How To Avoid Rerender All Child Components Which In Loop When Parent Component State Update

I m having one child component which is inside a loop of parent component. when one of the child components is updating the state of parent component, it is re-rendering the all ch

Solution 1:

for that you can use React.memo that will memoize your component if props remains the same. But given your code you need to make some extra changes:

• you have to apply useCallback to memoize onChangeHandle function;

• to memoize properly onChangeHandle you need to refactor it. you can't pass selectedChild directly, otherwise it memoizes its value. use setSelectedChild passing as argument a function that takes selectedChild instead.

• your Child should receive isSelected as boolean value instead of function. otherwise props will remain the same and Child never updates;

  import React, { useState, memo, useCallback } from "react";
  
  function Parent() {
    const [selectedChild, setSelectedChild] = useState([]);
  
    const onChangeHandle = useCallback((event, id) => {
      setSelectedChild(selectedChild => {
        const checked = event.target.checked;
        let updatedArray = [...selectedChild];
        if (checked) {
          if (!selectedChild.includes(id)) {
            updatedArray.push(id);
          }
        } else {
          var index = updatedArray.indexOf(id);
          if (index !== -1) {
            updatedArray.splice(index, 1);
          }
        }
        return updatedArray;
      });
    }, []);
  
    const dummy = id => {
      return selectedChild.includes(id);
    };
  
    const renderChildren = () =>
      [1, 2, 3].map((value, index) => {
        return (
          <Child
            key={index}
            index={index}
            value={value}
            handle={onChangeHandle}
            isSelected={dummy(index)}
          />
        );
      });
  
    return (
      <div>
        <table>
          <tbody>{renderChildren()}</tbody>
        </table>
        <div>{selectedChild}</div>
      </div>
    );
  }
  
  const Child = memo(({ index, value, handle, isSelected }) => {
    console.log("rendering");
  
    return (
      <tr>
        <td>
          <input
            type="checkbox"
            checked={isSelected}
            onChange={event => handle(event, index)}
          />
        </td>
        <td>
          hello {index} {value}
        </td>
      </tr>
    );
  });
  
  export default function App() {
    return (
      <div className="App">
        <Parent />
      </div>
    );
  }
  

https://stackblitz.com/edit/so-memo-children?file=src/App.js

Solution 2:

The basic answer is use React.memo on Child.

const Child = memo(function Child(...) {...})

But to make memo work, the component needs to receive the same props if it shouldn't get rerendered. That means using useCallback on onChangeHandle:

 const onChangeHandle = useCallback((event, id) => {...}, [])

But since onChangeHandle uses selectedChild that always changes on checkbox change, you'll also need to ref it using useRef:

const selectedChildRef = useRef();
selectedChildRef.current = selectedChild;

and use reffed version inside of onChangeHandle.

The last thing that needs to be done is to change isSelected prop from function to just a flag since it needs to be run on each checkbox change:

isSelected={selectedChild.includes(index)}

https://codesandbox.io/s/newpro-forked-wxvqs

Solution 3:

You could implement shouldComponentUpdate (doc: https://reactjs.org/docs/react-component.html#shouldcomponentupdate) inside the definition of Child to have more control over when it rerenders. But that's only meant for cases where you have performance issues- generally you don't have to worry about it, and letting them all rerender is standard.