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How can I correctly use the React context API in a class based component? [duplicate]

In this example, I have this react class:
class MyDiv extends React.component
constructor(){
this.state={sampleState:'hello world'}
}
render(){
return <div>{this.state.sampleState}
}
}
The question is if I can add React hooks to this. I understand that React-Hooks is alternative to React Class style. But if I wish to slowly migrate into React hooks, can I add useful hooks into Classes?

High order components are how we have been doing this type of thing until hooks came along. You can write a simple high order component wrapper for your hook.
function withMyHook(Component) {
return function WrappedComponent(props) {
const myHookValue = useMyHook();
return <Component {...props} myHookValue={myHookValue} />;
}
}
While this isn't truly using a hook directly from a class component, this will at least allow you to use the logic of your hook from a class component, without refactoring.
class MyComponent extends React.Component {
render(){
const myHookValue = this.props.myHookValue;
return <div>{myHookValue}</div>;
}
}
export default withMyHook(MyComponent);

Class components don't support hooks -
According to the Hooks-FAQ:
You can’t use Hooks inside of a class component, but you can definitely mix classes and function components with Hooks in a single tree. Whether a component is a class or a function that uses Hooks is an implementation detail of that component. In the longer term, we expect Hooks to be the primary way people write React components.

As other answers already explain, hooks API was designed to provide function components with functionality that currently is available only in class components. Hooks aren't supposed to used in class components.
Class components can be written to make easier a migration to function components.
With a single state:
class MyDiv extends Component {
state = {sampleState: 'hello world'};
render(){
const { state } = this;
const setState = state => this.setState(state);
return <div onClick={() => setState({sampleState: 1})}>{state.sampleState}</div>;
}
}
is converted to
const MyDiv = () => {
const [state, setState] = useState({sampleState: 'hello world'});
return <div onClick={() => setState({sampleState: 1})}>{state.sampleState}</div>;
}
Notice that useState state setter doesn't merge state properties automatically, this should be covered with setState(prevState => ({ ...prevState, foo: 1 }));
With multiple states:
class MyDiv extends Component {
state = {sampleState: 'hello world'};
render(){
const { sampleState } = this.state;
const setSampleState = sampleState => this.setState({ sampleState });
return <div onClick={() => setSampleState(1)}>{sampleState}</div>;
}
}
is converted to
const MyDiv = () => {
const [sampleState, setSampleState] = useState('hello world');
return <div onClick={() => setSampleState(1)}>{sampleState}</div>;
}

Complementing Joel Cox's good answer
Render Props also enable the usage of Hooks inside class components, if more flexibility is needed:
class MyDiv extends React.Component {
render() {
return (
<HookWrapper
// pass state/props from inside of MyDiv to Hook
someProp={42}
// process Hook return value
render={hookValue => <div>Hello World! {hookValue}</div>}
/>
);
}
}
function HookWrapper({ someProp, render }) {
const hookValue = useCustomHook(someProp);
return render(hookValue);
}
For side effect Hooks without return value:
function HookWrapper({ someProp }) {
useCustomHook(someProp);
return null;
}
// ... usage
<HookWrapper someProp={42} />
Source: React Training

you can achieve this by generic High order components
HOC
import React from 'react';
const withHook = (Component, useHook, hookName = 'hookvalue') => {
return function WrappedComponent(props) {
const hookValue = useHook();
return <Component {...props} {...{[hookName]: hookValue}} />;
};
};
export default withHook;
Usage
class MyComponent extends React.Component {
render(){
const myUseHookValue = this.props.myUseHookValue;
return <div>{myUseHookValue}</div>;
}
}
export default withHook(MyComponent, useHook, 'myUseHookValue');

Hooks are not meant to be used for classes but rather functions. If you wish to use hooks, you can start by writing new code as functional components with hooks
According to React FAQs
You can’t use Hooks inside of a class component, but you can
definitely mix classes and function components with Hooks in a single
tree. Whether a component is a class or a function that uses Hooks is
an implementation detail of that component. In the longer term, we
expect Hooks to be the primary way people write React components.
const MyDiv = () => {
const [sampleState, setState] = useState('hello world');
render(){
return <div>{sampleState}</div>
}
}

You can use the react-universal-hooks library. It lets you use the "useXXX" functions within the render function of class-components.
It's worked great for me so far. The only issue is that since it doesn't use the official hooks, the values don't show react-devtools.
To get around this, I created an equivalent by wrapping the hooks, and having them store their data (using object-mutation to prevent re-renders) on component.state.hookValues. (you can access the component by auto-wrapping the component render functions, to run set currentCompBeingRendered = this)
For more info on this issue (and details on the workaround), see here: https://github.com/salvoravida/react-universal-hooks/issues/7

Stateful components or containers or class-based components ever support the functions of React Hooks, so we don't need to React Hooks in Stateful components just in stateless components.
Some additional informations
What are React Hooks?
So what are hooks? Well hooks are a new way or offer us a new way of writing our components.
Thus far, of course we have functional and class-based components, right? Functional components receive props and you return some JSX code that should be rendered to the screen.
They are great for presentation, so for rendering the UI part, not so much about the business logic and they are typically focused on one or a few purposes per component.
Class-based components on the other hand also will receive props but they also have this internal state. Therefore class-based components are the components which actually hold the majority of our business logic, so with business logic, I mean things like we make an HTTP request and we need to handle the response and to change the internal state of the app or maybe even without HTTP. A user fills out the form and we want to show this somewhere on the screen, we need state for this, we need class-based components for this and therefore we also typically use class based components to orchestrate our other components and pass our state down as props to functional components for example.
Now one problem we have with this separation, with all the benefits it adds but one problem we have is that converting from one component form to the other is annoying. It's not really difficult but it is annoying.
If you ever found yourself in a situation where you needed to convert a functional component into a class-based one, it's a lot of typing and a lot of typing of always the same things, so it's annoying.
A bigger problem in quotation marks is that lifecycle hooks can be hard to use right.
Obviously, it's not hard to add componentDidMount and execute some code in there but knowing which lifecycle hook to use, when and how to use it correctly, that can be challenging especially in more complex applications and anyways, wouldn't it be nice if we had one way of creating components and that super component could then handle both state and side effects like HTTP requests and also render the user interface?
Well, this is exactly what hooks are all about. Hooks give us a new way of creating functional components and that is important.

React Hooks let you use react features and lifecycle without writing a class.
It's like the equivalent version of the class component with much smaller and readable form factor. You should migrate to React hooks because it's fun to write it.
But you can't write react hooks inside a class component, as it's introduced for functional component.
This can be easily converted to :
class MyDiv extends React.component
constructor(){
this.state={sampleState:'hello world'}
}
render(){
return <div>{this.state.sampleState}
}
}
const MyDiv = () => {
const [sampleState, setSampleState] = useState('hello world');
return <div>{sampleState}</div>
}

It won't be possible with your existing class components. You'll have to convert your class component into a functional component and then do something on the lines of -
function MyDiv() {
const [sampleState, setSampleState] = useState('hello world');
return (
<div>{sampleState}</div>
)
}

For me React.createRef() was helpful.
ex.:
constructor(props) {
super(props);
this.myRef = React.createRef();
}
...
<FunctionComponent ref={this.myRef} />
Origin post here.

I've made a library for this. React Hookable Component.
Usage is very simple. Replace extends Component or extends PureComponent with extends HookableComponent or extends HookablePureComponent. You can then use hooks in the render() method.
import { HookableComponent } from 'react-hookable-component';
// 👇👇👇👇👇👇👇👇
class ComponentThatUsesHook extends HookableComponent<Props, State> {
render() {
// 👇👇👇👇👇👇
const value = useSomeHook();
return <span>The value is {value}</span>;
}
}

if you didn't need to change your class component then create another functional component and do hook stuff and import it to class component

Doesn't work anymore in modern React Versions. Took me forever, but finally resulted going back to go ol' callbacks. Only thing that worked for me, all other's threw the know React Hook Call (outside functional component) error.
Non-React or React Context:
class WhateverClass {
private xyzHook: (XyzHookContextI) | undefined
public setHookAccessor (xyzHook: XyzHookContextI): void {
this.xyzHook = xyzHook
}
executeHook (): void {
const hookResult = this.xyzHook?.specificHookFunction()
...
}
}
export const Whatever = new WhateverClass() // singleton
Your hook (or your wrapper for an external Hook)
export interface XyzHookContextI {
specificHookFunction: () => Promise<string>
}
const XyzHookContext = createContext<XyzHookContextI>(undefined as any)
export function useXyzHook (): XyzHookContextI {
return useContext(XyzHookContextI)
}
export function XyzHook (props: PropsWithChildren<{}>): JSX.Element | null {
async function specificHookFunction (): Promise<void> {
...
}
const context: XyzHookContextI = {
specificHookFunction
}
// and here comes the magic in wiring that hook up with the non function component context via callback
Whatever.setHookAccessor(context)
return (
< XyzHookContext.Provider value={context}>
{props.children}
</XyzHookContext.Provider>
)
}
Voila, now you can use ANY react code (via hook) from any other context (class components, vanilla-js, …)!
(…hope I didn't make to many name change mistakes :P)

Yes, but not directly.
Try react-iifc, more details in its readme.
https://github.com/EnixCoda/react-iifc

Try with-component-hooks:
https://github.com/bplok20010/with-component-hooks
import withComponentHooks from 'with-component-hooks';
class MyComponent extends React.Component {
render(){
const props = this.props;
const [counter, set] = React.useState(0);
//TODO...
}
}
export default withComponentHooks(MyComponent)
2.Try react-iifc： https://github.com/EnixCoda/react-iifc

Functional components inside vs outside of React class

I am wondering what the effects of coupling / nesting a functional component inside a React class component without explicitly passing props to it via params and using this.props via the parent class are. I understand that having a functional component outside of the React class component is easier to test and read, but am curious to know what the exact difference between using this.props vs props via params are in terms of performance / renders.
For example:
class Foo extends React.Component {
bar = () => { return (<p>{this.props.baz}</p>) }
render() {
return (
<h1>Hello, {this.props.name}</h1>
{this.bar()}
)
}
}
Vs.
class Foo extends React.Component {
render() {
return (
<h1>Hello, {this.props.name}</h1>
<Bar baz={'foobar'}
)
}
}
function Bar(props) {
return <p>{props.baz}</p>
}

Both give the same result except when you care about code reusability.
If you care about reusing the Bar function then you better keep it outside the class so you can import it elsewhere.
Example:
If Bar renders a success or warning message. you'll want to keep the same design for all warning messages in the system.
Now if every component has its own warning message code, you'll not be able to edit the warning message design easily also you'll have to keep rewriting the same code over and over again.

React - Updating State's Component From Another File

Suppose that I had a React component such as
import { getEventListener } from 'controls';
class SomeComponent extends React.Component {
render() {
return (<div onClick={this.handleEvent}></div>)
}
handleEvent = (event) => {
getEventListener(event, this);
}
}
And I have another file such as
export function getEventListeners(event, component) {
component.setState({x: 1};
}
If getEventListeners calls set state from the component to change one of the properties would it cause an issue?

This causes the design issue, passing the whole component by reference and accessing it in a function breaks the principle of least privilege. Calling the function with a context like getEventListener.call(this, event) would have the same problem.
If getEventListener isn't supposed to be reused between components, it shouldn't be extracted from a component where it's used. It uses this.setState method and clearly belongs to a class. In case multiple inheritance is involved, a mix-in can be used.
A solution that is idiomatic to React is reusable state updater function. It's decoupled from a component and supposed to be used for pure synchronous functions:
export const getEventListeners = event => state => {
// return state object that derives from an event
};
...
this.setState(getEventListeners(event));

In reactjs how can I call a method from another component?

Just trying reactjs and came across the situation that I want to call a method on another component:
class MyComp extends React.Component {
callMe(){
...
}
}
So mycomp2:
import MyComp from 'myComp';
class MyComp2 extends React.Component {
test(){
MyComp.callMe();
}
}
How can I do this ?

If the method callMe doesn't use this, you can declear it as static to use it like that.
class MyComp extends React.Component {
static callMe(){
...
}
}
If not, you can use ref to make it work.
You may need:
https://facebook.github.io/react/docs/refs-and-the-dom.html

Cannot. You cannot do that in react component. The only way is to move that function to their common ancestor.
Of course, there is another way, why not consider redux? let's react component talk through redux state. that way, you will never have this puzzle.

It depends on how your components are related to one another. If one is a parent and the other is a child then the function can be passed as a prop from parent to child, e.g.
class Parent extends React.Component {
callMe() {
console.log('called');
}
render() {
return (
<Child doSomething={() => this.callMe()} />
);
}
}
class Child extends React.Component {
render() {
<button onClick={this.props.doSomething}>Calls the parent fn</button>
}
}
Alternatively if they are not a parent-child relationship, then you need to introduce a parent component in which the function can live, then pass it via props in the same way.
If neither of these approaches seems to fit, then let us know what behaviour you are trying to achieve that requires this, and there my well be a different way to structure your code.

How to access child component functions via refs

The docs for React state that component functions can be accessed by a parent component via refs. See: https://facebook.github.io/react/tips/expose-component-functions.html
I am attempting to use this in my application but run into an "undefined is not a function" error when the child function is called. I'm wondering if this has anything to do with using the ES6 format for React classes because I don't see any other differences between my code and the docs.
I have a Dialog component that looks like the following pseudocode. The Dialog has a "Save" button that calls save(), which needs to call the save() function in the child Content component. The Content component collects information from child form fields and performs the save.
class MyDialog extends React.Component {
save() {
this.refs.content.save(); <-- save() is undefined
}
render() {
return (
<Dialog action={this.save.bind(this)}>
<Content ref="content"/>
</Dialog>);
}
}
class Content extends React.Component {
save() {
// Get values from child fields
// and save the content
}
}
I could instead pass a prop (saveOnNextUpdate) down to Content and then execute save whenever it is true, but I would rather figure out how to get the method detailed in the React doc above to work.
Any ideas on how to get the doc approach to work or access the child component function in a different way?

Redux connect accepts an option parametre as the forth parameter. In this option parameter you can set the flag withRef to true. Then you can access functions to refs by using getWrappedInstance(). Like this:
class MyDialog extends React.Component {
save() {
this.refs.content.getWrappedInstance().save();
}
render() {
return (
<Dialog action={this.save.bind(this)}>
<Content ref="content"/>
</Dialog>);
}
}
class Content extends React.Component {
save() { ... }
}
function mapStateToProps(state) { ... }
module.exports = connect(mapStateToProps, null, null, { withRef: true })(Content);
Read more about it here: https://github.com/reactjs/react-redux/blob/master/docs/api.md#connectmapstatetoprops-mapdispatchtoprops-mergeprops-options
Worth reading this article about use of refs and consider if there's better approaches: https://facebook.github.io/react/docs/refs-and-the-dom.html#dont-overuse-refs

An alternative way to do this would be to use some other prop name (other than ref). I've found that this also works well if you're using a library like styled-components or emotion For example in a connected MyComponent:
<MyComponent
...
innerRef={(node) => { this.myRef = node; }}
/>

As it turns out, m90 was right -- this was a different issue entirely. I'm posting the solution in case someone runs into the same problem in the future.
My application is built with Redux, and the problem stems from using the react-redux connect function to connect a component to the store/global state. For some reason, exporting a component and connecting it to the store makes it impossible to access the functions inside of it. In order to get around this, I had to remove all use of global state from Content so that I could export it as a "dumb" component.
To be more clear, Content.js looked like this:
var connect = require('react-redux').connect;
class Content extends React.Component {
save() {
// Get values from child fields
// and save the content
// Use of this.props.stateObject
}
}
function mapStateToProps(state) {
const {
stateObject
} = state;
return {
stateObject
};
}
module.exports = connect(mapStateToProps)(Content);
Removing the use of global state (and therefore the use of connect and mapStateToProps allowed me to export the component using:
module.exports = Content;
Accessing this.refs.content.save() magically worked after doing this.