Если я правильно понял вашу проблему, вы можете использовать оператор observeEvent
:
---
title: "Untitled"
runtime : shiny
output:
flexdashboard::flex_dashboard:
orientation: columns
vertical_layout: fill
---
```{r setup, include=FALSE}
library(flexdashboard)
code <- "This is code"
```
Column {data-width=650}
-----------------------------------------------------------------------
### Chart A
```{r}
actionButton("upload","Upload",width = 150)
actionButton("clear_upload","Clear",width = 150)
verbatimTextOutput("code")
get_code <- eventReactive(input$upload,{
code
})
observeEvent(input$upload, {output$code <- renderPrint(get_code())})
observeEvent(input$clear_upload, {output$code <- renderPrint("")})
```