Useeffect in hooks

Catching loach 2021-05-03 14:44:20
useeffect hooks


useEffect

side effect : Changes to the environment are side effects , Such as modification document.title,useEffect Is in render Post run . purpose

  • As componentDidMount Use ,[ ] As the second parameter
  • As componentDidUpdate Use , You can specify a dependency
  • As componentWillUnmount Use , adopt return

These three uses can exist at the same time

simulation componentDidMount

import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";
function App() {
const [n, setN] = useState(0);
const onClick = () => {
setN((i) => i + 1);
};
useEffect(() => {
console.log(" Executed the first time you render ");
}, []);
return (
<div>
n: {n}
<button onClick={onClick}>+1</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
 Copy code 

The second parameter is [ ] Represents execution only on the first rendering , give the result as follows :

react2.gif

simulation componentDidUpdate

import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";
function App() {
const [n, setN] = useState(0);
const onClick = () => {
setN((i) => i + 1);
};
useEffect(() => {
console.log("n Execute when changing ");
}, [n]);
return (
<div>
n: {n}
<button onClick={onClick}>+1</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
 Copy code 

n Execute when changing , give the result as follows :

react2.gif

When no parameters are accepted , On behalf of any state All changes are implemented

 useEffect(() => {
console.log(' Any one of them state All changes are implemented ')
})
 Copy code 

simulation componentWillUnmount

useEffect simulation componentWillUnmount, use return Just fine , Examples are as follows :

import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";
const App = props => {
const [childVisible,setChildVisible] = useState(true)
const hide = () => {
setChildVisible(false)
}
const show = () => {
setChildVisible(true)
}
return (
<div>
{childVisible ? <button onClick={hide}>hide</button> : <button onClick={show}>show</button>}
{childVisible ? <Child/> : null}
</div>
)
}
const Child = (props) => {
useEffect(() => {
console.log(' Rendering or changing ')
return ()=>{
console.log('child Destroyed ')
}
})
return (
<div>
Child
</div>
)
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
 Copy code 

When child Disappearing from the page means being destroyed , give the result as follows :

react2.gif

useEffect summary

// It's only executed the first time it's rendered :
useEffect(()=>{
console.log(' Execute this sentence after the first rendering ')
},[])
// n or m Execute when changing
useEffect(()=>{
console.log('n Changed , The value is '+n)
console.log('m Changed , The value is '+m)
},[n,m])
// No reference means any state All changes are implemented
useEffect(()=>{
console.log('n Changed , The value is '+n)
console.log('m Changed , The value is '+m)
})
// To destroy a component, use return
 Copy code 
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