Bookdown: Cross-reference figure in another file - r

Question

I have taken the plunge and am preparing a manuscript for publication entirely in RStudio using bookdown. In the main text I would like to cross-reference figures in a separate supporting information .Rmd file.
Let's say this is my main text file called main.Rmd:
---
title: "Main text"
output:
bookdown::pdf_book:
toc: no
---
Here is the main text file. I would like to refer to \#ref(fig:supporting-figure).
Here is the supporting text called supporting.Rmd, and the figure to refer to, saved in the same folder:
---
title: "Supporting info"
output:
bookdown::pdf_book:
toc: no
---
Here is the supporting text.
```{r supporting-figure}
plot(cars)
```
How can I cross-ref supporting-figure in the main text?
I have checked the section on cross-references in Yihui's bookdown manual, but I cannot see how to extend this to cross-references between files.
I also found this question:
Cross-reference figure in a separate Rmarkdown (PDF) file
but the accepted answer does not work for me (perhaps because I am using bookdown rather than base Rmarkdown?)

Answer 1

I am not entirely sure how you are compiling these two files into a single bookdown document, because as it stands they are just two separate R Markdown documents. But there are two issues:
Figures need a caption to be cross-referenced
You can only cross-reference figures which have a caption assigned with fig.cap, as explained here:
If we assign a figure caption to a code chunk via the chunk option
fig.cap, R plots will be put into figure environments, which will be
automatically labeled and numbered, and can also be cross-referenced.
Incorrectly configured bookdown project:
From what I can tell, you have not got the project configured correctly for bookdown:
The main file should be called index.Rmd
Supporting files should not have any YAML
You should include site: bookdown::bookdown_site in the YAML of the main documene
Check out this answer for some tips on a minimal bookdown file.
Solution
index.Rmd
---
title: "Main text"
site: bookdown::bookdown_site
output:
bookdown::pdf_book:
toc: no
---
Here is the main text file. I would like to refer to \#ref(fig:supporting-figure).
supporting.Rmd
Here is the supporting text.
```{r supporting-figure, fig.cap= "Some Figure"}
plot(cars)
```

Answer 2

I ran into the same issue and came up with this solution if you aim at compiling 2 different pdfs. It relies on LaTeX's xr package for cross references: https://stackoverflow.com/a/52532269/576684