My component fetches and displays posts using infinite scrolling (via an IntersectionObserver). The API call that the component makes is dependent on the current number of fetched posts. This is passed as an offset to the API call.
The desired effect is that, if the posts array is empty, its length is 0 and my API will return the first 5 posts. When the last post in the UI intersects with the viewport, another fetch is made, but this time the API call is passed an offset of 5, so the API skips the first 5 posts in the collection and returns the next 5 instead.
Here is my code:
export default function Feed() {
const [posts, setPosts] = useState([]);
const fetchPosts = async () => {
const newPosts = await postAPI.getFeed(posts.length);
setPosts((prevPosts) => [...prevPosts, ...newPosts]);
};
useEffect(fetchPosts, []);
const observerRef = useRef(
new IntersectionObserver(([entry]) => {
if (entry.isIntersecting) {
observerRef.current.unobserve(entry.target);
fetchPosts();
}
})
);
const observePost = useCallback((node) => node && observerRef.current.observe(node), []);
return (
<>
{posts.map((post) => {
const key = post._id;
const isLastPost = key === posts.at(-1)._id;
const callbackRef = !isLastPost ? undefined : observePost; //only if last post in state, watch it!
return <PostCard {...{ key, post, callbackRef }} />;
})}
</>
);
}
However, every time the fetchPosts function is called, the value it uses for posts.length is 0 - the number the function was originally created with.
Can someone explain to me why the closure over posts.length is stale here? I thought that every time a component re-renders, all nested functions within it were recreated from scratch? As such, surely the fetchPosts function should be using the latest value of posts.length every time it is called? Any help is appreciated! :)
You are creating a new IntersectionObserver on every render - not a good idea. However, only the first of all these created observers, the one you store in the ref (the ref that is never updated), is the one on which you call observe(), and that first observer uses a stale closure over the initial value of posts.
Instead, I would suggest creating the intersection observer(s) inside the observePost function:
const [posts, setPosts] = useState([]);
const fetchPosts = useCallback(async () => {
const newPosts = await postAPI.getFeed(posts.length);
setPosts((prevPosts) => [...prevPosts, ...newPosts]);
}, [posts.length]);
// ^^^^^^^^^^^^
useEffect(fetchPosts, []);
const observePost = useCallback((node) => {
if (!node) return;
const observer = new IntersectionObserver(([entry]) => {
if (entry.isIntersecting) {
observer.unobserve(entry.target);
fetchPosts();
}
});
observer.observe(node);
}, [fetchPosts]);
// ^^^^^^^^^^
Important here is the dependency of the observePost callback on the fetchPosts callback which has a dependency on the length of posts, to avoid getting stale. It's probably also possible to solve this with refs, but I don't think they're necessary here.
Related
I'm currently running into some issues whilst developing a Typescript React App.
Underneath is my current code..
But it's not behaving like I would want it to behave. :)
So what I would like to achieve is that the data with getData(depth) runs whenever the component is being loaded and afterwards every 5 seconds.
But when the Depth changes with the Dropdown.item buttons, it should re-render and the getData() should be ran with the new depth value that we just set in the state.. and keep on rendering afterwards with the new value...
I've been struggling with this, so any help is very much appreciated!!
Thank you!
import React, { useState, useEffect } from "react";
const chart = () => {
const [depth, setDepth] = useState(20);
const [chartData, setChartData] = useState({})
//Getting the data when the app initially renders and should keep rendering every 5 seconds after that.
//When the value of the depth changes, we should stop getting the data with the old depth //value and should start a new interval of 5 seconds and just keep running with the new //depth value
//When first entering the app, this should run immediately with the initial depth state //(20)
useEffect(() => {
const interval = setInterval(() => {
//this code is not the actual code, just an example of what is running
const data = getData(depth)
//just fetched the new data, now setting it..
setChartData(data)
}, 5000);
return () => clearInterval(interval);
}, []);
return (
<div>
<div>
<DropdownButton id="dropdown-basic-button" title="Depth Percentage">
<Dropdown.Item onClick={() => setDepth(5)}>5%</Dropdown.Item>
<Dropdown.Item onClick={() => setDepth(20)}>20%</Dropdown.Item>
</DropdownButton>
</div>
<div>
//Rendering the Chart here....
</div>
</div>
);
};
export default chart;
That's because useEffect hook take a second params called dependency array, where this dependency array is what matter for the inner callback(inisde useEffect) to access the latest values you want.
So your are not being totally truthful here, if the inner callback depends on depth to be in its latest update then you should include it in the dependency array
useEffect(() => { ... }, [ depth ]);
that's for the depth but writing this code will immediately cause problems because for each new depth value the inner callback will be called and the setInterval will re-run again (causing many many...many of intervals).
To solve this you should avoid using setInterval alll together in hooks based code.
If having interval is really important I have a suggestion for you
const [intervalCount, setIntervalCount] = useState(0);
const [depth, setDepth] = useState(20);
const [chartData, setChartData] = useState({})
useEffect(() => {
// when depth change re-fetch data and set it
const data: any = getData(depth);
setChartData(data);
}, [depth])
// simulate set interval behavior
// each 5 s this function will be re-invoked
useEffect(() => {
// re-fetch data and set it
const data: any = getData(depth);
setChartData(data);
// wait 5 s before cause a re-render
setTimeout(() => {
setIntervalCount(count => count + 1);
}, 5000);
}, [intervalCount]);
Updated: After rading from Dan Abramov blog
you can find a better elegant solution that use setInterval and hooks
Making setInterval Declarative with React Hooks
He made a custom hook called useInterval
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 be like
useInterval(() => {
// Your custom logic here
setCount(count + 1);
}, 1000);
I am currently making a Graph component that fetches data from an API, parses the data to be used with a graph library, and then renders the graph. I have all of that working right now, but the issue I am having is with adding the ability to filter. The filtering I am currently doing is done by the parent of the Graph component, which will set the filters prop in the component which is then processed by a useEffect. But this seems causes some portions to re-render and I am trying to prevent. Below is what I have roughly speaking.
Rough example of Parent:
const Parent = (props) => {
const [filters, setFilters] = useState({});
//there are more state values than just this one also cause
//the same problem when their setState is called.
return (
<Graph filters={filters} />
<FilterComponent
onChange={(value) => setFilters(value)}
/>
)
}
export default Parent
Rough example of Child:
const Graph = (props) => {
const [nodes, setNodes] = useState({});
const [links, setLinks] = useState({});
const [velocity, setVelocity] = useState(0.08);
const createGraph = async () => {
//fetches the data, processes it and then returns it.
//not including this code as it isn't the problem
return {
nodes: nodes,
links: links,
};
}
//loads the graph data on mount
useEffect(() => {
const loadGraph = async () => {
const data = await createGraph();
setNodes(data.nodes);
setLinks(data.links);
};
loadGraph();
}, []);
//filters the graph on props change
useEffect(() => {
//this function uses setNodes/setLinks to update the graph data
filterGraph(props.filter);
}, [props.filters]);
return (
<ForceGraph2D
graphData={{
nodes: nodes,
links: links,
}}
d3VelocityDecay={velocity}
cooldownTicks={300}
onEngineStop={() => setVelocity(1)}
/>
);
}
export default Graph
My main issue is that whenever the FilterComponent updates, while I want it to update the graph data, this seems to re-render the Graph component. This causes the graph to start moving. This graph library creates a graph which kinda explodes out and then settles. The graph has a cooldown of 300, and after which it isn't supposed to move, which is where onEngineStop's function is called. But changing the filter state in Parent causes the graph to regain it's starting velocity and explode out again. I want to be able to change the filter state, update the graph data, without re-rendering it. I've looked into useMemo, but don't know if that's what I should do.
I'm fairly new to React having just started two weeks ago, so any help is greatly appreciated! Also, this is my first post on stackOverflow, so I apologize if I didn't follow some community standards.
Edit
I was asked to include the filterGraph function. The function actually was designed to handle different attributes to filter by. Each node/link has attributes attached to them like "weight" or "size". The filterComponent would then pass the attr and the value range to filter by. If a component falls outside that range it becomes transparent.
const Graph = (props) => {
...
//attr could be something like "weight"
//val could be something like [5,10]
const filterGraph = ({ attr, val }) => {
for (const [id, node] of Object.entries(nodes)) {
const value = nodes[id][attr];
if (val.length == 2) {
if (val[0] > value || val[1] < value) {
const color = nodes[id]["color"] || "#2d94adff";
nodes[id]["color"] = setOpacity(color, 0)
);
} else {
const color = nodes[id]["color"] || "#2d94adff";
nodes[id]["color"] = setOpacity(color, 1)
);
}
}
}
setNodes(Object.values(this.nodes));
}
...
}
In your example you mention that filterGraph uses setNodes/setLinks. So everytime the filter changes (props.filters) you will do 2 setState and 2 rerenders will be triggered. It can be that React will batch them so it will only be 1 rerender.
Depending on what filterGraph does exactly you could consider let it return filteredNodes en filteredLinks without putting the filterGraph in a useEffect.
Then pass the filteredNodes en filteredLinks to the graphData like graphData={{
nodes: filteredNodes,
links: filteredLinks,
}}
This way you won't trigger extra rerenders and the data will be filtered on every render. which is already triggered when the props.filters change. This is an interesting article about deriving state https://kentcdodds.com/blog/dont-sync-state-derive-it
Since you also mention that there are more state values in the parent you could make the component a pure component, which means it won't get rerendered when the parent renders but the props that are being passed don't change
https://reactjs.org/docs/react-api.html#reactmemo
Also it's better to include createGraph in the useEffect it's being used or wrap it in a useCallback so it won't be recreated every render.
const Graph = React.memo((props) => {
const [nodes, setNodes] = useState({});
const [links, setLinks] = useState({});
const [velocity, setVelocity] = useState(0.08);
//loads the graph data on mount
useEffect(() => {
const createGraph = async () => {
//fetches the data, processes it and then returns it.
//not including this code as it isn't the problem
return {
nodes: nodes,
links: links,
};
}
const loadGraph = async () => {
const data = await createGraph();
setNodes(data.nodes);
setLinks(data.links);
};
loadGraph();
}, []);
const { filteredNodes, filteredLinks } = filterGraph(props.filter)
return (
<ForceGraph2D
graphData={{
nodes: filteredNodes,
links: filteredLinks,
}}
d3VelocityDecay={velocity}
cooldownTicks={300}
onEngineStop={() => setVelocity(1)}
/>
);
})
export default Graph
so i have a bit of a weird problem i dont know how to solve.
In my code i have a custom hook with a bunch of functionality for a fetching a list
of train journeys. I have some useEffects to that keeps loading in new journeys untill the last journey of the day.
When i change route, while it is still loading in new journeys. I get the "changes to unmounted component" React error.
I understand that i get this error because the component is doing an async fetch that finishes after i've gone to a new page.
The problem i can't figure out is HOW do i prevent it from doing that? the "unmounted" error always occur on one of the 4 lines listed in the code snippet.
Mock of the code:
const [loading, setLoading] = useState(true);
const [journeys, setJourneys] = useState([]);
const [hasLaterDepartures, setHasLaterDepartures] = useState(true);
const getJourneys = async (date, journeys) => {
setLoading(true);
setHasLaterDepartures(true);
const selectedDateJourneys = await fetchJourney(date); // Fetch that returns 0-3 journeys
if (condition1) setHasLaterDepartures(false); // trying to update unmounted component
if (condition2) {
if (condition3) {
setJourneys(something1); // trying to update unmounted component
} else {
setJourneys(something2) // trying to update unmounted component
}
} else {
setJourneys(something3); // trying to update unmounted component
}
};
// useEffects for continous loading of journeys.
useEffect(() => {
if (!hasLaterDepartures) setLoading(false);
}, [hasLaterDepartures]);
useEffect(() => {
if (hasLaterDepartures && journeys.length > 0) {
const latestStart = ... // just a date
if (latestStart.addMinutes(5).isSameDay(latestStart)) {
getJourneys(latestStart.addMinutes(5), journeys);
} else {
setLoading(false);
}
}
}, [journeys]);
I can't use a variable like isMounted = true in the useEffect beacuse it would reach inside the if statement and reach a "setState" by the time i'm on another page.
Moving the entire call into a useEffect doesn't seem to work either. I am at a loss.
Create a variable called mounted with useRef, initialised as true. Then add an effect to set mounted.current to false when the component unmounts.
You can use mounted.current anywhere inside the component to see if it's mounted, and check that before setting any state.
useRef gives you a variable you can mutate but which doesn't cause a rerender.
When you use useEffect hook with action which can be done after component change you should also take care about clean effect when needed. Maybe example help you, also check this page.
useEffect(() => {
let isClosed = false
const fetchData = async () => {
const data = await response.json()
if ( !isClosed ) {
setState( data )
}
};
fetchData()
return () => {
isClosed = true
};
}, []);
In your use case, you probably want to create a Store that doesn't reload everytime you change route (client side).
Example of a store using useContext();
const MyStoreContext = createContext()
export function useMyStore() {
const context = useContext(MyStoreContext)
if (!context && typeof window !== 'undefined') {
throw new Error(`useMyStore must be used within a MyStoreContext`)
}
return context
}
export function MyStoreProvider(props) {
const [ myState, setMyState ] = useState()
//....whatever codes u doing with ur hook.
const exampleCustomFunction = () => {
return myState
}
const getAllRoutes = async (mydestination) => {
return await getAllMyRoutesFromApi(mydestination)
}
// you return all your "getter" and "setter" in value props so you can use them outside the store.
return <MyStoreContext.Provider value={{ myState, setMyState, exampleCustomFunction, getAllRoutes }}>{props.children}</MyStoreContext.Provider>
}
You will wrap the store around your entire App, e.g.
<MyStoreProvider>
<App />
</MyStoreProvider>
In your page where you want to use your hook, you can do
const { myState, setMyState, exampleCustomFunction, getAllRoutes } = useMyStore()
const onClick = async () => getAllRouters(mydestination)
Considering if you have client side routing (not server side), this doesn't get reloaded every time you change your route.
My component relies on local state (useState), but the initial value should come from an http response.
Can I pass an async function to set the initial state? How can I set the initial state from the response?
This is my code
const fcads = () => {
let good;
Axios.get(`/admin/getallads`).then((res) => {
good = res.data.map((item) => item._id);
});
return good;
};
const [allads, setAllads] = useState(() => fcads());
But when I try console.log(allads) I got result undefined.
If you use a function as an argument for useState it has to be synchronous.
The code your example shows is asynchronous - it uses a promise that sets the value only after the request is completed
You are trying to load data when a component is rendered for the first time - this is a very common use case and there are many libraries that handle it, like these popular choices: https://www.npmjs.com/package/react-async-hook and https://www.npmjs.com/package/#react-hook/async. They would not only set the data to display, but provide you a flag to use and show a loader or display an error if such has happened
This is basically how you would set initial state when you have to set it asynchronously
const [allads, setAllads] = useState([]);
const [loading, setLoading] = useState(false);
React.useEffect(() => {
// Show a loading animation/message while loading
setLoading(true);
// Invoke async request
Axios.get(`/admin/getallads`).then((res) => {
const ads = res.data.map((item) => item._id);
// Set some items after a successful response
setAllAds(ads):
})
.catch(e => alert(`Getting data failed: ${e.message}`))
.finally(() => setLoading(false))
// No variable dependencies means this would run only once after the first render
}, []);
Think of the initial value of useState as something raw that you can set immediately. You know you would be display handling a list (array) of items, then the initial value should be an empty array. useState only accept a function to cover a bit more expensive cases that would otherwise get evaluated on each render pass. Like reading from local/session storage
const [allads, setAllads] = useState(() => {
const asText = localStorage.getItem('myStoredList');
const ads = asText ? JSON.parse(asText) : [];
return ads;
});
You can use the custom hook to include a callback function for useState with use-state-with-callback npm package.
npm install use-state-with-callback
For your case:
import React from "react";
import Axios from "axios";
import useStateWithCallback from "use-state-with-callback";
export default function App() {
const [allads, setAllads] = useStateWithCallback([], (allads) => {
let good;
Axios.get("https://fakestoreapi.com/products").then((res) => {
good = res.data.map((item) => item.id);
console.log(good);
setAllads(good);
});
});
return (
<div className="App">
<h1> {allads} </h1>
</div>
);
}
Demo & Code: https://codesandbox.io/s/distracted-torvalds-s5c8c?file=/src/App.js
i have a problem with hooks.
i'm using react-hooks, i have a button which at onClick getting data from api and setState with it.
Problem is:
when i click to button first time i get response from api but can't set it to state. When click to button second time i can setState. Why it happened ?
here is my component look like:
function App() {
const [a, setA] = useState(null);
const fetchData = () => {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
console.log(a)
})
}
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
i tried to using fetchData function like that:
function fetchData() {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
console.log(a)
})
}
but it's not helped too
a is a const. It's impossible to change it, so there's no way your console.log statement at the end of fetchData could ever log out something different than the value that a had when fetchData was created.
So that's not what setA does. The point of setA isn't to change the value of the const, but to cause the component to rerender. On that new render, a new set of variables will be created, and the new a will get the new value. Move your console.log out to the body of your component, and you'll see it rerender with the new value.
In short: Your code appears to be already working, you just have the wrong logging.
If your scope is to fetch data, use this:
const [data, setData] = useState("");
useEffect(async () => {
const result = await axios(
'here will be your api',
);
setData(result.data);
});
Now your response will be stored in data variable.
I would not use an effect for it, effects are useful if the props or state changes and can thereby substitute lifecycle methods like componentDidMount, componentDidUpdate, componentWillUnmount, etc.. But in your case these props haven't changed yet (you want to change your state though). Btw, be aware that #Asking's approach will fetch data on EVERY rerender (props or state change). If you want to use useEffect, be sure to add the second parameter to tell React when to update.
Normally, your code should work, I haven't tested but looks legit. Have you used developer tools for react to check if the state/hook has changed? Because if you say it did not work because of the console.log printing: Have in mind that setA() is an async function. The state was most likely not yet changed when you try to print it. If you haven't react developer tools (which I highly recommend) you can check the real state by adding this code in the function:
useEffect(() => console.log(a), [a]);
I have a few real improvements to your code though:
function App() {
const [a, setA] = useState(null);
const fetchData = useCallback(async () => {
let data = {
id: 1
}
const res = await axios.post(baseUrl, data);
setA(res.data);
}, []);
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
By adding useCallback you ensure that the function is memorized and not declared again and again on every rerender.
#Nicholas Tower has already answered your question. But if you are looking for code
function App() {
const [a, setA] = useState(null);
const fetchData = () => {
let data = {
id: 1
}
axios.post(baseUrl, data)
.then(res => {
if (res.data) {
setA(res.data)
}
})
}
console.log(a)
return (
<div className="App">
<div>
<button onClick={fetchData}></button>
</div>
</div>
);
}
export default App;
just place log before return (. This should do