Add Ref to Grid

[paul-sachs]

Add reference prop so we can get the internal div element. Useful for measurement and scroll control.

We could use React.forwardRef but that does a few things:

• limits version of compatible react
• breaking API change (ref would now be the div, vs the class object)

[github-actions]

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[STRML]

That's pretty useful, especially since ReactDOM.findDOMNode() is discouraged now.

It looks like this is pretty unsurprising / used elsewhere (https://github.com/atlassian/react-beautiful-dnd/blob/master/docs/guides/using-inner-ref.md), so I'll merge.

[STRML]

Thanks for the contribution!

[ben-elsen]

@paul-sachs Could you provide an example of how to implement this because I tried but I keep getting the error:

Warning: findDOMNode is deprecated in StrictMode. findDOMNode was passed an instance of DraggableCore which is inside StrictMode. Instead, add a ref directly to the element you want to reference. Learn more about using refs safely here: https://fb.me/react-strict-mode-find-node

[paul-sachs]

@ben-elsen not sure what you mean by implement. To use the new prop:

const ref = useRef();
return (
  <GridLayout innerRef={ref} />
);

This change didn't remove any references to findDOMNode within the source. More work would need to be done to remove the usage of findDOMNode.

[ben-elsen]

@paul-sachs I misunderstood this feature.. I thought it was intended to fix the warning. Sorry :-)