I hope this won't get flagged as a duplicate because none of identical problems reflect my issue totally.
So the issue i'm having is i have a useEffect hook like this:
useEffect(()=>{
console.log("opening websocket");
websocket.current=new WebSocket(`ws://127.0.0.1:8000/chat/${curMessage.receiverUsername}/${curUserName}/`);
websocket.current.onopen= (event)=>{
console.log("open:",event)
}
websocket.current.onclose= (event)=>{
setSentMessages([]);
console.log("close:",event,sentMessages);
}
websocket.current.onmessage=(event)=>{
console.log("new message",event);
console.log(JSON.parse(event.data));
setSentMessages([...sentMessages,JSON.parse(event.data)]);
setCont([...cont,1,2]);
console.log(sentMessages);
}
setSentMessages([]);
},[curMessage,curUserName]);
the variable curMessage gets it value from redux state and since it's in the dependency array in the useEffect changes should be reflected immediately the changes are made in the redux state,this work correctly except for the setSentMessages([]) at the last line in the useEffect,the state sentMessages never gets updated immediately the useEffect is triggered which results to still getting old values instead of an empty array,everything else works fine and updates in the useEffect when it gets triggered,i have tried making placing setSentMessages([]) in another useEffect which is dependent on another state but still the same result.
const [sentMessages, setSentMessages] = useState([
// some initial state here
]);
const sentMessagesRef = useRef([
// some initial state here
]);
useEffect(() => {
// Keep ref in sync with state
sentMessagesRef.current = sentMessages;
}, [sentMessages]);
useEffect(
() => {
websocket.current.onmessage = (event) => {
// Use up-to-date value of sentMessages here,
// not the value at the time of declaring this function.
setSentMessages([...sentMessagesRef.current, JSON.parse(event.data)]);
}
setSentMessages([]);
},
[curMessage, curUserName]
);
I believe your issue is that setSentMessages([]) is in fact working, but the onmessage function then overwrites that state with the outdated value. The state is updated immediately, but your callback uses a stale reference of sentMessages. Copying the state to a ref and then accessing that fixes it.
Related
I am using React with functional components in combination with useState() and RxJs.
I'm subscribing to a BehaviorSubject in my useEffect[] and everytime a new message is published, I want to check the current state of my component to decide which steps to take.
But: Even though in my program flow I can clearly see that my state has a certain value, the subscribe callback always only shows the initial empty value. When I stop execution in the middle of the callback, I can see that the "outdated" state is in the closure of the callback.
Why is this?
I've broken it down to those essential code parts:
function DesignView() {
const [name, setName] = useState("");
useEffect(() => {
console.log(name); // <--- This always shows correctly, of course
}, [name]);
useEffect(() => {
// even if this is the ONLY place I use setName() ... it doesn't work
setName("Test Test Test Test");
let subscription = directionService.getDirection().subscribe(() => {
console.log(name); // <--- this only ever shows "" and never "Test Test Test Test"
// no matter at what point of time the published messages arrive!
});
return () => {
subscription.unsubscribe();
}
}, []);
return (
...
);
}
The cause of this problem is that a non-react callback only ever sees a static copy of the state. The same problem appears in the useEffect cleanup function.
Solution:
Either
Add a ref to the state variable. Change the ref.current whenever the state changes and use the ref in the callback
Add the state variable to the dependency array of useEffect and unsubscribe/subscribe every time
I am building an app to understand the useState hook. This app simply has a form for entering username. I am trying to save the entered username. So, I have used react useState. And I tried to await the updating function of the useState in the event handler.
const usernameChangeHandler = async (event) => {
await setEnteredUsername(event.target.value);
console.log(enteredUsername, enteredAge);
};
And when I tried to log the username it doesn't show us the current state but the previous state. Why?
const usernameChangeHandler = async (event) => {
await setEnteredUsername(event.target.value);
console.log(enteredUsername, enteredAge);
};
enteredUsername is never going to change. It's a closure variable that's local to this single time you rendered the component. It's usually a const, but even if it was made with let, setEnteredUsername does not even attempt to change its value. What setEnteredUsername does is ask react to rerender the component. When the render eventually happens, a new local variable will be created with the new value, but code from your old render has no access to that.
If you need to run some code after calling setEnteredUsername, but you don't actually care if the component has rerendered yet, the just use the value in event.target.value, since you know that's going to be the new value of the state:
const usernameChangeHandler = (event) => {
setEnteredUsername(event.target.value);
console.log(event.target.value, enteredAge);
}
If instead you need to make make sure that the component has rerendered and then do something after that, you can put your code in a useEffect. Effects run after rendering, and you can use the dependency array to make it only run if the values you care about have changed:
const [enteredUsername, setEnteredUsername] = useState('');
useEffect(() => {
console.log('rendering complete, with new username', enteredUsername);
}, [enteredUsername]);
const usernameChangeHandler = (event) => {
setEnteredUsername(event.target.value);
};
the act of setting state is asynchronous; therefore, console logging directly after setting your state will not accurately provide you with how state currently looks. Instead as many have suggested you can utilize the useEffect lifecycle hook to listen for changes in your enteredUserName state like so:
useEffect(() => {
console.log(enteredUsername);
}, [enteredUsername]);
listening for changes within the useEffect will allow you to create side effects once state has updated and caused your component to rerender. This in turn will trigger your useEffect with the enteredUsername dependency, as the enteredUserName state has changed.
So, I'm using hooks to manage the state of a set of forms, set up like so:
const [fieldValues, setFieldValues] = useState({}) // Nothing, at first
When setting the value, the state doesn't update:
const handleSetValues = values => {
const _fieldValues = {
...fieldValues,
...values
}
setFieldValues(_fieldValues)
console.log(fieldValues) // these won't be updated
setTimeout(() => {
console.log(fieldValues) // after ten seconds, it's still not updated
},10000)
}
If I call the function a second time, it'll have updated, but that's not gonna work for me.
I never saw behaviour like this in class components.
Is it meant to... like, not update? Or just update whenever it feels like it? Really confusing behaviour.
setFieldValues(_fieldValues) is an async call, means you won't able to get the result in the very next line of this.
You can use useEffect hook.
useEffect(() => {
// do your work here
}, [fieldValues]);
It seems from your question that you have background of Class components of React, so useEffect is similar to componentDidMount and componentDidUpdate lifecycle methods.
useEffect calls whenever the state in the dependency array (in your case [fieldValues]) changes and you get the updated value in useEffect body.
You can also perform componentWillUnmount work in useEffect as well.
Have a brief guide.
setFieldValues is an asynchronous function, so logging the value below the statement will not have any effect.
Regarding using setTimeout, the function would capture the current value of props being passed to it and hence that would be the value printed to the console. This is true to any JS function, see the snippet below:
function init(val) {
setTimeout(() => {
console.log(val);
}, 1000);
}
let counterVal = 1;
init(counterVal);
counterVal++;
So how can we print the values when the value changes? The easy mechanism is to use a useEffect:
useEffect(() => {
console.log(fieldValues)
}, [fieldValues]);
I want to update state every second inside setinterval() but it doesn't work.
I am new to react hook so can not understand why this is happening.
Please take a look at the following code snippet and give me advice.
// State definition
const [gamePlayTime, setGamePlayTime] = React.useState(100);
let targetShowTime = 3;
.........................
// call function
React.useEffect(() => {
gameStart();
}, []);
.............
const gameStart = () => {
gameStartInternal = setInterval(() => {
console.log(gamePlayTime); //always prints 100
if (gamePlayTime % targetShowTime === 0) {
//can not get inside here
const random = (Math.floor(Math.random() * 10000) % wp("70")) + wp("10");
const targetPosition = { x: random, y: hp("90") };
const spinInfoData = getspinArray()[Math.floor(Math.random() * 10) % 4];
NewSpinShow(targetPosition, spinInfoData, spinSpeed);
}
setGamePlayTime(gamePlayTime - 1);
}, 1000);
};
The reason why you did not get updated state is because you called it inside
useEffect(() => {}, []) which is only called just once.
useEffect(() => {}, []) works just like componentDidMount().
When gameStart function is called, gamePlaytime is 100, and inside gameStart, it uses the same value however the timer works and the actual gamePlayTime is changed.
In this case, you should monitor the change of gamePlayTime using useEffect.
...
useEffect(() => {
if (gamePlayTime % targetShowTime === 0) {
const random = (Math.floor(Math.random() * 10000) % wp("70")) + wp("10");
const targetPosition = { x: random, y: hp("90") };
const spinInfoData = getspinArray()[Math.floor(Math.random() * 10) % 4];
NewSpinShow(targetPosition, spinInfoData, spinSpeed);
}
}, [gamePlayTime]);
const gameStart = () => {
gameStartInternal = setInterval(() => {
setGamePlayTime(t => t-1);
}, 1000);
};
...
You're creating a closure because gameStart() "captures" the value of gamePlayTime once when the useEffect hook runs and never updates after that.
To get around this, you must use the functional update pattern of React hook state updating. Instead of passing a new value directly to setGamePlayTime(), you pass it a function and that function receives the old state value when it executes and returns a new value to update with. e.g.:
setGamePlayTime((oldValue) => {
const someNewValue = oldValue + 1;
return someNewValue;
});
Try this (essentially just wrapping the contents of your setInterval function with a functional state update):
const [gamePlayTime, setGamePlayTime] = React.useState(100);
let targetShowTime = 3;
// call function
React.useEffect(() => {
gameStart();
}, []);
const gameStart = () => {
gameStartInternal = setInterval(() => {
setGamePlayTime((oldGamePlayTime) => {
console.log(oldGamePlayTime); // will print previous gamePlayTime value
if (oldGamePlayTime % targetShowTime === 0) {
const random = (Math.floor(Math.random() * 10000) % wp("70")) + wp("10");
const targetPosition = { x: random, y: hp("90") };
const spinInfoData = getspinArray()[Math.floor(Math.random() * 10) % 4];
NewSpinShow(targetPosition, spinInfoData, spinSpeed);
}
return oldGamePlayTime - 1;
});
}, 1000);
};
https://overreacted.io/making-setinterval-declarative-with-react-hooks/
Dan Abramov article explains well how to work with hooks, state, and the setInterval() type of API!
Dan Abramov! Is one of the React maintaining team! So known and I personally love him!
Quick explanation
The problem is the problem of how to access the state with a useEffect() that executes only once (first render)!
Note: For a deep explanation of why the state isn't updated within the useEffect callback and other inside useEffect callbacks. Check the last section about closures and react re-render ...
The short answer is: by the use of refs (useRef)! And another useEffect() that run again when update is necessary! Or at each render!
Let me explain! And check the Dan Abramov solution! And you'll get better the statement above at the end! With a second example that is not about setInterval()!
=>
useEffect() either run once only, or run in each render! or when the dependency update (when provided)!
Accessing state can be possible only through a useEffect() that run and render each relevant time!
Or through setState((state/*here the state*/) => <newStateExpression>)
But if you want to access the state inside useEffect() => re-run is necessary! meaning passing and executing the new callback!
That doesn't work well with setInterval! If you clear it and re-set it each time! the counter get reset! Leading to no execution if the component is re-rendering fast! And make no sense!
If you render only once! The state is not updated! As the first run, run a one callback! And make a closure! The state is fixed! useEffect(() => { <run once, state will stay the same> setInterval(() => { <state fixed as closure of that time> }) }, []).
For all such kind of situation! We need to use useRef! (refs)!
Save to it a callback that holds the state! From a useEffect() that rerenders each time! Or by saving the state value itself in the ref! Depending on the usage!
Dan abramov solution for setInterval (simple and clean)
That's what you are looking for!
useInteval hook (by Dan Abramov)
import React, { useState, useEffect, useRef } from 'react';
function useInterval(callback, delay) {
const savedCallback = useRef();
// Remember the latest callback.
useEffect(() => {
savedCallback.current = callback;
}, [callback]);
// Set up the interval.
useEffect(() => {
function tick() {
savedCallback.current();
}
if (delay !== null) {
let id = setInterval(tick, delay);
return () => clearInterval(id);
}
}, [delay]);
}
Usage
import React, { useState, useEffect, useRef } from 'react';
function Counter() {
let [count, setCount] = useState(0);
useInterval(() => {
// Your custom logic here
setCount(count + 1);
}, 1000);
return <h1>{count}</h1>;
}
We can see how he kept saving the new callback at each re-render! A callback that contains the new state!
To use! it's a clean simple hook! That's a beauty!
Make sure to read Dan article! As he explained and tackled a lot of things!
setState()
Dan Abramov mentioned this in his article!
If we need to set the state! Within a setInteral! One can use simply setState() with the callback version!
useState(() => {
setInterval(() => {
setState((state/*we have the latest state*/) => {
// read and use state
return <newStateExpression>;
})
}, 1000);
}, []) // run only once
we can even use that! Even when we are not setting a state! Possible! Not good though! We just return the same state value!
setState((state) => {
// Run right away!
// access latest state
return state; // same value (state didn't change)
});
However this will make different react internal code part run (1,2,3), And checks! Which ends by bailing out from re-rendering! Just fun to know!
We use this only when we are updating the state! If not! Then we need to use refs!
Another example: useState() with getter version
To showcase the how to work with refs and state access! Let's go for another example! Here is another pattern! Passing state in callbacks!
import React from 'react';
function useState(defaultVal) {
// getting the state
const [state, setState] = React.useState(defaultValue);
// state holding ref
const stateRef = React.useRef();
stateRef.current = state; // setting directly here!
// Because we need to return things at the end of the hook execution
// not an effect
// getter
function getState() {
// returning the ref (not the state directly)
// So getter can be used any where!
return stateRef.current;
}
return [state, setState, getState];
}
The example is of the same category! But here no effect!
However, we can use the above hook to access the state in the hook simply as below!
const [state, useState, getState] = useState(); // Our version! not react
// ref is already updated by this call
React.useEffect(() => {
setInteval(() => {
const state = getState();
// do what you want with the state!
// it works because of the ref! Get State to return a value to the same ref!
// which is already updated
}, 1000)
}, []); // running only once
For setInterval()! The good solution is Dan Abramov hook! Making a strong custom hook for a thing is a cool thing to do! This second example is more to showcase the usage and importance of refs, in such state access need or problem!
It's simple! We can always make a custom hook! Use refs! And update the state in ref! Or a callback that holds the new state! Depending on usage! We set the ref on the render (directly in the custom hook [the block execute in render()])! Or in a useEffect()! That re-run at each render or depending on the dependencies!
Note about useEffect() and refs setting
To note about useEffect()
useEffect => useEffect runs asynchronously, and after a render is painted on the screen.
You cause a render somehow (change state, or the parent re-renders)
React renders your component (calls it)
The screen is visually updated
THEN useEffect runs
Very important of a thing! useEffect() runs after render() finishes and the screen is visually updated! It runs last! You should be aware!
Generally, however! Effects should be run on useEffect()! And so any custom hook will be ok! As its useEffect() will run after painting and before any other in render useEffect()! If not! As like needing to run something in the render directly! Then you should just pass the state directly! Some people may pass a callback! Imagine some Logic component! And a getState callback was passed to it! Not a good practice!
And if you do something somewhere of some such sense! And talking about ref! Make sure the refs are updated right! And before!
But generally, you'll never have a problem! If you do then it's a smell! the way you are trying to go with is high probably not the right good way!
Closure and more. Why the state variables are not having the latest value?
The why you can't simply access the state value directly boils down to the closure notion. Render function at every re-render go totally with a new call. Each call has its closures. And at every re-render time, the useEffect callback is a new anonymous function. With its new scope of value.
And here the dependency array matter. To access the closure of this call. And so the recent state of this call. You have to make useEffect use the new callback. If dependencies change then that would happen. If not that wouldn't.
And if you do [] then useEffect() would run the first time only. every new call after the first render. Would always have useEffect get a new anonymous function. But none of them is effective or used (run).
The same concept applies to useCallback and many other hooks. And all of that is a result of closures.
callbacks inside the useEffect callback
ex: event listners, setInterval, setTimeout, some(() => {}), api.run(() => {})
Now even if you update the useEffect callback through dependency change. And let's say you did some event listener or setInterval call. But you did it conditionally so if already running no more setting it. The setInterval callback or event listener callback wouldn't get access to the recent state value. Why? you already guessed. They were created in the first run, first call space and all the state values are closure passed down at that **time**. And in the later updates and calls. It's a new render function call. And a whole different useEffect callback as well. Sure it's the same function code. But not same functions at all. The initial callback of the initial run on that render function call. Is a function that was created back then. And then the event listener callback or the setInterval callback was created back then. And they are still in memory and referred to. The event listener part of the event listener API objects instances, and the object that made the registration and the setInterval part of the node runtime. They had the state closure having the value of the time. And never get to see or know about any other re-render call. **Unless somehow you. inject something within. That still referencesorcan access the latest value(references or globals). And those values come from the hooks (useState) and their internal machinery. All it would have is theclosures of the time of the creation.**
Funny metaphor
Most people falling into this trap. Look at the code and saying hey why when the state updates it's not getting the newer value. And the answer is. The state value is something coming from useState, it's not a global variable. And even though you are looking at the same code. The first call and later calls are all different spaces (beings). The only thing making it possible for us to work with functions in this sense is the hooks. And how they store the state and bring it back.
And a good metaphor would be: to get to an office and do some contract and deal. And then later come back and enter the same office, but wait not really the same office rather one that looks the same. But a new office took its place (moved out, moved in, same activity). And you wonder why they didn't recognize you. Yup a bit of an off of a metaphor. But still good a bit.
On the whole I hope that gave you a good sense!
You shouldn't use setInterval with hooks. Take a look at what Dan Abramov, one of the maintainers of React.js, said regarding an alternative on his blog: https://overreacted.io/making-setinterval-declarative-with-react-hooks/
If I write
function Component() {
const [isLoading, setLoading] = useState(true);
const request = () => {
setLoading(true)
console.log(isLoading)
setLoading(false)
console.log(isLoading)
}
}
It will log out 'true' both times. Why isn't the state updating in the console? Eventhough it works fine in the DOM.
this.setState({ ...} will show the new state value
Setting a new value in state by using setState or useState hook is an asynchronous process.
If you want to log the new value once it has changed, you have to couple it with the useEffect hook
useEffect(() => {
console.log(isLoading)
}, [isLoading]);
Arnaud's answer is 100% correct, but what confused me when playing around with this stuff was that calling setState twice didn't have the same effect as trying to console.log directly after an update.
This is because of how React handles state updates internally, and even though your log may be wrong, your state is actually updated appropriately.