React ref.current is null

问题

I'm working on an agenda/calendar app with a variable time range. To display a line for the current time and show blocks for appointments that have been made, I need to calculate how many pixels correspond with one minute inside the given time range.

So for example: If the agenda starts at 7 o'clock in the morning and ends at 5 o'clock in the afternoon, the total range is 10 hours. Let's say that the body of the calendar has a height of 1000 pixels. That means that every hour stands for 100 pixels and every minute for 1,66 pixels.

If the current time is 3 o'clock in the afternoon. We are 480 minutes from the start of the agenda. That means that the line to show the current time should be at 796,8 pixels (480 * 1,66) from the top of the calendar body.

No problems with the calculations but with getting the height of the agenda body. I was thinking to use a React Ref to get the height but I'm getting an error: ref.current is null

Below some code:

class Calendar extends Component {
    calendarBodyRef = React.createRef();

    displayCurrentTimeLine = () => {
        const bodyHeight = this.calendarBodyRef.current.clientHeight; // current is null
    }

    render() {
        return (
            <table>
                <thead>{this.displayHeader()}</thead>
                <tbody ref={this.calendarBodyRef}>
                    {this.displayBody()}
                    {this.displayCurrentTimeLine()}
                </tbody>
            </table>
        );
    }
}

回答1:

So the thing about refs is that they aren't guaranteed to be set on first render. You can be sure they are set during and after componentDidMount so you have two ways going forward.

You could use the callback style ref and set state based on that. E.g. instead of passing your ref as the prop, you can pass in a reference to a function like this.handleRef and it would do some logic in there:

  handleRef = r => {
    this.setState({ bodyHeight: r.clientHeight})
    this.calendarBodyRef .current = r;
  };

Or, you could keep your current set up but you would have to move your clientHeight bit to a lifecycle function like:

  componentDidMount() {
    this.setState({ bodyHeight: this.calendarBodyRef .current.clientHeight });
  }

Ultimately, you can't immediately read the current value of a ref like that, you would have to check it after the render and then read your bodyHeight from state.

回答2:

If avoiding storage of the calculated body height in component state is preferred, then another approach would be to introduce an second ref (ie elementDisplayHeightRef) like so:

class Calendar extends React.Component {

    /* Create a ref for the body */
    calendarBodyRef = React.createRef();

    /* Create a ref for element where height will be displayed */
    elementDisplayHeightRef = React.createRef();

    displayCurrentTimeLine = () => {

        /* Calculate body height from ref */
        const bodyHeight = this.calendarBodyRef.current.clientHeight;    

        /* Update display */
        this.elementDisplayHeightRef.current.innerText = `bodyHeight:${bodyHeight}`
    }

    render() {
        return (
            <table>
                <thead></thead>
                <tbody ref={this.calendarBodyRef}>
                    <td><td>Some row</td></td>
                    {/* Bind display ref */ }
                    <tr><td ref={this.elementDisplayHeightRef}></td></tr>
                </tbody>
            </table>
        );
    }

    /* Add did mount life cycle hook, and trigger display of body height */
    componentDidMount() {

      this.displayCurrentTimeLine()
    }
}

This approach calls displayCurrentTimeLine() during the componentDidMount() life cycle hook (which itself is called after the first render()) to ensure that both refs are fully initialised, before the component logic interacts with them in displayCurrentTimeLine().

Hope that helps!

来源：https://stackoverflow.com/questions/55248483/react-ref-current-is-null

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reactjs

ref