Avoid unnecessary load in Shiny app

Question

I have an eventReactive function that loads in two datasets. Each dataset can be chosen, but they both loaded within the same eventReactive. However, if the user only changes one of the two datasets, I don't want them both to load but only the one that has changed. I know that this would be much easier with two action buttons but I need to have it in one. Please find example code below.

So in the example, I only want to execute that part of the code again for which the variable has changed. ui.r:

library(shiny)

# Define UI for dataset viewer application
shinyUI(fluidPage(

  # Application title
  titlePanel("Reactivity"),

  # Sidebar with controls to provide a caption, select a dataset,
  # and specify the number of observations to view. Note that
  # changes made to the caption in the textInput control are
  # updated in the output area immediately as you type
  sidebarLayout(
    sidebarPanel(
      textInput("caption", "Caption:", "Data Summary"),
      actionButton('Go','Go'),

      selectInput("dataset", "Choose a dataset:", 
                  choices = c("rock", "pressure", "cars")),
      selectInput("dataset2", "Choose a dataset:", 
                  choices = c("rock", "pressure", "cars")),

      numericInput("obs", "Number of observations to view:", 10)
    ),


    # Show the caption, a summary of the dataset and an HTML 
     # table with the requested number of observations
    mainPanel(
      h3(textOutput("caption", container = span)),
      verbatimTextOutput('one'),
      verbatimTextOutput('two'),
      verbatimTextOutput("summary"), 

      verbatimTextOutput("summary2")#, 

      # tableOutput("view"),
      # tableOutput("view2")
    )
  )
))

server.r

library(shiny)
library(datasets)

# Define server logic required to summarize and view the selected
# dataset
shinyServer(function(input, output) {

  # By declaring datasetInput as a reactive expression we ensure 
  # that:
  #
  #  1) It is only called when the inputs it depends on changes
  #  2) The computation and result are shared by all the callers 
  #   (it only executes a single time)
  #
  datasetInput <- eventReactive(input$Go,{

    ##Only run if input$dataset has changed##
    withProgress(message='Loading 1:This should take very long....',value=0,{
      for(i in 1:5){
        Sys.sleep(1)
        incProgress(i/5)
      }
    x<-switch(input$dataset,
           "rock" = rock,
           "pressure" = pressure,
           "cars" = cars)

    })
###only run if input$dataset2 has changed
    withProgress(message='Loading 2:This should take even longer....',value=0,{
      for(i in 1:5){
        Sys.sleep(1.5)
        incProgress(i/5)
      }       
      y<-switch(input$dataset2,
                         "rock" = rock,
                         "pressure" = pressure,
                         "cars" = cars)
    })




    return(list(x=x,y=y,input1=input$dataset,input2=input$dataset2))
  })

  # The output$caption is computed based on a reactive expression
  # that returns input$caption. When the user changes the
  # "caption" field:
  #
  #  1) This function is automatically called to recompute the 
  #     output 
  #  2) The new caption is pushed back to the browser for 
  #     re-display
  # 
  # Note that because the data-oriented reactive expressions
  # below don't depend on input$caption, those expressions are
  # NOT called when input$caption changes.
  output$caption <- renderText({
    input$caption
  })

  # The output$summary depends on the datasetInput reactive
  # expression, so will be re-executed whenever datasetInput is
  # invalidated
  # (i.e. whenever the input$dataset changes)
  output$summary <- renderPrint({
    dataset <- datasetInput()$x
    summary(dataset)
  })

  output$summary2 <- renderPrint({
    dataset <- datasetInput()$y
    summary(dataset)
  })

  # The output$view depends on both the databaseInput reactive
  # expression and input$obs, so will be re-executed whenever
  # input$dataset or input$obs is changed. 
  output$view <- renderTable({
    head(datasetInput()$x, n = input$obs)
  })
  output$view2 <- renderTable({
    head(datasetInput()$y, n = input$obs)
  })
  output$one<-renderText(datasetInput()$input1)
  output$two<-renderText(datasetInput()$input2)

})

I already tried something like this: if(datasetInput()$input1 != input$dataset){execute...} But it doesn't work, probably because you are using a value within a reactive function that is created in the same function...

Answer 1

You could separate the two data loading steps into reactive expressions on which your eventReactive has a dependency. That way when you click Go it calls the two reactive expressions if they have changed input values, but doesn't if the input has not updated. Eg:

library(shiny)
library(datasets)

ui = fluidPage(
  # Application title
  titlePanel("Reactivity"),
  sidebarLayout(
    sidebarPanel(
      textInput("caption", "Caption:", "Data Summary"),
      actionButton('Go','Go'),
      selectInput("dataset", "Choose a dataset:", 
                  choices = c("rock", "pressure", "cars")),
      selectInput("dataset2", "Choose a dataset:", 
                  choices = c("rock", "pressure", "cars")),

      numericInput("obs", "Number of observations to view:", 10)
    ),

    mainPanel(
      h3(textOutput("caption", container = span)),
      verbatimTextOutput('one'),
      verbatimTextOutput('two'),
      verbatimTextOutput("summary"), 
      verbatimTextOutput("summary2")
    )
  )
)

server = function(input, output) {
  dat1 = reactive({
    withProgress(message='Loading 1:This should take very long....',value=0,{
      for(i in 1:5){
        Sys.sleep(1)
        incProgress(i/5)
      }
      switch(input$dataset,
             "rock" = rock,
             "pressure" = pressure,
             "cars" = cars)
    })
  })

  dat2 = reactive({
    withProgress(message='Loading 2:This should take even longer....',value=0,{
      for(i in 1:5){
        Sys.sleep(1.5)
        incProgress(i/5)
      }       
      switch(input$dataset2,
             "rock" = rock,
             "pressure" = pressure,
             "cars" = cars)
    })
  })

  datasetInput <- eventReactive(input$Go,{
    x = dat1()
    y = dat2()
    return(list(x=x,y=y,input1=input$dataset,input2=input$dataset2))
  })

  output$caption <- renderText({
    input$caption
  })

  output$summary <- renderPrint({
    dataset <- datasetInput()$x
    summary(dataset)
  })

  output$summary2 <- renderPrint({
    dataset <- datasetInput()$y
    summary(dataset)
  })

  output$view <- renderTable({
    head(datasetInput()$x, n = input$obs)
  })

  output$view2 <- renderTable({
    head(datasetInput()$y, n = input$obs)
  })

  output$one<-renderText(datasetInput()$input1)

  output$two<-renderText(datasetInput()$input2)
}

shiny::shinyApp(ui,server)