Reactive input not working with ggvis and Shiny

Question

Simple example of a Shiny app using ggvis. Trying to use a pulldown to filter a variable. So here I'm trying to filter by mtcars' gear (either 3, 4, or 5), then plotting x and y of mpg and hp for each of the unique values of gear.

I get the initial plot drawn with a default of '3' selected, but if I change the value via the pulldown nothing happens. I think I know where things are going wrong (commented in the code), but I've tried just about everything I can think of and have no idea what the actual mistake I'm making is.

Thanks

ui.R

# ui.R

library(shiny)

shinyUI(fluidPage(
  titlePanel("Car Thing"),

  sidebarLayout(
    sidebarPanel(

        uiOutput("choose_gear")                     
    ),

    mainPanel(
       ggvisOutput("ggvis")         
    )
  )
))

server.R

#server.R

library(shiny)
library(ggvis)
library(dplyr)

gear_nos <- sort(unique(mtcars$gear))

    shinyServer(function(input, output, session) {

        output$choose_gear <- renderUI({        
            selectInput("gears", "Choose Gear", gear_nos, selected="3")                                 
        })              

        # I'm pretty sure this is where I'm messing something up
        pickedGear <- reactive({
            mtcars %>% filter(gear == input$gears)
        })

        if(is.null(dim(pickedGear))){
            pickedGear <- mtcars[mtcars$gear == 3,]
        }

        pickedGear %>% ggvis(~mpg, ~hp) %>% layer_points(fill := "green") %>% bind_shiny("ggvis")

})

Answer 1

I think this might be what you want.

Note that it took me quite awhile to figure out the validate piece that eliminates an extraneous error message (incorrect string: length(0) 32 expected) on startup initialization of the shinyServer code, but I will remember it for the future now I guess.

library(shiny)
library(ggvis)
library(dplyr)
# library(googleVis) # used observe instead now

u <- shinyUI(fluidPage(
  titlePanel("Car Thing"),

  sidebarLayout(
    sidebarPanel(
      uiOutput("choose_gear")                     
    ),
    mainPanel(
      ggvisOutput("ggvis")         
    )
  )
))
gear_nos <- sort(unique(mtcars$gear))

s <- shinyServer(function(input, output, session) {

  output$choose_gear <- renderUI({
      selectInput("gears", "Choose Gear", gear_nos, selected="3")
  })              

  pickedGear <- reactive({
    shiny::validate(need(input$gears, message=FALSE))
    mtcars %>% filter(gear == input$gears)
  })

  # could also replace "observe" with this from googlevis : "output$ggvis <- renderGvis({" 
  observe({
   pickedGear() %>% ggvis(~mpg,~hp) %>% layer_points(fill:="green") %>% bind_shiny("ggvis")
  })
})
shinyApp(u,s)

Yielding: