How come I cannot access my global variable in my shiny_prerendered document?

[my_username]

Hello everyone,

I am writing an RMarkdown document with runtime = shiny_prerendered, but having trouble accessing what should be a global variable.

Basically, I want the user to upload a file to the document. Once that happens, the variable FILE_VALID will switch from FALSE to TRUE. If FILE_VALID == TRUE, a text message should render elsewhere in the document. However, the message does not render even when the value of FILE_VALID changes.

I did my best to create a reprex of the document using online tutorials for guidance, but I think I was unsuccessful. Therefore, I hope you will accept my RMarkdown code as a stand-in reprex.

If anyone can spot the bug(s) in my code, would you please share your insight?

---
title: "Reprex"
output: html_document
---
```{r setup, include=FALSE}
library(shiny)
library(rmarkdown)
knitr::opts_chunk$set(echo = TRUE)
```
```{r load_file}
fluidPage(
  fileInput('file', 'Load any file and then click "Verify file":'),
  actionButton('verify', 'Verify file')
)

FILE_VALID <<- FALSE
```
```{r, context = 'server'}
observeEvent(input$file, {
  if (isTruthy(input$load_file_upload) == FALSE) {
    FILE_VALID <<- TRUE
  }
})
```
```{r show_text}

fluidPage(
  textOutput('text')
)
```
```{r, context = 'server'}
output$text <- renderText({
  if (FILE_VALID == TRUE) {
    print("hi")
  }
})
```
```

[cderv]

I think you are just missing some context about reactivity

Chapter 3 Basic reactivity | Mastering Shiny

FILE_VALID is not a reactive value in your example so renderText() won't be rerun once you upload the file. You're missing some reactivity I believe.

Quick example without using the upload widget

---
title: "Reprex"
output: html_document
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

```{r load_file}
fluidPage(
  actionButton('clickme', 'Click me')
)
```

```{r context = "server"}
FILE_VALID <- reactiveVal(FALSE)
observeEvent(input$clickme, {
    FILE_VALID(TRUE)
})
```

```{r show_text}
fluidPage(
  textOutput('text')
)
```

```{r context = "server"}
output$text <- renderText({
  if (isTruthy(FILE_VALID())) {
    print("hi")
  }
})
```

I would suggest looking at documentation about reactive values.

Disclaimer: I am not expert in shiny, just wanted to check shiny prerendered was working ok.