test if reactive value does not exist or is empty

[mattbroerman]

I am trying to build an app that joins together different data sources conditional on user input. But I need to test the existence of reactives to do that, and can't figure it out.

I have one main dataset1 that I want to always pass back to the user (for a download, but here, a renderPrint) and then another dataset2 that I join to it, if it exists.

Here's a shiny reprex, key line at isTruthy:

library(shiny)
library(tibble)
library(dplyr)
library(magrittr)

app_ui <- function() {
  tagList(fluidPage(
    actionButton("get1", "Get dataset1"),
    actionButton("get2", "Get dataset2"),
    verbatimTextOutput("view")
  ))
}

app_server <- function(input, output) {
  dataset1 <-
    eventReactive(input$get1, {
      mtcars %>% head() %>% rowid_to_column()
    })
  
  dataset2 <-
    eventReactive(input$get2, {
      chickwts %>% head() %>% rowid_to_column()
    })
  
  dataset_joint <- reactive({
    if (isTruthy(dataset2())) {
      dataset1() %>%
        left_join(dataset2())
    } else {
      dataset1()
    }
  })
  
  output$view <-
    renderPrint({
      dataset_joint()
    })
}

shinyApp(app_ui, app_server)

I get the behavior I want if I replace isTruthy(dataset2()) with isTruthy(input$get2). But in my actual use-case, I am passing datasets between modules and testing the existence of a dataset is easier than passing around both the input and dataset and then testing the input.