Modify chunk options via knit_hooks - r

Question

I want to define a custom chunk hook to certain r-markdown chunks, which override certain chunk options (include and eval, basically).
Use case is that I add this chunk option to a chunk, which will hide code and output per default (include = FALSE) and evaluates the chunk only if a file exists eval = <some logic>. This works if I hardcode it (cf. to chunk export_disp_mpg_by_hand but not via a custom chunk option.
But regardless of how I try it, the options are not considered.
Code
---
title: "Chunks Options"
author: "Me"
output: html_document
---
```{r setup, include = FALSE, message = FALSE}
library(ggplot2)
library(knitr)
library(here)
image_path <- "figs"
if (!dir.exists(here(image_path))) {
dir.create(here(image_path))
}
knit_hooks$set(export_fig = function(before, options, envir) {
if (before) {
fn <- gsub("^export_", "", options$label)
options$include <- FALSE
options$eval <- !file.exists(here(image_path, paste0(fn, ".png")))
### Does not work either
## opts_current$set(include = FALSE,
## eval = !file.exists(here(image_path, paste0(fn, ".png"))))
}
options
})
```
# Plot
```{r make_plot}
(pl <- ggplot(mtcars, aes(disp, mpg)) +
geom_point())
```
```{r export_disp_mpg, export_fig = TRUE}
ggsave(here(image_path, "disp_mpg.png"), pl)
```
```{r export_disp_mpg_by_hand, include = FALSE, eval = !file.exists(here(image_path, "disp_mpg.png"))}
ggsave(here(image_path, "disp_mpg.png"), pl + geom_smooth(method = "lm"))
```
Expected Behaviour
Chunk export_disp_mpg should never be shown in the rendered output and the the file it creates should only be created if it is not existing.

Answer 1

So close, I found the solution. In case anybody is interested, I leave the question here with the follwong working answer.
Instead of using knit_hooks, one should use opts_hooks:
```{r setup, include = FALSE}
opts_hooks$set(export_fig = function(options) {
fn <- gsub("^export_", "", options$label)
options$include <- FALSE
options$eval <- !file.exists(here(image_path, paste0(fn, ".png")))
options
})
```