Update vignette about knit_print for pkg author

[cderv]

Context

R 3.6 and R 4.0 have change things for S3 methods lookup and the search is not more used.

• About the change : https://developer.r-project.org/Blog/public/2019/08/19/s3-method-lookup/index.html
• CRAN doc: https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Registering-S3-methods

This has caused so issue in certain packages that where not following closely the advice in the vignette
vegawidget/vegawidget#132 (comment)

The vignette should be updated with more examples and mention of the importance of exporting due to this new behavior
https://cran.r-project.org/web/packages/knitr/vignettes/knit_print.html#for-package-authors

Different solutions:

Import the generic and package export S3 method

@importFrom knitr knit_print
@export
knit_print.class <-

The package must be in Import in the DESCRIPTION file

Register the method dynamically

• No easy way with base R and it uses internal function.
• vctrs has a helper for that: register_s3_method - that can be copied - pasted as MIT licenced
• R 3.6 + offers a function .S3method()

See examples for some of this:

• tibble
  • https://github.com/tidyverse/tibble/blob/dd29fb76b6b0335a08658c14483fe3cbb81cf7d7/R/aaa-pillar.R#L12
  • https://github.com/tidyverse/tibble/blob/75bcebc86b8a36889a0fb47bfe72275bfb1f4fb2/R/zzz.R#L17
• htmlwidget
  • https://github.com/ramnathv/htmlwidgets/blob/9df7c9e4ec669a73c83559849dc8b2e5d1821d25/R/knitr-methods.R#L4
• gt
  • https://github.com/rstudio/gt/blob/bae32f432f190a904e26c3d1396b63ea65e5106a/R/zzz.R#L2

Delayed registration

R 3.6 introduce a new way to use S3method in NAMESPACE: S3method(knitr::knit_print, class)

With rogygen2 it is

@exportS3Method knitr::knit_print
knit_print.class <- 

[yihui]

R 3.6 introduce a new way to use S3method in NAMESPACE: S3method(knitr::knit_print, class)

Oh I didn't know that. That seems to be the simplest method. From what I understand, it doesn't require knitr to be in Imports, I'd advocate this method. The requirement for R >= 3.6.0 doesn't sound too bad to me (v3.6.0 was released on 2019-04-26, which feels like a century ago now? :)

But other methods can certainly be mentioned in the vignette.

[github-actions]

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