly clean up, you should add a removeEventListener too.
return () => window.removeEventListener("mousemove", logNum);
Now if you debug this piece of code, you'll notice that the effect triggers on every rerender. That is because a new logNum function gets created on every rerender, no matter wether num changes or not. To prevent that, you can use useCallback to make the logNum reference stable:
const logNum = useCallback(() => console.log(num), [num]);
An alternative to all of this would be to use a reference to the current state:
const actualNum = useRef(num);
// that works no matter when and how this is executed
console.log(actualNum.current);