Thanks In Advance.
Am new to react hooks (REact Native)
Iam maintaining 2 state array, 1 has the list of item to be displayed and another one is the list user selects from 1st array.
const [myList, setMyList] = React.useState([
{
ID: 0,
AddName: 'Zion 1'
},
{
ID: 1,
AddName: 'Zion 2',
},
{
ID: 2,
AddName: 'Zion 3',
}
])
const [userSelectedList, setUserSelectedList] = React.useState([])
How do i have a function for a flatlist render onclick, if user selected 1 item it should get in to the state userSelectedList and if user click that again the item should be removed from userSelectedList.
Am not keeping my code here as its so shaggy. Can anyone help me out with the exact login and syntax to work with.
const itemOnClick =(item) =>{
//logic --setState
}
State mutation is a problem here.
const [userSelectedList, setUserSelectedList] = React.useState(null)
const itemOnClick =(item) =>{
item !== null ? setUserSelectedList(item) :setUserSelectedList(null)
}
but if you are using select element I preferred to make a react reference.
const [userSelectedList, setUserSelectedList] = React.useState([]);
const itemOnClick =(item) => {
if (userSelectedList.length === 0) {
setUserSelectedList(item)
return;
}
const arrId = myList.map(item => item.id);
if(arrId.includes(item.id)) {
const newArrUserSelected = userSelectedList.filter(val => val.id !== item.id)
setUserSelectedList(newArrUserSelected)
} else {
setUserSelectedList([...userSelectedList, item])
}
}
Hope to help you!
Related
I ran into an asynchronous useState problem.
I have a situation where I first need to add an object to the state array in the handler. And then add this state to the localStorage.
setFavoritedSongs ((prev) => [...prev, {name: "Lala", length: "3:20"}]);
localStorage.setItem("storageItemName", JSON.stringify(favoritedSongs));
If I delete the entire localStorage first and run the handler. So an empty array is added to my localStorage (the state shows me updated). After the action again, the required object is finally added to my array.
I tried something like this, but still the same problem.
const tempArray = favoritedSongs.push({ name: "Lala", length: "3:20" });
localStorage.setItem(storageItemName, JSON.stringify(tempArray));
How do you solve this, please?
/// EDIT
I have something like this
const FavoriteSong = () => {
const song = { id: 1, name: "Lala", length: "3:20" };
const [favoritedSongs, setFavoritedSongs] = useState([]);
const [isFavorited, setIsFavorited] = useState(false);
useEffect(() => {
if (localStorage.getItem("storageItemName")) {
const storageSongs = JSON.parse(
localStorage.getItem("storageItemName") || ""
);
setFavoritedSongs(storageSongs);
const foundSong = storageSongs?.find((song) => song.id === song.id);
foundSong ? setIsFavorited(true) : setIsFavorited(false);
}
}, [song]);
const handleClick = () => {
if (isFavorited) {
const filteredSong = favoritedSongs.filter(
(song) => song.id !== song.id
);
localStorage.setItem("storageItemName", JSON.stringify(filteredSong));
setIsFavorited(false);
} else {
setFavoritedSongs((prev) => [...prev, song]);
localStorage.setItem("storageItemName", JSON.stringify(favoritedSongs));
setIsFavorited(true);
}
};
return <div onClick={handleClick}>CLICK</div>;
};
export default FavoriteSong;
Just place your localStorage.set logic inside a useEffect to make sure it runs after the state actually changes.
useEffect() => {
localStorage.setItem(...);
}, [favoritedSongs]};
For that you can Use the condition If data in the array then It will set in localStorage otherwise not
const tempArray = favoritedSongs.push({ name: "Lala", length: "3:20" });
tempArray.length && localStorage.setItem(storageItemName, JSON.stringify(tempArray));
.
setFavoritedSongs ((prev) => [...prev, {name: "Lala", length: "3:20"}]);
FavoritedSongs.length(your state name) && localStorage.setItem("storageItemName", JSON.stringify(favoritedSongs));
Considering this state, I need to select some data from it:
const initialState: PlacesStateT = {
activeTicket: null,
routes: {
departure: {
carriageType: 'idle',
extras: {
wifi_price: 0,
linens_price: 0,
},
},
arrival: {
carriageType: 'idle',
extras: {
wifi_price: 0,
linens_price: 0,
},
},
},
};
so, I came up with two approaches:
first:
const useCoaches = (dir: string) => {
const name = mapDirToRoot(dir);
const carType = useAppSelector((state) => state.places.routes[name].carriageType);
const infoT = useAppSelector((state) => {
return state.places.activeTicket.trainsInfo.find((info) => {
return info.routeName === name;
});
});
const coaches = infoT.trainInfo.seatsTrainInfo.filter((coach) => {
return coach.coach.class_type === carType;
});
return coaches;
};
and second:
const handlerActiveCoaches = (name: string) => (state: RootState) => {
const { carriageType } = state.places.routes[name];
const { activeTicket } = state.places;
const trainInfo = activeTicket.trainsInfo.find((info) => {
return info.routeName === name;
});
return trainInfo.trainInfo.seatsTrainInfo.filter((coach) => {
return coach.coach.class_type === carriageType;
});
};
const useActiveInfo = (dir: string) => {
const routeName = mapDirToRoot(dir);
const selectActiveCoaches = handlerActiveCoaches(routeName);
const coaches = useAppSelector(selectActiveCoaches);
return coaches;
};
Eventually, if the first one works ok then the second one gives a lot of useless re-renders in component. I suspect that there are problems with selectActiveCoaches closure, maybe react considers that this selector is different on every re-render but I am wrong maybe. Could you explain how does it work?
selectActiveCoaches finishes with return seatsTrainInfo.filter(). This always returns a new array reference, and useSelector will force your component to re-render whenever your selector returns a different reference than last time. So, you are forcing your component to re-render after every dispatched action:
https://react-redux.js.org/api/hooks#equality-comparisons-and-updates
One option here would be to rewrite this as a memoized selector with Reselect:
https://redux.js.org/usage/deriving-data-selectors
enter image description here
I want to count "indie" and "action" at the same time when the button is clicked. However, the only real application is "action". Please tell me how.
This is my solution to your problem
import React, { useState, useEffect } from "react";
const games = [
{ id: 1, genre: ["indie", "action"] },
{ id: 2, genre: ["indie"] },
{ id: 3, genre: ["action"] }
];
function ButtonComponent(props) {
const { genre, fn } = props;
return <button onClick={() => fn(genre)}>Click</button>;
}
function TestPage() {
const [genre, setGenre] = useState({ indie: 0, action: 0 });
const addGenrecount = (genres) => {
setGenre((previousState) => {
let { indie, action } = previousState;
genres.forEach((genre) => {
if (genre === "indie") indie = indie + 1;
if (genre === "action") action = action + 1;
});
return { indie, action };
});
};
useEffect(() => console.log("genre", genre), [genre]); // Logs to the console when genre change
return games.map((game) => {
const { id, genre } = game;
return <ButtonComponent key={id} genre={genre} fn={addGenrecount} />;
});
}
export default TestPage;
You may also go to codesandbox to test the demo
https://codesandbox.io/s/xenodochial-dirac-q01h4?file=/src/App.js:0-968
Just Friendly Tip:
If you need help regarding react I recommend to upload your code to codesandbox so that we can easily reproduce or solve the problem
I transformed a class component into a functional component but it looks like it does not work in a way it suppose to work and I can not find what is wrong. When I create a new object there is no name for the object and when I try to mark the object as a complete it removes all created objects at ones. I created a codesandbox here. Unfortunately, I am not too much familiar with functional component. Any help would be appreciated.
Here is my codesandbox sample:
https://codesandbox.io/s/crazy-sid-09myu?file=/src/App.js
Your Todos:
const [todos, setTodos] = useState([
{ id: uuid(), name: "Task 1", complete: true },
{ id: uuid(), name: "Task 2", complete: false }
]);
onAddHandler:
const addTodo = () =>
setTodos([...todos, { id: uuid(), name: "New Task", complete: false }]);
onSetCompleteHandler:
const setCompleteHandler = id =>
setTodos(
todos.map(todo => {
if (todo.id === id) {
return {
...todo,
complete: todo.complete ? 0 : 1
};
}
return todo;
})
);
I have created your new todos. Check out this link
Todos App
I have updated your code, please check the URL https://codesandbox.io/s/determined-morning-n8lgx?file=/src/App.js
const onComp = id => {
for (let i = 0; i < todos.length; i++) {
if (todos[i].id === id) {
let t = { ...todos[i] };
t.complete = !t.complete;
todos[i] = t;
}
}
setTodos([...todos]); // Here todos reference need to be changed
};
And also
const onSubmit = event => {
event.preventDefault();
setTodos([
...todos,
{
id: generateNewId(),
name: newTodoName,
complete: false
}
]);
setNewTodoName("");
};
While using hooks we need to be careful about state variable updates. While manipulating arrays and objects use the spread operator to create new references which invokes child components and re-render current component.
I need advice on where to perform data filtering to achieve best performance. Let's say I receive a big array of products from one endpoint of a remote API and product categories from another endpoint. I store them in Redux state and also persist to Realm database so that they are available for offline usage.
In my app, I have a Stack.Navigator that contains 2 screens: ProductCategories and ProductsList. When you press on a category it brings you to the screen with products that fall under that category. Currently, I perform the data filtering right inside my component, from my understanding it fires off every time the component is rendered and I suspect this approach slows down the app.
So I was wondering if there is a better way of doing that? Maybe filter the data for each category in advance when the app is loading?
My code looks as follows:
const ProductCategories = (props) => {
const isFocused = useIsFocused();
useEffect(() => {
if (isFocused) {
setItems(props.productCategories);
}
}, [isFocused]);
return (
...
);
};
const mapStateToProps = (state) => ({
productCategories: state.catalog.productCategories,
});
const ProductsList = (props) => {
const isFocused = useIsFocused();
const productsFilteredByCategory = props.products.filter((product) => {
return product.category === id;
});
useEffect(() => {
if (isFocused) {
setItems(productsFilteredByCategory);
}
}, [isFocused]);
return (
...
)
const mapStateToProps = (state) => ({
products: state.catalog.products,
});
You have to normalize (you can see main principles here) data in redux, to the next view:
// redux store
{
categories: {
1: { // id of category
id: 1,
title: 'some',
products: [1, 2, 3] // ids of products
},
...
},
products: {
1: { // id of product
id: 1,
title: 'some product',
},
...
}
}
Then you can create few selectors which will be even without memoization work much faster then filter, because time of taking data from object by property is constant
const getCategory = (state, categoryId) => state.categories[categoryId]
const getProduct = (state, productId) => state.products[productId]
const getCategoryProducts = (state, categoryId) => {
const category = getCategory(state, categoryId);
if (!category) return [];
return category.products.map((productId) => getProduct(state, productId))
}