Display error message when api call comes back empty in Shiny?

Question

I have an interactive visualization that connects to a city government's police data API.

When certain combinations of inputs are selected, my API call comes back empty and I get a nasty red error message (as my plot inputs are unavailable).

Can someone tell me how to display a more informative error message along the lines of, "there are no incidents matching your selection, please try again"? I would like this error message to appear as a showNotification and my ggplot not to render.

Below is an extremely stripped down version of what I am doing. Note how when a combination like "AVONDALE" and "CHEMICAL IRRITANT" is selected, the chart renders, whereas when a combination like "ENGLISH WOODS" and "TASER-BEANBAG-PEPPERBALL-40MM FOAM" is selected, an error message is returned. This error message is what I would like to address with a showNotification alert.

Note that this uses the Socrata API, so the package RSocrata must be installed and loaded.

install.packages("RSocrata")
library(shiny)
library(reshape2)
library(dplyr)
library(plotly)
library(shinythemes)
library(tibble)
library(RSocrata)

# Define UI for application that draws a histogram
ui <- fluidPage(
  navbarPage("Example", 
             theme = shinytheme("united"),
             tabPanel("Plot",
                      sidebarLayout(
                        sidebarPanel(

                          # neighborhood selector
                          selectizeInput("neighbSelect", 
                                         "Neighborhoods:", 
                                         choices = c("AVONDALE", "CLIFTON", "ENGLISH WOODS"), 
                                         multiple = FALSE)),

                          # incident description selector
                          selectizeInput("incSelect", 
                                         "Incident Type:", 
                                         choices = c("CHEMICAL IRRITANT", "TASER-BEANBAG-PEPPERBALL-40MM FOAM"), 
                                         multiple = FALSE))
                        ),

                        # Output plot
                        mainPanel(
                          plotlyOutput("plot")
                        )
                      )
             )

# Define server logic
server <- function(input, output) {
  forceInput <- reactive({
    forceInput <- read.socrata(paste0("https://data.cincinnati-oh.gov/resource/e2va-wsic.json?$where=sna_neighborhood= '", input$neighbSelect, "' AND incident_description= '", input$incSelect, "'"))
  })

# Render plot
  output$plot <- renderPlotly({
    ggplot(data = forceInput(), aes(x = sna_neighborhood)) +
      geom_histogram(stat = "count")
  })
}

# Run the application 
shinyApp(ui = ui, server = server)

Thank you so much for any help anyone can offer!

Answer 1

Im going to give an example with the shinyalert library to have the popup. Here I added the sample choice TEST to indicate no data:

#install.packages("RSocrata")
library(shiny)
library(reshape2)
library(dplyr)
library(plotly)
library(shinythemes)
library(tibble)
library(RSocrata)
library(shinyalert)

# Define UI for application that draws a histogram
ui <- fluidPage(
  useShinyalert(),
  navbarPage("Example", 
             theme = shinytheme("united"),
             tabPanel("Plot",
                      sidebarLayout(
                        sidebarPanel(

                          # neighborhood selector
                          selectizeInput("neighbSelect", 
                                         "Neighborhoods:", 
                                         choices = c("AVONDALE", "CLIFTON", "ENGLISH WOODS","TEST"), 
                                         multiple = FALSE)),

                        # incident description selector
                        selectizeInput("incSelect", 
                                       "Incident Type:", 
                                       choices = c("CHEMICAL IRRITANT", "TASER-BEANBAG-PEPPERBALL-40MM FOAM"), 
                                       multiple = FALSE))
             ),

             # Output plot
             mainPanel(
               plotlyOutput("plot")
             )
  )
)

# Define server logic
server <- function(input, output,session) {

  forceInput <- reactive({
    forceInput <- read.socrata(paste0("https://data.cincinnati-oh.gov/resource/e2va-wsic.json?$where=sna_neighborhood= '", input$neighbSelect, "' AND incident_description= '", input$incSelect, "'"))

    if(nrow(forceInput)==0){
      shinyalert("Oops!", "No data returned", type = "error")
      forceInput <- NULL
    }
    forceInput
  })

  # Render plot
  output$plot <- renderPlotly({
    req(forceInput())
    ggplot(data = forceInput(), aes(x = sna_neighborhood)) +
      geom_histogram(stat = "count")
  })
}

# Run the application 
shinyApp(ui = ui, server = server)