React Hooks Introduction

13/04/2019 reveal.js
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REACT HOOKS ⚓REACT HOOKS ⚓Felix Kühl - JobMatchMe GmbH

INTRODUCTION TO HOOKSINTRODUCTION TO HOOKS
statefull component
useState
class NameInput extends React.Component {
constructor(){
state = { name: "Max" }
}
handleChange = (e) => {
this.setState({name: e.target.value})
}
render(){ return <input type="text" onChange={this.handleChange} value={this.state} /> }
}
const NameInput = () => {
const [name, setName] = useState("Max")
const handleChange = (e) => {
setName(e.target.value)
}
return <input type="text" onChange={handleChange} value={name} />
}

WHY USESTATE IS ACUTALLYWHY USESTATE IS ACUTALLY
AWESOMEAWESOME
const Parent = () => {
const [name, setName] = useState()
return <Child setParentState={setName} />
}
const Child = ({setParentState}) => {
const upperCaseMe = () => {
setParentState((state) => state.toUpperCase())
})
return <button onClick={upperCaseMe}>Upper case my name!</button>
}

componentDidMount & componentDidUpdate
class TitleInput extends React.Component {
constructor(){
state = { title: "Hello" }
}
handleChange = (e) => {
this.setState({title: e.target.value})
}
componentDidMount() {
document.title = this.state.title
}
componentDidUpdate() {
document.title = this.state.title
}
render(){ return <input type="text" onChange={this.handleChange} value={this.state.title} /> }
}

useEﬀect
const TitleInput = () => {
const [title, setTitle] = useState("Hello")
const handleChange = (e) => {
setTitle.target.value)
}
useEffect(() => { document.title = title })
return <input type="text" onChange={handleChange} value={title} />
}

componentDidMount
class WidthComponent extends React.Component {
constructor(){
state = { width: window.innerWidth }
}
handleResize = () => {
this.setState({width: window.innerWidth})
}
componentDidMount() {
window.addEventListener('resize', this.handleResize)
}
componentWillUnmount() {
window.removeEventListener('resize', this.handleResize)
}
render(){ return <p>{this.state.width}</p> }
}

useEﬀect
const WidthComponent = () => {
const [width, setWidth] = useState(window.innerWidth)
const handleResize = () => {
setWidth(window.innerWidth)
}
useEffect(() => {
window.addEventListener('resize', handleResize)
return window.removeEventListener('resize', handleResize)
}, [])
return <p>{width}</p>
}

And what about componentDidUpdate?

And what about componentDidUpdate?
useEffect(() => { if(iMightChange !== null) { doSomething() } }, [iMightChange])

useContext
const Consumer = () => {
const value = useContext()
return (
<Context.Consumer>
{value => <p>Context says {value}</p>}
</Context.Consumer>
)
}
const Consumer = () => {
const value = useContext()
return (
<p>Context says {value}</p>
)
}

useMemo
const memoizedValue = useMemo(() => someExpensiveCalculation(x, y), [x, y]);

REUSE AND COMPOSEREUSE AND COMPOSE
Are hooks kind of like mixins
const MyLocation = () => {
const location = useLocation()
return <p>You are at: {location}</p>
}
const useLocation = () => {
const [location, setLocation] = useState("loading...")
useEffect(() => {
if (navigator.geolocation) {
navigator.geolocation.getCurrentPosition(() =>
setLocation("lat: "+position.coords.latitude+" lng: "+position.coords.longitude)
);
} else {
return "Geolocation is not supported by this browser.";
}
})
return location
}

HOOKS IN REAL LIFEHOOKS IN REAL LIFE
Our (old) data provider
/* Context */
const DataContext = React.createContext()
/**
* Global user data provider
*
**/
class DataProvider extends React.Component {
constructor(props) {
super(props)
this.state = {
status: 'init',
data: parseData({}),
actions: {fetchData: this.fetchData}
}
}
/**
* Fetch Data only when we are authenticated. This might not happen on initial mounting
/

... now with hooks! ✨
/* Context */
export const DataContext = React.createContext()
/**
* Data Reducer
*/
const reducer = (state, action) => { ... }
/**
* Global user data provider
*
**/
const DataProvider = props => {
/**
* Fetch Data from API endpoint
*/
const fetchData = async () => { ... }
const [state, dispatch] = useReducer(reducer, {
'i i '

Class vs Hooks
async componentDidMount() {
if (this.props.authState === 'signedIn') {
await this.fetchData()
}
}
async componentDidUpdate(prevProps, prevState) {
if (
prevProps.authState !== this.props.authState &&
this.props.authState === 'signedIn'
) {
await this.fetchData()
}
}
useEffect(() => {
if (props.authState === 'signedIn') {
fetchData()
}
}, [props.authState])

HOOKS UNDER THE HOODHOOKS UNDER THE HOOD
react / packages / react-reconciler / src / ReactFiberHooks.js
// TODO: Not sure if this is the desired semantics, but it's what we
// do for gDSFP. I can't remember why.
if (workInProgressHook.baseUpdate === queue.last) {
workInProgressHook.baseState = newState;
}
...

https://medium.com/the-guild/under-the-hood-of-reacts-hooks-system-eb59638c9dba

State object
State hooks queue
{
foo: 'foo',
bar: 'bar',
baz: 'baz',
}
{
memoizedState: 'foo',
next: {
memoizedState: 'bar',
next: {
memoizedState: 'bar',
next: null
}
}
}

FINAL THOUGHTSFINAL THOUGHTS

React Hooks Introduction

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