如何从基于 class 的组件中的反应 function 调用返回的 const

Question

Here I have a Loading screen as a functional react component that I try to render conditionally in the App component. The concept of this loading screen is that I have a boolean variable that will be used to conditionally render the home page after the loading screen ends.

 import React from 'react'; import { useState, useEffect } from 'react'; import { useSpring, animated } from 'react-spring'; import BarLoader from 'react-spinners/BarLoader'; import Logo from "../assets/images/logo.svg"; const LoadingScreen = () => { const spinner = ` display: block; margin: 0 auto; width: 150px; height: 2.5px; `; const style = useSpring({opacity: 1, from: {opacity: 0}}); const [isLoading, setIsLoading] = useState(false); useEffect(() => { setIsLoading(true); setTimeout(() => { setIsLoading(false) }, 4000) }, []) const LoadingTemplate = () => { <animated.div className="loading-screen" style={style}> <div className="loader-wrapper"> <img className="splash-logo" src={Logo} alt="Marouane Edghoughi" /> <BarLoader color="#384BEB" css={ spinner } loading={isLoading} /> </div> </animated.div> } return { LoadingTemplate, isLoading } } export default LoadingScreen;

When I try to call the boolean variable and the screen template in the following code:

 render() { const {LoadingTemplate, isLoading} = LoadingScreen(); return ( <Router> { isLoading? <LoadingTemplate />: <Container className="theme p-0" fluid={true}> {/* something will be displayed here */} </Container> } </Router> ); } }

I just get this error:

Error: Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for one of the following reasons:

1. You might have mismatching versions of React and the renderer (such as React DOM)
2. You might be breaking the Rules of Hooks
3. You might have more than one copy of React in the same app See https://reactjs.org/link/invalid-hook-call for tips about how to debug and fix this problem.

The function is working properly if I try to call it from a functional component. This is my first time trying it with a class.

Any help would be greatly appreciated ^_^

Answer 1

The error message is 100% correct. Hooks can be used only in Function Components, and cannot be used like this in class components. The underlying mechanics of the two types are different. Hooks are a feature of functional components and rely on those mechanics, and are therefore not compatible with class components.

You may not realize that you are using a hook, but LoadingScreen actually is one: It returns a value other than a React Element, and you are calling it as a function (ie const x = LoadingScreen() ), rather than using it as a component (ie <LoadingScreen /> ).

That's definitely not allowed in class components. You could use a function component instead:

const Component = () => {

    const {LoadingTemplate, isLoading} = LoadingScreen();

    return (
      <Router>
        {
        isLoading ?

            <LoadingTemplate />

          :

          <Container className="theme p-0" fluid={true}>
            {/* something will be displayed here */}
          </Container>

        }
        
      </Router>
    );
  }
}

Or you can try these methods to get around this limitation. If you do decide to use a function component instead, then you should use useLoadingScreen to follow the React hook naming conventions.