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Calling a child function from a parent component in React:

1. Wrap the Child component in forwardRefa .
2. Use the hook in child useImperativeHandleto add a function to Child .
3. Use ref to call Child 's function from Parent , for example .childRef.current.childFunction()

import {forwardRef, useImperativeHandle, useRef} from 'react';

const Child = forwardRef((props, ref) => {
  useImperativeHandle(ref, () => ({
    childFunction1() {
      console.log('child function 1 called');
    },
    childFunction2() {
      console.log('child function 2 called');
    },
  }));

  return (
    <div>
      <h2>child content</h2>
    </div>
  );
});

export default function Parent() {
  const childRef = useRef(null);

  const handleClick = () => {
    childRef.current.childFunction1();

    childRef.current.childFunction2();
  };

  return (
    <div>
      <Child ref={childRef} />

      <h2>parent content</h2>

      <button onClick={handleClick}>Call child functions</button>
    </div>
  );
}

forwardRefWe forward the ref from the Parent component to the Child using .

forwardRefThe method accepts a function that takes props and ref as arguments.

The function we pass to forwardRefshould return a React node.

We need to forward the ref to the Child so that we can use the hook to customize the instance value of the Child that is exposed to the Parent component useImperativeHandlewhen using ref .

useImperativeHandle(ref, () => ({
  childFunction1() {
    console.log('child function 1 called');
  },
  childFunction2() {
    console.log('child function 2 called');
  },
}));

The parent component that renders <Child ref={childRef} />will be able to call childFunction1 as childRef.current.childFunction1().

Alternatively, we can use a more indirect approach.

Calling a child function from a parent component in React:

1. ParentDeclare a count state variable in the component.
2. Add the count variable useEffectto the dependencies of the hook in Child.
3. Increment the count in the Parent to re-run the child useEffect.

import {useEffect, useState} from 'react';

const Child = ({count}) => {
  useEffect(() => {
    const childFunction1 = () => {
      console.log('child function 1 called');
    };

    const childFunction2 = () => {
      console.log('child function 2 called');
    };

    // 👇️ don't run on initial render
    if (count !== 0) {
      childFunction1();

      childFunction2();
    }
  }, [count]);

  return (
    <div>
      <h2>child content</h2>
    </div>
  );
};

export default function Parent() {
  const [count, setCount] = useState(0);

  const handleClick = () => {
    setCount(current => current + 1);
  };

  return (
    <div>
      <Child count={count} />

      <h2>parent content</h2>

      <button onClick={handleClick}>Call child functions</button>
    </div>
  );
}

ParentThe component declares a count state variable and passes it as a prop Child.

We added the count variable useEffectto the dependencies of the hook, so every time it changes, useEffectthe function we passed to will run.

useEffect(() => {
  const childFunction1 = () => {
    console.log('child function 1 called');
  };

  const childFunction2 = () => {
    console.log('child function 2 called');
  };

  // 👇️ don't run on initial render
  if (count !== 0) {
    childFunction1();

    childFunction2();
  }
}, [count]);

The Child component useEffectdeclares and calls 2 functions in the hook.

Parent can run logic in Child 's hooks by changing countstate variables .useEffect

请注意, we useEffectcheck if count is not equal to 0 before calling the function in .

useEffectThe hook runs when the component mounts and every time one of its dependencies changes.

If we don't want to run logic on mount, check if the count variable is not equal to 0 before calling the function.