In R Shiny how do I check the value of a reactive and print a message-CodePudding

I am trying to figure out how to debug in Shiny. I would like to print a message to the console if a reactive takes a specific value. I know I can put a message inside of the reactive but I am curious to know if/how I do it from outside of the of reactive call it self. I tried to use an observeEvent() and it seems to trigger every time the reactive changes. What is the right way to print a message based on the value in a reactive?

library(shiny)

ui <- fluidPage(
  textInput("name_first", "What's your first name?"),
  textInput("name_last", "What's your last name?"),
  textOutput("greeting")
)

server <- function(input, output, server) {
  
  dude <- reactive(paste(input$name_first, input$name_last))
  
  # why does this not work?
  observeEvent(dude() == "a zombie", {message("run")})
  
  output$greeting <- renderText(dude())
  
}

shinyApp(ui, server)

CodePudding user response：

You can use req() to trigger an event only if a reactive has a certain value.

observeEvent(dude(),{
    req(dude() == "a zombie")
    message("run")
})

For debugging shiny apps in general, you can have a look at the browser() function

CodePudding user response：

A possibility:

  isZombie <- reactive({
    if(dude() == "a zombie") TRUE # no need of 'else NULL'
  })
  
  observeEvent(isZombie(), {message("run")})