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 canβt 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 canβt 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';
// ππππππππ
class ComponentThatUsesHook extends HookableComponent<Props, State> {
render() {
// ππππππ
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, β¦)!
(β¦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-iifcοΌ https://github.com/EnixCoda/react-iifc
I have a component and I want to call a method checking the state whenever it changes. This is my component with a dummy method to demonstrate what I want to do (animate the view offscreen if onboarding.show === false):
export class Onboarding extends Component {
animateView() {
// i want to call this method when
// the state changes
// something like;
if (!this.props.onboarding.show) {
Animated.spring(...);
}
}
render() {
const { onboarding, finish } = this.props;
return (
<Animated.View>
...
</Animated.View>
);
}
}
...
export default connect(
state => {
return {
onboarding: state.onboarding,
};
},
dispatch => {
return {
};
}
)(Onboarding);
Is there a way to subscribe to the changes in state?
== UPDATE ==
as requested, here's what my slideOffScreen method does:
slideOffScreen() {
Animated.timing(this.state.offsetX, {
toValue: -Dimensions.get('window').width,
duration: 350,
easing: Easing.elastic(),
}).start();
}
The react-redux connect method wraps the component with a container component that is aware of the store's state changes. Whenever the state changes, connect re-renders the wrapped component (Onboarding in your case).
According to the redux docs:
Technically, a container component is just a React component that uses
store.subscribe() to read a part of the Redux state tree and supply
props to a presentational component it renders. You could write a
container component by hand, but we suggest instead generating
container components with the React Redux library's connect()
function, which provides many useful optimizations to prevent
unnecessary re-renders.
If your component doesn't re-rendered when the state changes, check if you're not mutating the state instead of replacing it. Redux checks if the state changed by shallowly comparing the old state, and the new state (comparing only the references, and not the values).
For example, to add an item to an array, you can't use array.push(item) because that won't create a new array, just mutate the existing one. Instead you'll have to use something like array.concat(item), which does.
To update objects, you can see in the redux docs under handling actios example, you can see that to create a new state:
We don't mutate the state. We create a copy with Object.assign().
Object.assign(state, { visibilityFilter: action.filter }) is also
wrong: it will mutate the first argument. You must supply an empty
object as the first parameter. You can also enable the object spread
operator proposal to write { ...state, ...newState } instead.
Looks like this works:
componentWillReceiveProps(props) {
if (!props.onboarding.show) {
this.slideOffScreen();
}
}
not sure if there's a way to do it through the redux API
I want to call a child component's function from its parent. I have a way to do it, but I want to know if I'm missing a better way.
From Ashley Grant's blog post about accessing a custom element's viewModel from a custom attribute, I see that Aurelia adds au to the element and you can access the viewModel through that. So, if I add a nested component with a ref, like this:
<template>
<nested-element ref="childElement"></nested-element>
</template>
I can call a function on it like this:
this.childElement.au.controller.viewModel.someFunction();
This feels roundabout. I was hoping I would be able to access a nested element's viewModel through the parameters to a hook that the parent implements, such as created(owningView, myView) but I can't find a path to it.
Have I missed a better way?
Edit: I forgot to add that I need a return value from the function I'm calling, so having access to the viewmodel itself is preferable
ref gives you the element. view-model.ref gives you the element's view model.
<template>
<nested-element view-model.ref="childViewModel"></nested-element>
</template>
Call it like this in the parent view-model:
this.childViewModel.someFunction();
If you only have one instance of the nested-element or don't care if multiple nested-elements respond to the event. Then you could use standard Javascript event functionality for this:
bar.html
<template>
<h1>${value}</h1>
<input type="text" value.bind="value"></input>
<foo>text</foo>
</template>
bar.ts
import {bindable} from 'aurelia-framework';
export class Bar {
#bindable value;
public valueChanged(newValue, oldValue) {
var event = new CustomEvent("some-event-name", { "detail": { message: "Hello from Bar", oldValue, newValue } });
document.dispatchEvent(event);
}
}
foo.html
<template>
<h1>${value}</h1>
</template>
foo.ts
import {bindable} from 'aurelia-framework';
export class Foo {
constructor() {
document.addEventListener("some-event-name", (e) => {
console.log('hello here is Foo, recieved event from Bar : ', e);
}, true);
}
}
In React Native I want to use global variables when I am moving between different screens
Can anyone help me how to achieve it?
The global scope in React Native is variable global. Such as global.foo = foo, then you can use global.foo anywhere.
But do not abuse it! In my opinion, global scope may used to store the global config or something like that. Share variables between different views, as your description, you can choose many other solutions(use redux,flux or store them in a higher component), global scope is not a good choice.
A good practice to define global variable is to use a js file. For example global.js
global.foo = foo;
global.bar = bar;
Then, to make sure it is executed when project initialized. For example, import the file in index.js:
import './global.js'
// other code
Now, you can use the global variable anywhere, and don't need to import global.js in each file.
Try not to modify them!
Try to use global.foo = bar in index.android.js or index.ios.js, then you can call in other file js.
You can use the global keyword to solve this.
Assume that you want to declare a variable called isFromManageUserAccount as a global variable you can use the following code.
global.isFromManageUserAccount=false;
After declaring like this you can use this variable anywhere in the application.
You can consider leveraging React's Context feature.
class NavigationContainer extends React.Component {
constructor(props) {
super(props);
this.goTo = this.goTo.bind(this);
}
goTo(location) {
...
}
getChildContext() {
// returns the context to pass to children
return {
goTo: this.goTo
}
}
...
}
// defines the context available to children
NavigationContainer.childContextTypes = {
goTo: PropTypes.func
}
class SomeViewContainer extends React.Component {
render() {
// grab the context provided by ancestors
const {goTo} = this.context;
return <button onClick={evt => goTo('somewhere')}>
Hello
</button>
}
}
// Define the context we want from ancestors
SomeViewContainer.contextTypes = {
goTo: PropTypes.func
}
With context, you can pass data through the component tree without having to pass the props down manually at every level. There is a big warning on this being an experimental feature and may break in the future, but I would imagine this feature to be around given the majority of the popular frameworks like Redux use context extensively.
The main advantage of using context v.s. a global variable is context is "scoped" to a subtree (this means you can define different scopes for different subtrees).
Do note that you should not pass your model data via context, as changes in context will not trigger React's component render cycle. However, I do find it useful in some use case, especially when implementing your own custom framework or workflow.
Set up a flux container
simple example
import alt from './../../alt.js';
class PostActions {
constructor(){
this.generateActions('setMessages');
}
setMessages(indexArray){
this.actions.setMessages(indexArray);
}
}
export default alt.createActions(PostActions);
store looks like this
class PostStore{
constructor(){
this.messages = [];
this.bindActions(MessageActions);
}
setMessages(messages){
this.messages = messages;
}
}
export default alt.createStore(PostStore);
Then every component that listens to the store can share this variable
In your constructor is where you should grab it
constructor(props){
super(props);
//here is your data you get from the store, do what you want with it
var messageStore = MessageStore.getState();
}
componentDidMount() {
MessageStore.listen(this.onMessageChange.bind(this));
}
componentWillUnmount() {
MessageStore.unlisten(this.onMessageChange.bind(this));
}
onMessageChange(state){
//if the data ever changes each component listining will be notified and can do the proper processing.
}
This way, you can share you data across the app without every component having to communicate with each other.
If you just want to pass some data from one screen to the next, you can pass them with the navigation.navigate method like this:
<Button onPress={()=> {this.props.navigation.navigate('NextScreen',{foo:bar)} />
and in 'NextScreen' you can access them with the navigation.getParam() method:
let foo=this.props.navigation.getParam(foo);
But it can get really "messy" if you have more than a couple of variables to pass..
The way you should be doing it in React Native (as I understand it), is by saving your 'global' variable in your index.js, for example. From there you can then pass it down using props.
Example:
class MainComponent extends Component {
componentDidMount() {
//Define some variable in your component
this.variable = "What's up, I'm a variable";
}
...
render () {
<Navigator
renderScene={(() => {
return(
<SceneComponent
//Pass the variable you want to be global through here
myPassedVariable={this.variable}/>
);
})}/>
}
}
class SceneComponent extends Component {
render() {
return(
<Text>{this.props.myPassedVariable}</Text>
);
}
}