I'm writing a custom hook that takes an id as input and then should do the following tasks:
get data for that id synchronously from a store
subscribe to changes in this store and update data accordingly
I came up with the following implementation which has the downside that I am setting state directly from inside the effect hook, causing a second render.
function useData({id}) {
// set initial data on first render
const [data, setData] = useState(Store.getData(id));
useEffect(() => {
// when id changes, set data to whatever is in store at the moment
// here we call setData directly from inside useEffect, causing a second render
setData(Store.getData(id));
// subscribe to changes to update data whenever it changes in the store
return Store.onChange(id, setData);
}, [id]);
return data;
}
A second approach I tried was to add a dummy state that is only there to cause a re-render. In this approach I am directly returning the data received from Store.getData(). This ensures that I will get the freshest data with every render and the useEffect ensures that every onChange trigger will cause a new render.
function useData({id}) {
// adding some dummy state that we only use to force a render
const [, setDummy] = useState({});
const refresh = useCallback(() => {
setDummy({});
}, []);
useEffect(() => {
// subscribe to changes cause a refresh on every change
return Store.onChange(id, refresh);
}, [id, refresh]);
return Store.getData[id];
}
The second approach works well but it feels weird to add this dummy state. Sure, I could put this into another useRefresh hook but I am not sure if this would really be a good practice.
Is there any better way of implementing this, without calling setData directly from inside useEffect and without relying on some unused dummy state?
So by now you use useState inside your hook just to re-trigger rendering of host component once store is changed. How about taking change handler from the outside?
function useData(id, onChanged) {
useEffect(() => {
return store.onChange(id, onChanged);
}, [id, onChanged]);
return store.getData(id);
}
Related
Hey I've go a problem with this code.
const [itemsCart, setCart] = useState([]);
useEffect(() => {
(async function (){
const buffer = await JSON.parse(window.sessionStorage.getItem("itemsCart"))
setCart(buffer);
console.log(buffer);
console.log(itemsCart);
})()
}, []);
useEffect(() => {
window.sessionStorage.setItem("itemsCart", JSON.stringify(itemsCart));
}, [itemsCart]);
The buffer gets the data, the state variable dosen't. I assume there must be a problem with synchronization however I'm not able to fix that.
The output:
here
This happens because react will wait until all script in useEffect is called and after that, setState will trigger rerender. Because there can be multiple setStates and we want to rerender it only once. That means, you are logging old value in console.log(itemsCart) before its actually there after rerender.
You can logi it with second useEffect before updating sessionStorage and you will see, that state is changed. Or you can create new useEffect for this
useEffect(()=>{
console.log(itemsCart)
},[itemsCart]);
this works:
const [itemsCart, setCart] = useState(JSON.parse(window.localStorage.getItem("itemsCart")));
useEffect(() => {
console.log(itemsCart);
window.localStorage.setItem("itemsCart", JSON.stringify(itemsCart));
}, [itemsCart]);
To run the second useEffect(), itemsCart needs to be modified before via useState(). I can't see in your first useEffect() when you call setItemsCart().
This question is not correct and the approach to solve the problem is not correct as well(whatever problem you are trying to solve).
React has different design.
You are trying to get the items and then set them once you get it using useEffect.
The best approach would be to pass your array as a prop from higher order component and then use your useEffect once it has been triggered by dependencies(passed prop)
Make useEffect hook run before rendering the component
According to the thread below,
useCustomHook being called on every render - is something wrong with this
It says it is completely normal to keep calling the custom hook function every time React re-renders.
My questions are, if it affects on a performance side when returning an array from this Custom Hook function( Not when fetching API and receiving data ) which contains a lot of values.
If so, how to prevent it ( How to let this Custom Hook function run only once )?
Here is my Custom Hook code, it returns an array which contains around 5000 string values.
function FetchWords(url: string) {
const [data, setData] = useState<string[]>([]);
useEffect(() => {
fetch(url)
.then((words) => words.text())
.then((textedWords) => {
setData(textedWords.replace(/\r\n/g, "\n").split("\n"));
});
}, []);
const expensiveData = useMemo(() => data, [data]);
return expensiveData;
}
export default FetchWords;
My Main js
const wordLists: any[] = useFetch(
"https://raw.githubusercontent.com/charlesreid1/five-letter-words/master/sgb-words.txt"
);
CustomHooks should start with word use...
You don't need useMemo in your hook, simply return data state.
Your hook makes the fetch call only once, so no problem there as the effect has empty dependency, so it runs once after first render.
The hook stores the array of 5000 entries once in data state and returns the same reference each time your custom hook is called during component re-renders. There is no copy operation, so you don't need to worry about that.
If you only want to fetch 100 entries for example, then your backend needs to provide that api.
Hope this resolves your queries as it is not very clear what is your doubt.
If you are worried about bringing all this data at the same time, you can indicate from the backend that they send you a certain number of records and from the frontend you can manage them with the pagination.
the use of useMemo is superfluous.
the useEffect that you are using will only be rendered ONCE, that is, it will only call the 5,000 registers that you mention only once
How do I cause re-rendering to the component
function Bookmarks({ data }: any) {
const bookmarkedBlogs = useBookmarks().getBookmarkedBlogs(data.allMdx.nodes);
.....
}
when bookmarks change in the hook
function useBookmarks() {
const [bookmarks, setBookmarks, accessDenied] = useLocalStorage<BlogType['id'][]>('bookmarks', []);
const getBookmarkedBlogs = (blogs: BlogType[]) => {
return blogs.filter(checkIsBookmarked)
};
because as of now, even if I toggle bookmarks, the getBookmarkedBlogs function doesn't execute except in the initial render of the component.
How the implementation of useLocalStorage, and how you toggle bookmarks?
localStorage changes don't notify your every hooks except your make a observer model
if you don't make observer model, toggle bookmark in other hooks or other ways wouldn't notify your this hook, so it don't rerun
Your hook doesn't actually work.
It seems like you want a hook that subscribes to updates from some source outside the React render tree.
Here's a simple example of something that works like that:
function MyComp() {
let eventValue = useEventListener()
return (
<div>
{eventValue}
</div>
)
}
function useEventListener() {
let [ value, setValue ] = React.useState(1)
React.useEffect(() => {
setTimeout(() => setValue(5), 1000)
}, [])
return value
}
What makes it work is that the custom hook invokes a built-in hook when data should change. The React Hooks framework handles the rest.
Instead of a setTimeout, you could subscribe to an event stream, or listen for IPC messages, or something else. But:
The custom hook has to actually return something
The custom hook can only trigger a re-render by invoking a builtin hook
The custom hook must set up its subscribe-to-changes logic on its first invocation, which is often best handled by useEffect configured to run once
I am new to React and I have some doubt regarding useState hook.I was recently working on an API based recipe react app .The problem I am facing is when I submit something in search form a state change should happen but the state is not changing but if I resubmit the form the state changes.
import React,{useState,useEffect} from "react";
import Form from "./componnents/form";
import RecipeBlock from "./componnents/recipeblock"
import './App.css';
function App() {
const API_id=process.env.REACT_APP_MY_API_ID;
const API_key=process.env.REACT_APP_MY_API_KEY;
const [query,setQuery]=useState("chicken");
const path=`https://api.edamam.com/search?q=${query}&app_id=${API_id}&app_key=${API_key}`
const [recipe,setRecipe]=useState([]);
useEffect(() => {
console.log("use effect is running")
getRecipe(query);
}, []);
function search(queryString){
setQuery(queryString);
getRecipe();
}
async function getRecipe(){
const response=await fetch(path);
const data=await response.json();
setRecipe(data.hits);
console.log(data.hits);
}
queryString in search() function holds the value of form input,Every time I submit the form this value is coming correctly but setQuery(queryString) is not changing the query value or state and if I resubmit the form then it change the state.
The code you provided something doesn't make sense.
useEffect(() => {
console.log("use effect is running")
getRecipe(query);
}, []);
Your getRecipe doesn't take a variable. But from what I am understanding whenever you search you want to set the Query then get the recipe from that Query.
With the useEffect you can pass in a parameters to check if they changed before running a function. So update the setQuery then when the component reloads it will fire the useEffect if query has changed. Here is the code to explain:
useEffect(() => {
console.log("use effect is running")
getRecipe(query); <-- this doesn't make sense on your code
}, [query]);
function search(queryString){
setQuery(queryString);
}
By doing this when the state updates it causes the component to re-render and therefore if query has changed it will call your getRecipe function.
The main issue in your code is that you are running getRecipe() directly after setQuery(queryString). setQuery(queryString) is asynchronous and will queue a state change. When you then run getRecipe() directly after, the state will still hold the old value of query (and path) and therefore does not fetch the new data correctly.
One solution would be to call getRecipe() within a useEffect() dependent on path.
useEffect(() => {
getRecipe();
}, [path]);
function search(queryString){
setQuery(queryString);
// getRecipe() <- removed
}
With [path] given as dependencies for useEffect(), getRecipe() will be called automatically whenever path changes. So we don't have to call it manually from search() and therefore can remove getRecipe() from the function body. This also makes the current useEffect() (without [path] dependency) redundant, so it can be removed.
Another solution would be to provide the new query value through the getRecipe() parameters, removing the dependency upon the state.
function search(queryString){
setQuery(queryString);
getRecipe(queryString);
}
async function getRecipe(query) {
const path = `https://api.edamam.com/search?q=${query}&app_id=${API_id}&app_key=${API_key}`;
const response = await fetch(path); // <- is no longer dependent upon the state
const data = await response.json();
setRecipe(data.hits);
}
This does require moving the path definition inside getRecipe().
I have a function inside of my functional component that uses a value saved in state. However, when it is called, it has the original value in state, not the updated value. When I look at my component in Chrome React Dev Tools, I see that the updated value is stored in state. Aren't functions supposed to get the latest state value in React? I didn't think I'd have to wrap my functions in a useEffect every time some value in state they depend on changes. Why is this happening?
const Editor = (props) => {
const [template, setTemplate] = useState(null);
const [openDialog, setOpenDialog] = useState(false);
useEffect(() => {
if (props.templateId) {
getTemplate(props.templateId));
}
},[]);
const getTemplate = (templateId) => {
{...make API to get template...}
.then((response) => {
if (response.template) setTemplate(response.template);
});
}
/* THIS FUNCTION SAYS TEMPLATE IS ALWAYS NULL */
const sendClick = async () => {
if (template) {
await updateTemplate();
} else {
await initializeTemplate();
}
setOpenDialog(true);
};
}
UPDATE: I figured out the issue. The sendClick function is being used inside an object that I have in state. When that object is created, it creates a version of the sendClick function based on the state at that time. I realized I needed to refactor my code so that the function is not stored within my object in state so that the function will always have the latest state values.
Please correct the code there its setTemplate(template)); not getTemplate(template));
I'm guessing that you have that right in the source code... if Yes then,
You have got into a trap that all developers new to React fall into.
This code is betraying you ...
useEffect(() => {
if (props.template) {
setTemplate(template)); // Mentioned as getTemplate(template));
}
},[]); // Here is where you make the mistake
The second argument you pass to the useEffect is called as Dependencies. Meaning if your useEffect is dependent on any state or any variable or function, Ii should be pass as the second argument inside the []. By now you should have got the answer.
Clearly, your useEffect is dependent on template. You should pass that inside the [].
So the code will be : -
useEffect(() => {
if (props.template) {
setTemplate(template)); // Mentioned as getTemplate(template));
}
},[template]);
Now React will automatically run the function every time the value of template changes therefore, updates template.
For more information about useEffect ...
Refer React Documentation
Refer the useEffect API