How to do nested selections selectInput() in RMarkdown with Shiny?

Question

In a snippet of code below from my RMarkdown \ flexdashboard code with shiny , I need to modify the choices for the second selectInput() function, based on the selection made at the first selectInput() function.

selectInput('theme', 'Select theme:',
            choices = c(dtThemes$Theme %>% unique()))  

selectInput('question', 'Select Question:', 
            choices = c(dtQuestions$Question %>% unique())) # This works
            #choices = strQuestions)  # This does not work

strQuestions <- reactive({
    nQuestions <- dtThemes[Theme == input$theme, Q2018]
    dtQuestions[nQuestion %in% nQuestions, strQuestion]
})

How do I do that?

Encapsulating code inrenderUI() did not help:

  renderUI({
    selectInput('question', 'Select Question:', 
                strQuestions(), width="100%") 
  })

Answer 1

I've further studied this post: Create reactive selectInput - flexdashboard with Shiny and was able to figure out how to make my code work:

selectInput('theme', 'Select theme:',
            choices = c("All", dt$Theme %>% unique()), selected="All")

dt.subset<- reactive({
    if (input$theme=="All") {
      dt
    }else{
      dt[Theme == input$theme]
    }
  })
 renderUI({
    selectInput('question', 'Select Question:', 
                choices = (dt.subset()$Question ) %>% unique(), 
                selected =  ( (dt.subset()$Question ) %>% unique() )[1]) 
  })

}

The trick is that you need to have a default selected value. Otherwise,the code crashes.

Note also I'musing data.table for dt.