Effect Hook

1、Effect Hook It allows you to perform side effects in function components （ Used to simulate lifecycle hooks in class components ）

2、React Side effects in operation ：

• Hair ajax Request data acquisition
• Set up a subscription / Start timer
• Manually change the real D0M

3、 Grammar and instructions ：

useEffect(()=>{
    
// Any operation with side effects can be performed here 
return()=>{
    // Execute before the component is unloaded 
// Do some finishing work here , For example, clear the timer / Unsubscribe, etc 
}},[statevalue]) //  If you specify yes [], The callback function will only - Time render() After execution 

4. You can put useEfect Hook Think of it as a combination of the following three functions

• componentDidMount()
• componentDidUpdate()
• componentwillUnmount()