如何使用函数组件向 ref 公开函数？

Question

I want to create a component lets say a calendar functional component which can expose some functions like nextMonth , prevMonth , etc. So that the parent component using it can access these functions using the ref . It was easy with class components as the instance methods are automatically exposed. How do I achieve the same with functional components? I know I can use render props pattern for this. But any other solution?

Rough example:

function Calendar() {
   const nextMonth = () => {...}
   const prevMonth = () => {...}

   return (
      <div>.....</div>
   )
}


function Parent() {
   const cal = createRef();

   const onNext = () => {
     cal.current.nextMonth();
   }

   return (
     <>
        <Calendar ref={cal} />
        <button onClick={onNext}>Next</button>
     </>
   )
}

Answer 1

You can use useImperativeHandle hook.

const Calendar = React.forwardRef((props, ref) => {
  const [month, setMonth] = useState(0)
  const nextMonth = () => { setMonth((prev) => prev + 1) }
  const prevMonth = () => { setMonth((prev) => prev - 1) }

  useImperativeHandle(ref, () => ({
    nextMonth,
    prevMonth
  }));

  return (
    <div>{month}</div>
  )
})

const Parent = () => {
  const cal = useRef()

  const onNext = () => {
    cal.current.nextMonth()
  }

  return (
    <React.Fragment>
      <Calendar ref={cal} />
      <button type="button" onClick={onNext}>Next</button>
    </React.Fragment>
  )
}

Answer 2

Here's how you can do it, even though it's not really the React way. The trick is to set the current value of the ref in Calendar .

const Calendar = React.forwardRef((props, ref) => {
  const [month, setMonth] = useState(0)
  const nextMonth = () => { setMonth((prev) => prev + 1) }
  const prevMonth = () => { setMonth((prev) => prev - 1) }

  ref.current = { nextMonth, prevMonth } // eslint-disable-line

  return (
    <div>{month}</div>
  )
})

const Parent = () => {
  const cal = useRef()

  const onNext = () => {
    cal.current.nextMonth()
  }

  return (
    <React.Fragment>
      <Calendar ref={cal} />
      <button type="button" onClick={onNext}>Next</button>
    </React.Fragment>
  )
}

Answer 3

in functional components you can use forwardRef to pass ref to child component.

const Calendar = React.forwardRef((props, ref) => {
  const nextMonth = () => {...};
  const prevMonth = () => {...};

  return <div>.....</div>;
});

function Parent() {
  const cal = useRef(null);

  const onNext = () => {
    cal.current.nextMonth();
  };

  return (
    <>
      <Calendar ref={cal} />
      <button onClick={onNext}>Next</button>
    </>
  );
}

this work:

const Calendar = React.forwardRef((props, ref) => {
  const nextMonth = () => console.log("nextMonth");
  const prevMonth = () => console.log("prevMonth");
  ref.current = { nextMonth, prevMonth };
  return <div>.....</div>;
});

function Parent() {
  const cal = useRef(null);

  const onNext = () => {
    cal.current.nextMonth();
  };

  return (
    <>
      <Calendar ref={cal} />
      <button onClick={onNext}>Next</button>
    </>
  );
}

Answer 4

While @Matthieu Libeer answer is correct and this should work to you still consider making that in React way. By now your Calendar is the same uncontrolled component . By accessing it's internal state(and methods in case of class-based components) by ref you are loosing:

1. Ability to make pre-input validation(say, to discard selection for each second Sunday in month)
2. Replace it with different component in easy way

Once it would be fully controllable component + keeping consistency with native controls in props' names you would get something like

<Calendar value={currentData} onChange={this.validateAndSetDate} />

This way it can be easily replaced with compatible component(say <input type="date" /> ) as well as validation is much more easier. You still might need forwardRef say to .focus() imperatively but there will be less pain with other aspects.