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VisualizaçõesReact context - state not true
Im making a project and got stucked at context file. I want my state planet to be true so i can use it in my component file. At the start it becomes true showing the right index of the array but in the end its becoming undefined. Can someone point me out what is causing "planet" state to be undefined at the end? Im talking about if statement - if (planet) {...}. Im attaching code from context file and screen of my browser console. Would be nice if someone also told me if i can change state in its own setting function e.g. setPlanets(planet[0]).
import React from "react";
import { useState, useEffect } from "react";
export const Context = React.createContext({
onChooseMoon: () => {},
onChooseMars: () => {},
});
const ContextProvider = (props) => {
const [planet, setPlanets] = useState();
useEffect(() => {
fetch("./data.json")
.then((response) => {
return response.json();
})
.then((data) => {
const transformedData = data.destinations.map((destination) => {
return {
name: destination.name,
img: destination.images.png,
desc: destination.description,
dist: destination.distance,
travel: destination.travel,
};
});
setPlanets(transformedData);
});
}, []);
if (planet) {
setPlanets(planet[0]);
console.log(planet[0]);
console.log("hello");
}
const setMoonHandler = () => {
setPlanets(planet[0]);
};
const setMarsHandler = () => {
setPlanets(planet[1]);
};
return (
<Context.Provider
value={{
planet: planet,
onChooseMoon: setMoonHandler,
onChooseMars: setMarsHandler,
}}
>
{props.children}
</Context.Provider>
);
};
export default ContextProvider;
1 Respostas
Responde à pergunta0
Here's a breakdown of what's going on:
- Component mounts, value of
planetisundefined,useEffectis triggered. WhenuseEffectrunssetPlanets(transformedData)rerender is triggered. - Value of
planetis now whatevertransformedDatawas (I'm assuming an array of planet objects). Sinceplanetis truthy,setPlanets(planet[0])runs, causing a rerender. - Value of
planetis now whatevertransformedData[0]was (a single planet). Sinceplanetis still truthy,setPlanets(planet[0])runs again, causing a rerender. - Value of
planetis now whatevertransformedData[0][0]was. SincetransformedData[0]was not an array and probably doesn't have the key0, the value ofplanetis nowundefinedagain.
The issue is that if ever planet becomes defined, running setPlanet(planet[0]) will run until the value of planet becomes undefined again.
I think what you meant to do (judging by the naming) was to have two separate state variables. One to hold all of the planets, and one to hold a single (active/focused) planet.
Here's an example:
const ContextProvider = (props) => {
const [activePlanet, setActivePlanet] = useState();
const [planets, setPlanets] = useState([]);
useEffect(() => {
fetch("./data.json") // same as before
}, []);
useEffect(() => {
if (!activePlanet && planets.length > 0) {
setActivePlanet(planets[0]);
console.log(planets[0]);
console.log("hello");
}
}, [activePlanet, planets]);
// ...
};
Also it's generally a good idea to wrap all state updating logic in hooks (or tied to events like clicks), so I've added a useEffect to wrap the setActivePlanet call.