如何在 R Shiny 中用 reactiveVal 和 ObserveEvent 替换反应性

Question

In studying R Shiny I see that you can use reactive() without an observeEvent() as shown in the demo code below. However I am trying to learn the use of the combined reactiveVal() and observeEvent() functions.

In the demo code, the user can opt to show only the first 4 rows of the data frame (called "data") via the radio button. Super simple. It works fine as written using only reactive() without observeEvent() in the server section. But I am trying to replace this with a combination of reactiveVal() and observeEvent() as shown in the commented-out (#) section in the demo code. What am I doing wrong? How can I make reactiveVal() and observeEvent() work together to give the same results?

Demo code:

library(shiny)

data <- 
  data.frame(
    ID = c(1,1,1,2,2,2,3,3,3),
    Period = c(1, 2, 3, 1, 2, 3, 1, 2, 3),
    Values = c(5, 10, 15, 0, 2, 4, 3, 6, 9)
  )

ui <- fluidPage(
  
  h4(strong("Base data frame:")), 
  tableOutput("data"),
  radioButtons(inputId = "showData",
               label = "Show from original dataframe:",
               choiceNames = c('All','First 4 rows'),
               choiceValues = c('All','Rows'),
               selected = 'All',
               inline = TRUE
              ),
  h4(strong("Reactive results:")), 
  tableOutput("choices"),
  )

server <- function(input, output, session) {
 
  output$data <- renderTable(data)
  
  # Choices <- reactiveVal()
  # 
  # observeEvent(
  #   input$showData, {
  #     if(input$showData == 'Rows'){Choices <- data[1:4,]} else {Choices <- data}
  #   }
  # )
  
  Choices <- reactive(if(input$showData == 'Rows'){data[1:4,]} else {data})
  
  output[["choices"]] <- renderTable({Choices()}
  )

}
shinyApp(ui, server)

Answer 1

Below is the resolved corrected code, incorporating Limey's comments:

library(shiny)

data <- 
  data.frame(
    ID = c(1,1,1,2,2,2,3,3,3),
    Period = c(1, 2, 3, 1, 2, 3, 1, 2, 3),
    Values = c(5, 10, 15, 0, 2, 4, 3, 6, 9)
  )

ui <- fluidPage(
  
  h4(strong("Base data frame:")), 
  tableOutput("data"),
  radioButtons(inputId = "showData",
               label = "Show from original dataframe:",
               choiceNames = c('All','First 4 rows'),
               choiceValues = c('All','Rows'),
               selected = 'All',
               inline = TRUE
              ),
  h4(strong("Reactive results:")), 
  tableOutput("choices"),
  )

server <- function(input, output, session) {
 
  output$data <- renderTable(data)
  
  # The below uses combo of reactiveValues() and observeEvent(), per Limey's comments:
  rv <- reactiveValues(choices=c()) # reactiveValues is for multiple reactive values, and reactiveVal is for a single reactive value
  observeEvent(input$showData, {
      if(input$showData == 'Rows'){rv$choices <- data[1:4,]} 
      else {rv$choices <- data}
      }
    )
  output[["choices"]] <- renderTable({ rv$choices })

}
shinyApp(ui, server)