React.Children.only expected to receive a single React element child

[jefffriesen]

If you try to recreate the example in the README, you get this error:

React.Children.only expected to receive a single React element child

The problem is that <Step> is taking text nodes in the example:

        <Scrollama onStepEnter={this.onStepEnter}>
          <Step data={1} key='1'>
            step 1
          </Step>
          <Step data={2} key='2'>
            step 2
          </Step>
        </Scrollama>

However, if you replace the text nodes any element, then everything works correctly:

        <Scrollama onStepEnter={this.onStepEnter}>
          <Step data={1} key='1'>
            <div>step 1</div>
          </Step>
          <Step data={2} key='2'>
            <div>step 2</div>
          </Step>
        </Scrollama>

React.Children.only(children) throws with a text node: https://github.com/jsonkao/react-scrollama/blob/master/src/Step.js#L25

I don't know if you want to allow text nodes, or update the README to wrap the text in elements.

[jsonkao]

Thank you, this is really helpful. I will work on allowing text nodes.

[lita-jerry]

Has this problem been solved? How can I solve this problem? Thank’s

[jefffriesen]

Adding a related example:

This works:

        <Scrollama onStepEnter={this.onStepEnter}>
          <Step data={1} key='1'>
            <div>
              <MyComponent />
             </div>
          </Step>
        </Scrollama>

This throws:

        <Scrollama onStepEnter={this.onStepEnter}>
          <Step data={1} key='1'>
             <MyComponent />
          </Step>
        </Scrollama>

Uncaught (in promise) TypeError: Cannot read property 'offsetHeight' of null
at Step.updateOffsetHeight

It would be nice to not have to wrap all components inside each Step

[jsonkao]

The direct child of <Step> must be a DOM element because React Scrollama uses a child's offsetHeight property to calculate the intersection margin. However, only HTMLElements have offsetHeight. That's why using a React component as a direct child of <Step> breaks the library.

For now, to use a custom React component inside <Step>, I recommend wrapping it with a <div> like so:

<Step>
  <div>
    <MyComponent />
  </div>
</Step>

I'm very open to hearing suggestions about how this can be resolved/improved. One solution could be that custom components could forward the ref to their DOM elements. React Scrollama could also check whether a child ref was a DOM element, and use findDOMNode() if it weren't. Both are unideal...

[jsonkao]

(Note to self: See the wording on the error message Transitions can only be applied to DOM elements, not components from Svelte).

[inspectordanno]

@jsonkao OMG!! This was the error that I was having that confused me! Have to wrap the step-child (haha pun) in a <div> if it is a React component. Got it!

I wouldn't be unopposed to having a ref exposed that I could forward to a custom component.

[maerzhase]

You can use custom components but you have to handle ref forwarding. This is all not specifics to the lib but basics on how react works

https://reactjs.org/docs/forwarding-refs.html#forwarding-refs-to-dom-components

[inspectordanno]

@maerzhase But isn't the issue that <Step/> needs a way to accept a ref? The ref needs to be defined in the user code, but then forwarded along to the <Step/> component. This would involve changing the library.

[maerzhase]

I am not sure what you mean by that. The step component is actually creating a ref for each step, thats why the child either needs to be a html element or a custom component that uses ref forwarding.

If your "MyComponent" would use ref forwarding you wouldn't need to wrap a div around it.

[jsonkao]

@inspectordanno I'm glad this helped! Sorry that the documentation is a bit scattered. @maerzhase I tried doing this once (I gave up) but I do vaguely remember either (a) needing to change <Step/> or (b) needing to force users to use React.forwardRef (which seems kind of too much to ask).

[gjhltn]

Sorry to be dim.

What should this look like with ref forwarding?

I'm doing

onst Scene = React.forwardRef(({
	currentStepIndex,
	stepIndex
}, ref) =>
	<Step ref={ref} data={stepIndex} >
            <div
              style={{
                margin: '50vh 0',
                border: '1px solid gray',
                opacity: currentStepIndex === stepIndex ? 1 : 0.2,
              }}
            >
              I'm a Scrollama Step of index {stepIndex}
            </div>
			</Step>)

const ScrollamaDemo = () => {
  const [currentStepIndex, setCurrentStepIndex] = useState(null);

  // This callback fires when a Step hits the offset threshold. It receives the
  // data prop of the step, which in this demo stores the index of the step.
  const onStepEnter = ({ data }) => {
	  alert('doesn't fire')
    setCurrentStepIndex(data);
  };

  return (
    <div style={{ margin: '50vh 0', border: '2px dashed skyblue' }}>
      <div style={{ position: 'sticky', top: 0, border: '1px solid orchid' }}>
        I'm sticky. The current triggered step index is: {currentStepIndex}
      </div>
      <Scrollama onStepEnter={onStepEnter} debug>
        {[1, 2, 3, 4].map((_, stepIndex) => (
			 <Scene currentStepIndex={currentStepIndex} stepIndex={stepIndex} key={stepIndex} />
        ))}
      </Scrollama>
    </div>
  );
};

But onSteoEnter doesn't fire.

and I get this error

Could not get step with id react-scrollama-0
String: "Could not get step with id react-scrollama-0"

[maerzhase]

@gswirrl you are using standard HTML elements and therefore you don't need React.forwardRef.
You only need React.forwardRef when you use a custom React component inside the <Step/>-component.

This works without React.forwardRef

<Scrollama>
  <Step>
    <div>
      Step 1
    </div>
  </Step>
  <Step>
    <div>
      Step 2
    </div>
  </Step>
</Scrollama>

This only works with React.forwardRef on FancyDiv

<Scrollama>
  <Step>
    <FancyDiv>
      Step 1
    </FancyDiv>
  </Step>
  <Step>
    <FancyDiv>
      Step 2
    </FancyDiv>
  </Step>
</Scrollama>

const FancyDiv = React.forwardRef((props, ref) => (
  <div ref={ref}>
    {props.children}
  </div>
));

[gjhltn]

@maerzhase thanks, I tried to simplifiy what I'm trying to do and reduced it to the simplest case - maybe I didn't do a good job of explaining.

What I'd tried to illustrate is that I need the <Step> to be inside another component. I want to use those other component instances as the children of Scrollama. ie in pseudo code

const FancyStep = () =>
    <Step ...>
       {/*some stuff here*/}
    </Step>


<Scrollama>
  <FancyStep />
  <FancyStep />
   <FancyStep />
</Scrollama>

[maerzhase]

@gswirrl in your example you are dropping all the props that the Scrollama component provides to the Step.

const FancyStep = props => <Step {...props}>{/*some stuff here*/}</Step>

Could you elaborate on "...that I need the <Step> to be inside another component..."? Why do you need it to be in your custom component? I am not aware of a convention that would force you to have it as opposed to the actual library example.

[gjhltn]

@maerzhase I appreciate your kind reply. Yes I am aware the simplified pseudocode drops the props. I was trying to describe the use case in the simplest possible way. Obviously in practice it is a very great deal more complicated than the pseudo code I posted.

If you nest step in another component it errors. In your example too.

I'm building a higher level abstraction and in a nutshell the steps compose a good bit of additional functionality and get passed around in ways which are easy if they are wrapped in a component and awkward otherwise.

My question though is really simple - is there a way to put a step inside another component which is the child of the scrolama component?

[maerzhase]

@gswirrl yes this should work as long as you don't drop the props that are provided to the child components of the Scrollama component.

If you still experience problems please provide a reproducable example and no pseudo-code—otherwise I cannot help you with your problem.

For the future we will definitely add more examples that will also include your use-case.

[gjhltn]

Thanks! I hoped it would work like this too but it doesn't seem to. Adapting the readme example...

import React, {useState} from "react"

import { Scrollama, Step } from 'react-scrollama';
	
const FancyStep = ({
	stepIndex,
	currentStepIndex
}) => <Step data={stepIndex} key={stepIndex}>
            <div
              style={{
                margin: '50vh 0',
                border: '1px solid gray',
                opacity: currentStepIndex === stepIndex ? 1 : 0.2,
              }}
            >
              I'm a Scrollama Step of index {stepIndex}
            </div>
	</Step>

const ScrollamaDemo = () => {
  const [currentStepIndex, setCurrentStepIndex] = useState(null);
  const onStepEnter = ({ data }) => {
    setCurrentStepIndex(data);
  };
  
  return (
    <div style={{ margin: '50vh 0', border: '2px dashed skyblue' }}>
      <div style={{ position: 'sticky', top: 0, border: '1px solid orchid' }}>
        I'm sticky. The current triggered step index is: {currentStepIndex}
      </div>
      <Scrollama onStepEnter={onStepEnter} debug>
	   {[1, 2, 3, 4].map((_, stepIndex) => (
	   <FancyStep key={stepIndex} stepIndex={stepIndex} currentStepIndex={currentStepIndex}/>
       ))}
      </Scrollama>
    </div>
  );
};

export default ScrollamaDemo;|

Which as I say, gives this error

Warning: Function components cannot be given refs. Attempts to access this ref will fail. Did you mean to use React.forwardRef()?

Check the render method of `Scrollama`.
FancyStep
_classCallCheck
div
ScrollamaDemo
_classCallCheck
PureComponent
ReactDevOverlay
_classCallCheck
AppContainer
Root

which is why, in my very first post above, I was trying to forward the reference, it it seems that that too does not in fact work, as while it fixes this error, as I noted originally, it fails again downstream of there.

I hope I'm just doing this wrong, but at the moment i ant find a way at all to have the Step in a nested component 🙁

many many thanks again for your help!

[maerzhase]

@gswirrl aaaaand in your example code your are still dropping the props. Sorry I didn't know anymore how to explain it to you—So I set up a codesandbox that illustrates what you are trying to achieve:

https://codesandbox.io/s/react-scrollama-custom-step-component-rnr7d?file=/src/App.js

[gjhltn]

@maerzhase thank you I'm most grateful for your patience and help. Youve actually nailed it here, but a bit differently than we both thought....

The sandbox works excellently, as you say, but when I ran that same code verbatim in my env, it didn't work. Turns out I am on react-scrollama": "2.2.16" and if I change the sandbox to that it errors too in the same way mine was doing.

I think that might be the issue? 😀

However.... that new 2.3 version introduces a new problem. I'm in next.js and ssr is dying because window isn't defined. That should be work roundable however. if it's useful I can open a separate issue for that?

[maerzhase]

@gswirrl hmm you are right, i must admit i joined the project rewriting the code-base which is now on the prerelease 2.3.0-beta.0. So i can't vouche for the last releases 😭

About SSR: Scrollama highly depends on window functionality. Currently the easiest fix is to use next-dynamic, but please open an issue so we can maybe try to find a solution to make ssr possible! Cheers!

[gjhltn]

@maerzhase man you're a star - the sandbox completely solved the mystery thank you.

Appreciate the next tips too - yes I shall do something like that - it's clearly GOT to be a client side thing 😀

once again tons of thanks for going to that extra bit of trouble. I'm really grateful.

[maerzhase]

@gswirrl hej man no problem! I really enjoy contributing in the open source community and its cool if this library supports you in building something great! if you need any support in anything else just hit us up!